A
TEXT-BOOK
OF TANNING

Pl. I.

E. & F. N. Spon, London & New York.

"INK-PHOTO." SPRAGUE & CO. LONDON.

LIME PITS AND RINSING TANKS.

A

TEXT-BOOK OF TANNING:

A TREATISE ON THE

CONVERSION OF SKINS INTO LEATHER,

BOTH PRACTICAL AND THEORETICAL.

BY

HENRY R. PROCTER, F.C.S.,

OF LOWLIGHTS TANNERY;
EXAMINER IN TANNING TO THE CITY AND GUILDS TECHNICAL INSTITUTE.

With 8 Plates and numerous Illustrations.

E. & F. N. SPON, 125, STRAND, LONDON.
NEW YORK: 35, MURRAY STREET.
1885.

[PREFACE.]

The aim of the following handbook is two-fold; to give, in a compendious form, such a summary of our scientific knowledge as may be useful to the practical tanner; and such a sketch of manufacturing processes as may enable the chemist to apply his knowledge to their improvement. Each may, therefore, find some superfluous matter, for which his indulgence is asked. The book is an expansion of a short article which appeared in Spons' 'Encyclopædia of the Industrial Arts,' and to some extent still bears traces of its origin; and, having been written under stress of limited leisure, and defective eyesight, is very far from being so perfect as I should desire. For the sake of completeness it has been necessary to describe many processes which are outside the range of my own manufacturing experience, and in doing so I have generally referred to the sources of my information. Chapters [III]. and [XXIV]. are written by Mr. C. G. Warnford Lock, to whose kind assistance I am much indebted. It may be well to state in conclusion, that while the work is not intended for a cram-book for technical students, it is hoped that it may be an assistance to teachers of the subject.

HENRY R. PROCTER.
Tynemouth,
August 1885.

[CONTENTS.]

A
TEXT-BOOK OF TANNING
ETC., ETC.

INTRODUCTORY NOTE.

Leather manufacture may be broadly divided into two stages: "tanning," in which the raw hide is converted into the imputrescible and more or less flexible material known as "leather"; and "currying," in which this leather is further manipulated, and treated with fatty matters, to soften and render it more waterproof, and to improve its appearance. Glove-kid, and certain other leathers, however, are not tanned at all, but "tawed," or prepared with a mixture in which alum and salt are the most active ingredients; chamois, "shammy," or "wash" leather, is produced by fulling with oil alone, and many leathers can scarcely be said to be curried, although more or less oil is used in the final processes of "finishing" or "dressing." The first subject to be treated of in this work will be the operation of tanning, properly so called, taking for example the tannage of sole- and belting-leather. This demands thorough explanation, in both its practical and theoretical aspects, not only because it is one of the most important branches of the trade, but because the principles involved are those which equally underlie all other tanning methods. The next to be dealt with will be the modifications of the process which are necessary in tanning the more flexible leathers used for boot-uppers, hose-pipes, and saddlery purposes; then the currying of these leathers; and finally, the manufacture of moroccos, Russian, and japanned leathers, calf- and glove-kid, &c.

[CHAPTER I.]

ANATOMICAL STRUCTURE OF HIDE.

Before speaking of actual processes of manufacture, it is necessary to devote some attention to the structure and chemical constitution of hide or skin, which forms the raw material. Although a great variety of skins are employed in tanning, they are all constituted on the same general type, and an anatomical description of the hide of the ox will apply almost equally to those of the calf, sheep, and goat; but from differences in thickness and closeness of texture, their practical uses differ widely. [Fig. 1] shows a section of ox-hide, cut parallel with the hair, magnified about 50 diam.: a, epithelial layer or epidermis, consisting of horny layer above, and rete malpighi below; b, pars papillaris, and c, pars reticularis of corium, derma, or true skin; d, hairs; e, sebaceous or fat-glands; f, sudoriferous or sweat-glands; g, opening of ducts of sweat-glands; h, erectores pili muscles, for erecting the hair.

The fresh hide consists of 2 layers: an outer, the epidermis; and an inner, the true skin. The epidermis is very thin as compared with the true skin which it covers, and is entirely removed preparatory to tanning; it nevertheless possesses important functions. It is shown in [Fig. 1] at a, and more highly magnified in [Fig. 2]. Its inner mucous layer b, the rete malpighi, which rests upon the true skin c, is soft, and composed of living nucleated cells, which are elongated in the deeper layers, and gradually become flattened as they approach the surface, where they dry up, and form the horny layer a. This last is being constantly worn away, and thrown off as dead scales of skin; and as constantly renewed from below, by the continued multiplication of the cells. It is from this epithelial layer that the hair, as well as the sweat- and fat-glands, are developed. It will be seen in [Fig. 1] that each hair is surrounded by a sheath, which is continuous with the epidermis. In embryonic development, a small knob of cells forms on the under side of the epidermis, and this enlarges, and sinks deeper into the true skin, while the root of the young hair is formed within it; this is shown in [Fig. 3], a b. Smaller projections also form on the stalk of the knob, and in due time produce the sebaceous glands. The process of development of the sudoriferous glands is very similar to that of the hairs. There is a great analogy between this process and that of the ordinary renewal of hair in the adult animal. At d¹ [Fig. 1], is seen an old and worn-out hair. It is shrunken and elongated, and is almost ready to fall out. It will be noticed that its sheath or follicle projects somewhat below the hair to the right. This is the first production of a young hair, and is quite analogous to the knob of epithelium which has been described as forming the starting-point of a hair in embryo. At d², the same process is seen further advanced, the young hair being already formed, and growing up into the old sheath. At d³, it is complete, the old hair having fallen out, and the young one having taken its place.

Fig. 1.

Fig. 2.

Fig. 3.

The hair itself is covered with a layer of overlapping scales, like the slates on a roof, but of irregular form. These give it a serrated outline at the sides, strongly developed in wool. Within these scales, which are sometimes called the "hair cuticle," is a fibrous substance, which forms the body of the hair; and sometimes, but not always, there is also a central and cellular pith, which is mostly transparent, though under the microscope it frequently appears black and opaque, from the optical effect of imprisoned air. On boiling or long soaking in water, alcohol, or turpentine, these air-spaces become saturated with the liquid, and then appear transparent.

The fibrous part of the hair is made up of long spindle-shaped cells, and contains the pigment which gives the hair its colour. The hair of the deer differs from that of most other animals in being almost wholly formed of polygonal cells, which, in white hairs, are usually filled with air. At its base, the hair swells into a bulb, which is hollow, and rests on a sort of projecting knob of the corium, called the hair-papilla. This has blood-vessels and nerves, and supplies nourishment to the hair. The hair-bulb is composed of round, soft cells, which multiply rapidly; as they grow, they press upward through the hair-sheath, become elongated and hardened, and form the hair. In dark hairs, both the cells of the hair itself and those of its follicle or sheath are strongly pigmented, but the hair much the more so, and hence the bulb has usually a distinct dark form. The dark-haired portions of a hide from which the hair has been removed by liming still remain coloured, from the pigmented cells of the hair-sheaths, which can only be got out completely by bating and scudding. The cells outside the bulb, shown at f, in [Fig. 4], pass upwards as they grow, and form a distinct coating around the hair, which is called the "inner root-sheath." This again consists of 2 separate layers, of which the inner is "Huxley's," the outer, "Henle's." They arise from the same cells in the base of the hair; but in the inner layer, these remain polygonal and nucleated, while in the outer, they become spindle-shaped and without nuclei. The inner root-sheath does not extend to the surface of the skin, but dies away below the sebaceous glands. This figure represents an ox-hair root, mag. 200 diam.: a, fibrous substance of hair; b, hair cuticle; c, inner root-sheath; d, outer root-sheath; e, dermic coat of hair-sheath; f, origin of inner sheath; g, bulb; h, papilla.

Fig. 4.

Outside the inner root-sheath is a layer of nucleated cells, continuous with those of the epidermis, and of the same character. This is the "outer root-sheath," and is shown at d, [Fig. 4]. This, together with the whole of the epidermis, is covered next the corium with an exceedingly fine membrane, called the "hyaline" or glassy layer. It is possible that this forms the very thin buff-coloured "grain" of tanned leather, which evidently is of different structure from the rest of the corium, since, if it gets scraped off before tanning, the exposed portion of the pars papillaris remains nearly white, instead of colouring. The whole of the hair-sheath is enclosed in a coating of elastic and connective-tissue fibres, which are supplied with nerves and blood-vessels, and form part of the corium. Near the opening of the hair-sheaths to the surface of the skin, the ducts of the sebaceous or fat-glands (e, [Fig. 1]), pass into them, and secrete a sort of oil to lubricate the hair. The glands themselves are formed of large nucleated cells, arranged somewhat like a bunch of grapes; one is shown highly magnified in [Fig. 5]: a, sebaceous gland; b, hair-stem; c, part of erector pili muscle. The upper and more central cells are most highly charged with fat, which is shown by the darker shading.

Fig. 5.

As already remarked, the sudoriferous or sweat-glands are also derived from the epidermis layer. They are shown at f, [Fig. 1], and on a larger scale (200 diam.) in [Fig. 6]: a, windings laid open in making section; they consist, in the ox and sheep, of a large wide tube, sometimes slightly twisted. In this, they differ considerably from those of man, which form a spherical knot of extremely convoluted tube. The walls of these glands are formed of longitudinal fibres of connective tissue of the corium, lined with a single layer of large nucleated cells, which secrete the perspiration. The ducts, which are exceedingly narrow, and with walls of nucleated cells like those of the outer hair-sheaths, sometimes open directly through the epidermis, as shown at g, [Fig. 1], but more frequently into the orifice of a hair-sheath, just at the surface of the skin. Each hair is provided with a slanting muscle (h, [Fig. 1]), called the arrector or erector pili, which is contracted by cold or fear, and causes the hair to "bristle," or stand on end; by forcing up the attached skin, it produces the effect known as "goose-skin." The muscle, which is of the unstriped or involuntary kind, passes from near the hair-bulb to the epidermis, and just under the sebaceous glands, which it compresses.

Fig. 6.

The corium or true skin is principally composed of interlacing bundles of white fibres, of the kind known as "connective tissue"; these are composed of fibrils of extreme fineness, cemented together by a substance of different composition from the fibres themselves. This may be demonstrated by steeping a small piece of hide for some days in a stoppered bottle in lime-, or baryta-water, in which the inter-fibrillar substance is soluble, and then teasing a small fragment of the fibre with needles on a glass microscope-slide, and examining with a power of at least 200-300 diam. In the middle portion of the skin, these bundles of fibre are closely interwoven; but next the body, they gradually become looser and more open, forming the pars reticularis (or netted part); and the innermost layer is a mere network of loose membrane, generally loaded with masses of fat-cells, and hence called adipose tissue.

It is this adipose tissue which is removed in the "fleshing" process. On the other hand, the outermost layer, just beneath the epidermis, is exceedingly close and compact, the fibre-bundles that run into it being separated into their elementary fibrils, which are so interlaced that they can scarcely be recognised. This is the pars papillaris, and forms the lighter-coloured layer, called (together with its very fine outer coating) the "grain" of leather. It is in this part that the fat-glands are embedded, while the hair-roots and sweat-glands pass through it into the looser tissue beneath.

Besides the connective-tissue fibres, the skin contains a small proportion of fine yellow fibres, called "elastic" fibres. If a thin section of hide be soaked for a few minutes in strong acetic acid, and then examined under the microscope, the white connective-tissue fibres become swollen and transparent, and the yellow fibres may then be seen, as they are scarcely affected by the acid. The hair-bulbs and sweat- and fat-glands are also rendered distinctly visible.

The nerves of the skin are very numerous, each hair being supplied with fibres passing into both the papilla and sheath. They also pass into the skin papillæ. They cannot readily be seen, without special preparation, and, so far as is known, exercise no influence on the tanning process. "Breaking the nerve" is a technical term, which signifies a thorough stretching and softening of the skin, but has nothing to do with nerves properly so called. The blood- and lymph-vessels are, from the present point of view, somewhat more important. They may often be seen in sections, and are lined with nucleated cells, similar to those of the glands. These are surrounded by coatings of unstriped muscular fibre, running both around and lengthways, and also by connective-tissue fibres. In the arteries, the muscular coating is much stronger than in the veins.

It may be thought that the space devoted to a discussion of the anatomical structure of the skin is disproportionately large; but there can be no doubt that, in order to make improvements, nothing is of more importance than a clear conception, even to the smallest details, of the materials and causes to be dealt with. The illustrations are from actual specimens, and enable the various parts of the hide to be identified under the microscope.

As this instrument is a most useful means of investigation in the tanning industry, and one likely to be of increasing importance, it will be well, before proceeding further, to say a few words, both on the selection of a suitable instrument, and on its manipulation in general.

To do useful work, it is not necessary to possess a very elaborate or expensive instrument, but it is essential that the microscope be well made and good of its kind. As high powers are often required in the examination, both of hide sections and of ferments, which are the principal objects of investigation in a tannery, it is of the first importance that the fine adjustment should be perfectly steady, without vibration or backlash. This, in the writer's experience, is never the case with cheap microscopes, in which the fine adjustment is made by a screw at the side of the tube moving the nose by means of a lever. A much more satisfactory arrangement is that in which the whole body of the microscope is raised or lowered by a screw in a pillar at the back of the stand on which it slides. A rack for the coarse adjustment is useful, but not essential. If a sliding tube only is provided, it must be tight enough not to slip, but must move easily up and down with a sort of screwing movement. A mechanical stage is not at all necessary, and for most purposes one of black glass is better as well as cheaper. The diaphragm for regulating the light should be as near level with the surface of the stage as possible, and when examined with a low power should appear in the centre of the field. For research work on the minuter ferments, an achromatic condenser and the finest oil- or water-immersion lenses are necessary, but directions for this are beyond the scope of the present work. It may, however, be mentioned that Prof. Flügge,[A] a first-class authority on the subject, especially recommends Abbé's illuminating apparatus as made by Zeiss.

[A] "Fermente und Mikroparasiten," Leipzig, 1883.

A frequent defect in cheap English microscopes is that the mirror for substage illumination does not bring the rays of a lamp to a focus exactly on the slide, but frequently some inches above it. This may be to a great extent overcome by the use of a bulls'-eye condenser between the lamp and the microscope. Another defect is that sometimes the centre of the mirror is not in a line with that of the microscope body.

The objectives (or lenses at the lower end of the microscope) are the most important part of the instrument, and however good it may be in all other respects, if these are defective the whole is useless. The most useful lenses for our purpose, if only two are to be selected, are a 1-in., magnifying about 50 diam., and a ¹/₄-in., magnifying about 200 to 400, according to the eye-piece; a ¹/₈-in. giving, say, twice this magnification will be needed to see the smaller bacteria distinctly, but it is possible just to see even the small putrefaction bacteria with a really fine ¹/₄-in. In any case, the highest power should be as perfect and of as large an angle as attainable. A good ¹/₄-in. should resolve Pleurosigma angulatum with direct light, and should show the movement of the granules of protoplasm in the round corpuscles which are present in saliva. In using the latter test, it must be remembered that the motion only lasts a very short time on a cold slide.

About 5l. is the very least for which a microscope can be obtained which is suitable for tanners' use; where it can be afforded, a better one is advisable.

Without disparaging other makers, it may be mentioned that the writer has generally used both the eye-pieces and objectives of Dr. Hartnack of Potsdam; and that they are moderate in price, at least for the dry combinations, and perfectly satisfactory for all technical purposes. Numbers 2, 5, and 8 objectives with No. 3 eye-piece, are sufficient for all ordinary work. If only 2 objectives are to be obtained, Nos. 3 and 7 would be perhaps the best selection. It is always better to use objectives on the stand, and with the eye-pieces for which they are intended, but in case Hartnack's objectives are used on an English stand (which is easily done by means of an adaptor ring), it is important to remember that they are constructed to work with a shorter tube than that customary on English microscopes, and that they will not perform well if its length is much more than 6 in.; these objectives are not provided with a movable adjustment for thickness of cover-glasses, which for technical purposes is not required, and in inexperienced hands is apt to prove troublesome. Extra-thin covers must therefore always be used. Where this adjustment is provided, the object must be accurately focused, and then, maintaining this focus with the fine focusing-screw, the collar must be cautiously turned till the best definition is obtained. Practically it will be best to make this adjustment accurately once for all, and to take care to use covers selected of a uniform thickness.

High-power objectives of wide angle (which condition is essential to good defining power) necessarily work extremely close to the object, and it is always best to use the thinnest cover-glasses which can be got. Even then, with such glasses as Hartnack's No. 8, unless the sections are very thin, it will be impossible to examine their lower parts; and one of the greatest difficulties of microscopic research is to obtain them thin enough. It will be obvious, from what has been said, that the greatest care is needed to avoid screwing the objective down on the cover, and so breaking one or both of them. One way to avoid this is to screw down as close as possible to begin with, and then focus upwards. Another plan, when the object on the slide is small, is to keep continuously moving the slide gently with the fingers, while looking into the tube. It is then easy to notice when the dust and small particles on the slide come into focus, and if the point should happen to be overstepped the contact will generally be felt before serious damage is done.

Illumination is one of the most important points in practical microscopy. With powers of not less than ¹/₂-in. focus, objects may generally be examined by light thrown upon them from above by a bulls'-eye condenser, or by good daylight. In this case they need not be transparent; and the plan is often convenient for a mere surface examination. In examining bodies illuminated in this way, prominences often appear as hollows and vice versâ, by a sort of optical illusion, which, once established, is very difficult to overcome. By remembering the direction of the light, and that this appears reversed in the microscope, it is easy to decide the truth.

For all finer work and higher powers, and most generally with the low powers also, it is necessary to render the object transparent, and to examine it by light transmitted from the mirror below the stage.

Good daylight is least trying to the eyes. Where artificial light must be used, that of a small paraffin lamp is best; and a blue chimney, or blue glass interposed between the stage and mirror, or lamp and microscope, spares the sight, and makes it easier to distinguish colours. The light should be sufficient, but not too dazzling. Work should never be prolonged after the least strain is felt, nor should the microscope be used for some little time after a meal. It is well to accustom oneself to keep both the eyes open while observing.

If it be required to see how far the cellular structures of the hide, such as hair-sheaths and fat-glands, are affected or destroyed in any stage of liming or bating, the following ready method may be employed. If a strip of hide be cut ²/₃ through from the grain side, as shown at a in [Fig. 7], and the flap be turned down, and held between the finger and thumb, the fibrous tissue will be put on the stretch, and will then allow a moderately thin shaving (including the grain and parts immediately below it) to be cut by a sharp razor. The hide should be held in the positions shown, and a steady drawing cut be made from flesh to grain, the razor being steadied on the tip of the forefinger, and its hollow surface flooded with water. If the thin section be now placed on a glass slide, moistened with a drop of water, and examined on the microscope under a strong light from above, with a 1-in. objective, the fat-glands will be seen as yellow masses, embedded in the white fibrous tissue. If a drop of a mixture of equal vols. of strong acetic acid, glycerin, and water be used to moisten the section, the fibrous tissue will become quite transparent, and whatever remains of the cellular tissue will be easily visible, and may even be studied under tolerably high powers if covered with a thin glass, and lighted by the mirror from below. (The cover-glass must be carefully cleaned by rubbing with a linen handkerchief, and placed in position with a pair of tweezers, one side being supported by a needle, which is gradually withdrawn, so as to avoid air-bubbles.) Care must be taken that this mixture does not touch the brass-work of the microscope; even the vapour is apt to tarnish, so that the preparation must not remain longer than necessary on the microscope. The same method is applicable for ascertaining the completeness of the tannage of leather, and to decide whether the hide fibre is really tanned, or only dyed. Actually tanned leather is unaffected by the acetic acid, but raw or only stained hide swells and becomes transparent.

Fig. 7.

To prepare the very thin sections necessary for detailed study of the hide, more complicated methods are required. Small slips of hide, not exceeding ¹/₄ in. wide, and cut exactly across the lie of the hair, are placed first in weak alcohol (equal parts methylated spirit and water), and, after a few hours, are removed into strong methylated spirit. It is then kept for some days in absolute alcohol, which must be repeatedly changed, until the hide is hard enough to give fine shavings, and may be cut either when held as above described, between cork or pith, or when embedded in paraffin wax. This is accomplished by placing the piece of hide in a little paper-box and covering it with melted paraffin (candle), which is just beginning to stiffen. The piece of hide may be fixed in position with a needle, which must of course be withdrawn before cutting. When hard, the paraffin is shaved away till the object is exposed, when it may be cut. The razor must be wet with alcohol, and the section be made exactly in the plane of the hair-roots, which may be seen with a hand-lens. (The use of a microtome for hide-sections is rarely successful, as it is almost impossible to fix the fragment of hide so that it is cut exactly with the hairs.) The slices may now be stained by placing them in a watch-glass with water and a few drops of the logwood or picrocarmine staining-mixtures sold by opticians, and afterwards either examined in glycerin, or, after soaking some hours in absolute alcohol, may be transferred to clove-oil, and afterwards to a slide, and covered with a drop of dammar varnish or Canada balsam dissolved in chloroform. The sections moistened with glycerin may also be mounted in Farrant's solution or glycerin jelly, under a cover-glass for permanent preservation. If picrocarmine be used, the connective-tissue fibres (gelatinous fibres) and the nuclei of the cells will be coloured red, and the cells themselves of both epidermis and glands, together with the muscles and elastic fibres, will be yellow.

Franz Kathreiner, who has made very elaborate researches on skin, and the changes which take place in it during the processes of tanning, employs a mixture of osmic and chromic acids for hardening, and at the same time staining the tissue. This mixture was first used by a German histologist with whose name I am not acquainted, in a research on the internal organs of hearing, and was applied by Kathreiner in 1879 to the investigation of skin, and communicated by him to the writer in the autumn of that year. His method is briefly as follows. The pieces of hide to be examined must, if salted, be well washed, or if dry, be thoroughly softened. For the study of hide in its unaltered and natural condition, it is essential that it be quite fresh, and taken from the animal as soon as possible after death. In any case the Panniculus adiposus or fatty layer is, as far as possible, removed with scissors, the hair cut short, and the skin cut up into little pieces of 3-4 millimetres wide by 10-12 millimetres long (about ¹/₈ in. by ¹/₂ in.); the hair must lie exactly across these pieces.

They are then placed for 4-8 days, according to the thickness of the hide, in about 12 times their volume of a solution consisting of

[B] Solution of osmic acid is best preserved in sealed tubes in the dark. If obtained in solution it is rarely of full strength, for which allowance may have to be made. Care must be taken to avoid inhaling its fumes, which are very irritating to the eyes and to the respiratory organs, producing severe catarrh.

This solution must be kept from dust and light, in a glass stoppered bottle, and in a cool place. On removing the hide-pieces from this solution, they are placed in about 12 times their volume of absolute alcohol for 4-8 days, during which time the spirit must be at least 3 times renewed. The sections are cut with a razor flooded with absolute alcohol, so that the thin shavings float without friction upon it. The hide-pieces may be held either between soft cork, or, as is generally preferable, simply between the forefinger and thumb as shown in [Fig. 7]. The cut must be made exactly parallel with the direction of the hair roots, and from the grain towards the flesh; and the sections cannot possibly be too thin. After lying for ¹/₂-1 hour in absolute alcohol, the sections are soaked till quite clear in clove oil (which must be pale and of the purest kind), and may then be mounted in dammar varnish, or solution of Canada balsam.

In these sections, fat and the oily contents of the fat glands are stained black, and the limits of the cells both of these glands and of other elements of the hide (rete malpighi, hair-bulb, &c.) are made very distinct, so as to be capable of the most delicate investigation under the highest powers; but the beginner will learn most easily to recognise the different tissues by studying at first some sections stained with picrocarmine as before described. The method is admirably adapted for the study of hide as affected by the limes and bates.

[CHAPTER II.]

CHEMICAL COMPOSITION OF HIDE.

The chemical composition of skin is very imperfectly understood. The bulk of the skin is, as has long been known, converted by boiling into gelatin or glue. The yellow fibres and cellular tissue remain undissolved. Müntz, who made some interesting researches on the subject, found that completely dried hide contained—3·086 per cent. of cellular tissue insoluble in hot water, 1·058 of fat, 0·467 of mineral matter, and 95·395 of matters soluble in hot water. Müntz counts the whole of the tissue soluble in hot water as converted into glue; but this is not strictly the case. Gelatin is not identical with the fibre of the hide, which is only converted into it by boiling. The nature of the change is not well understood; but it is either simply molecular, or depends on the addition of one or more molecules of water. The gelatin of bones seems identical with that of skin and connective tissue, but that of cartilage differs slightly from it, and is called chondrin. Raw hide, unhaired and purified, contains, according to Müntz—carbon, 51·43 per cent.; hydrogen, 6·64; nitrogen, 18·16; oxygen, 23·06; ash, 0·71; while gelatin has—carbon, 50·1 per cent.; hydrogen, 6·6; nitrogen, 18·3 (Mulder); carbon, 50 per cent.; hydrogen, 6·5; nitrogen, 17·5 (Fremy). Probably, however, neither substance was quite pure.

Gelatin is insoluble in alcohol, ether, and cold water, but swells in the last, absorbing about 40 per cent. It is soluble in hot water, but is reprecipitated on the addition of a sufficient quantity of alcohol, resembling in this respect gum, dextrin, and many other substances. It is soluble in glycerin, with the aid of heat, and in concentrated sulphuric acid in the cold. Moist gelatin exposed to the air rapidly putrefies. It first becomes very acid, from formation of butyric (and perhaps other) acids, but afterwards alkaline, from evolution of ammonia. Boiled with concentrated potash, it yields leucin (amidocaproic acid, C₆H₁₅NO₂), glycocin (sugar of gelatin), and other substances. The same products are obtained by boiling with sulphuric acid, and probably also more gradually, and in greater or less proportions, by the prolonged action of lime or barium hydrate, by putrefaction, and by any other influence which tends to resolve the gelatin molecule into its simpler parts. Gelatin is precipitated by all tannins, even from very dilute solution. A solution containing ²/₁₀₀₀₀ parts is rendered turbid by infusion of gall-nuts or gallotannic acid. The precipitate is soluble in excess of gelatin. Solution of gelatin dissolves considerable quantities of lime phosphate, hence this is always largely present in common glue. Gelatin is precipitated by mercuric chloride, in this respect resembling peptones; but not by potassium ferrocyanide, by which it is distinguished from albuminoids; and it differs from albumen in not being coagulated by heat. On the contrary, by prolonged boiling glue loses the property of gelatinising, and becomes soluble in cold water, being split up into two peptones; semi-glutin, which is insoluble in alcohol, and precipitated by platinic chloride; and hemicollin, which is soluble in alcohol, and not precipitated by platinic chloride. Both are precipitated by mercuric chloride (see Hofmeister, abst. Chem. Soc. Jour. 1881, p. 294). Gelatin or glue with about 3 per cent. of potassium dichromate becomes insoluble when exposed to the light, from the formation of a chromium compound. This reaction is the base of several modern photographic processes, and has been used for waterproofing and for cementing glass, &c.

The connective-tissue fibres are partially converted into gelatin by the action of strong acids and alkalies, as well as by heat. By weak acids, they are swollen and gradually dissolved, and Reimer[C] has found that the material may be reprecipitated by lime-water. It forms an irregular fibrous mass, which has not the sticky feel of gelatin, but is at once converted into that body by boiling. Rollet has demonstrated that when hide and other forms of connective tissue are soaked in lime- or baryta-water, the fibres become split up into finer fibrils, and as the action proceeds, these again separate into still finer, till the ultimate fibrils are as fine as can be distinguished under a powerful microscope. At the same time, the alkaline solution dissolves the substance which cemented the fibres together, and this may be recovered by neutralising the solution with acetic acid, when it comes down as a flocculent precipitate. This was considered by Rollet to be an albuminoid substance; but Reimer has shown that it is much more closely allied to the gelatigenous fibres, if indeed it is not actually produced from them by the action of the alkaline solution. Reimer used limed calf-skin for his experiments, and subjected it to prolonged cleansing with distilled water, so that all soluble parts must have been pretty thoroughly removed beforehand. He then digested it in closed glasses with lime-water for 7-8 days, and precipitated the clear solution with dilute acetic acid. He found that the same portion of hide might be used again and again, without becoming exhausted, which strongly supports the supposition that it is merely a product of the partial decomposition of the hide fibre. The substance, which he called "coriin," was purified by repeated solution in lime-water, and reprecipitation by acetic acid. It was readily soluble by alkalies, but insoluble in dilute acids, though in some cases it became so swollen and finely divided as to appear almost as if dissolved. It was, however, very soluble in common salt solution of about 10 per cent., though it was precipitated both by the addition of much water, and by saturating the solution with salt. Reimer found that a 10 per cent. salt solution was equally effective with lime-water in extracting it from the hide, and that it was partially precipitated on the addition of acid, and completely on saturating the acidified solution with salt. Other salts of the alkalies and alkaline earths acted in a similar manner, so that Reimer was at first deceived when experimenting with baryta-water, because, being more concentrated than lime-water, the coriin remained dissolved in the baryta salt formed on neutralising with acid, and it was necessary to dilute before a precipitate could be obtained. The slightly acid solution of coriin gave no precipitate with potassium ferrocyanide, nor was it precipitated by boiling, being thus distinguished from albuminoids. The neutral or alkaline solution was not precipitated by iron or mercuric chloride, copper sulphate, nor by neutral lead acetate; but was precipitated by basic lead acetate, basic iron sulphate, and excess of tannin. Its elementary composition is—carbon, 45·91: hydrogen, 6·57; nitrogen, 17·82; oxygen, 29·60; and Reimer proposes the following equation as representing its relation to hide fibre:—

[C] Dingler's Polyt. Journal, vol. 220, p. 167.

Hide Albumen.—The fresh hide, besides this coriin (which, very possibly, is only evolved by the action of the lime), contains a portion of actual albumen, viz. that of the blood serum and of the lymph, which is not only contained in the abundant blood-vessels, but saturates the fibrous connective tissue, of which it forms the nourishment. This albumen is mostly removed by the liming and working on the beam, which is preparatory to tanning. Probably for sole-leather, the albumen itself would be rather advantageous if left in the hide, as it combines with tannin, and would assist in giving firmness and weight to the leather. It is, however, for reasons which will be seen hereafter, absolutely necessary to get rid of any lime which may be in combination with it. The blood also must be thoroughly cleansed from the hide before tanning, as its colouring matter contains iron, and, in combination with the tannin, would give a bad colour.

The reactions of blood and lymph albumen are very similar to those of ordinary white of egg. It is precipitated by strong mineral acids, especially nitric, and also by boiling. The precipitate produced by strong hydrochloric acid redissolves by the aid of heat to a blue or purple solution. Tribasic phosphoric, tartaric, acetic, and most other organic acids, do not precipitate moderately dilute solutions of albumen, but convert it into a sort of jelly, which, like gelatin, does not coagulate, but liquefies on heating. It is precipitated by neutral salts of the alkali metals. Blood-albumen slightly acidified (with acetic acid) is precipitated by potassium ferrocyanide. It is not precipitated by dilute infusions of oak bark, but is rendered uncoagulable by heat, hence it cannot be employed to remove tannins from their solutions.

Elastic Fibres.—The elastic or yellow fibres of the hide are of a very stable character. They are not completely dissolved even by prolonged boiling, and acetic acid and hot solutions of caustic alkalies scarcely attack them. Probably they do not combine with tannin, and are very little changed in the tanning process.

Hair, Epidermis, and Glands.—These are, as has been seen, all derived from the epithelial layer, and hence, as might be inferred, have much in common in their chemical constitution. They are all classed by chemists under one name, "keratin," or horny tissue, and their ultimate analysis shows that in elementary composition they nearly agree. It is evident, however, that the horny tissues are rather a class than a single compound.

The keratins are gradually loosened by prolonged soaking in water, and, by continued boiling in a Papin's digester, are dissolved to an extract which does not gelatinise on cooling. Keratin is dissolved by caustic alkalies; the epidermis and the softer horny tissues are easily attacked, while hair and horn require strong solutions and the aid of heat to effect complete solution. The caustic alkaline earths act in the same manner as dilute alkaline solutions; hence lime easily attacks the epidermis, and loosens the hair, but does not readily destroy the latter. Alkaline sulphides, on the other hand, seem to attack the harder tissues with at least the same facility as the soft ones, the hair being often completely disintegrated, while the epidermis is still almost intact; hence their applicability to unhairing by destruction of the hair. Keratins are dissolved by fuming hydrochloric acid, with the production of a blue or violet coloration, like the albuminoids. They also resemble albumen, in the fact that their solution in sulphuric acid is precipitated by potassium ferrocyanide. By fusion with potash, or prolonged boiling with dilute sulphuric acid, keratin is decomposed, yielding leucin, tyrosin, ammonia, &c. The alkaline solution of keratin (hair, horns, &c.) is precipitated by acids, and, mixed with oil and baryta sulphate, is employed under Dr. Putz's patent as a filling material for leather, for which purpose it acts in the same way as the egg-yolks and meal used in kid-leather manufacture. Eitner attempted to use it for the same purpose with bark-tanned leather, but without much success. Putz has also proposed to precipitate the material after working its solution into the pores of the leather.

[CHAPTER III.]

COMMERCIAL TANNING MATERIALS.

Algarobilla.—The seed-pods of Prosopis pallida and P. Algarrobo are known as algarobilla, the two kinds being distinguished as negro and blanco. The trees are abundant in mountainous parts of South America, notably Chili and the Argentine Republic. The pods contain up to 50 per cent. of a bright-yellow tannin, somewhat resembling that of myrobalans. The friable tannin is readily soluble in cold water, and is so loosely held in the fibrous network of the pod, that great loss is sustained by careless handling. The commerce in algarobilla does not figure in the official trade returns; but J. Gordon & Co., Liverpool, obligingly state that they imported 50 tons, at an average value of 18l. 10s. a ton, in 1880. Widow Duranty & Son, also of Liverpool, are good enough to add that they received 160 tons in 1881, the first that had reached them for a long time. Havre imported 50 tons in 1881. The name algarrobo is also applied to Balsamocarpon brevifolium in Chili, and to Hymenæa Courbaril in Panama.

Chestnut-extract.—The wood of the chestnut (Castanea vesca) contains 14-20 per cent. of a dull-brown tannin. It is quite different from the bark and bark-extract of the American chestnut-oak (Quercus sessiliflora). Its extract is used largely to modify the colour produced by hemlock-extract, and for tanning and dyeing. The pulverised wood is also extensively employed in France. The imports are included in barks and extracts, [p. 39].

Cork-bark. See [Oak-barks].

Cutch, Catechu, or Terra Japonica (Fr., Cachou; Ger., Catechu).—The term kát, kut, or "cutch," is applied to the dried extract, containing 45-55 per cent. of dark-coloured mimo-tannic acid, prepared chiefly from 2 trees:—(1) Acacia Catechu [Mimosa Catechu, M. sundra], a tree of 30-40 ft., common in most parts of India and Burma, growing also in the hotter and drier districts of Ceylon, and abundant in tropical East Africa—the Soudan, Sennar, Abyssinia, the Noer country and Mozambique, though the utilisation of its tannin is restricted to India; (2) A. [M.] Suma, a large tree inhabiting South India (Mysore), Bengal, and Gujerat.

The process for preparing cutch varies slightly in different districts. The trees are reckoned to be of proper age when their trunks are about 1 ft. diam. They are then cut down, and the whole of the woody part, with the exception of the smaller branches and the bark, is reduced to chips: some accounts state that only the darker heart-wood is thus used. The chips are placed with water in earthen jars, arranged in a series over a mud fire-place, usually in the open air. Here the water is made to boil, the liquor as it becomes thick and strong being decanted into another vessel, in which the evaporation is continued until the extract is sufficiently inspissated, when it is poured into moulds made of clay, or of leaves pinned together in the shape of cups, or in some districts on to a mat covered with the ashes of burnt cow-dung, the drying in each case being completed by exposure to the sun and air. The product is a dark-brown extract, which is the usual form in which cutch is known in Europe.

In Kumaon, North India, a slight modification of the process affords a drug of very different appearance. Instead of evaporating the decoction to the condition of an extract, the inspissation is stopped at a certain point, and the liquor is allowed to cool, coagulate, and crystallise over twigs and leaves thrown into the pots for the purpose. By this method is obtained from each pot about 2 lb. of kath or catechu, of an ashy-whitish appearance. In Burma, the manufacture and export of cutch form one of the most important items of forest revenue. The quantity of cutch exported from the province in 1869-70 was 10,782 tons, valued at 193,602l., of which nearly half was the produce of manufactories situated in British territory. The article is imported in mats, bags, and boxes, often enveloped in the large leaf of Dipterocarpus tuberculatus. It is brought down from Berar and Nepal to Calcutta. That of Pegu has a high reputation.

Our imports of cutch in 1880 were 5155 tons, value 173,040l., from the British East Indies; 539 tons, 15,572l., from other countries; total, 5694 tons, 188,612l. Our exports in the same year were:—892 tons, 28,527l., to Germany; 676 tons, 24,562l., to the United States; 478 tons, 15,505l., to France; 303 tons, 10,537l., to Holland; 177 tons, 5859l., to Russia; 141 tons, 4835l., to Belgium; 245 tons, 8719l., to other countries; total, 2912 tons, 98,544l. The approximate London market value of Pegu cutch is 21-42s. a cwt.

An astringent extract prepared from the areca nut (Areca Catechu) is said to contribute to commercial cutch; if so, it is a totally distinct product from those just described.

Divi-divi, or Libi-dibi.—These names are applied to the seed-pods of Cæsalpinia coriaria, a tree of 20-30 ft., indigenous to several of the West Indies, Mexico, Venezuela, and North Brazil, and naturalised in Madras and Bombay Presidencies, and in the North-West Provinces. The pod may be known by its drying to the shape of a letter S; it contains 30-50 per cent. of a peculiar tannin, somewhat similar to that of valonia. It is cheap, and may be used in admixture with barks; but it is dangerously liable to undergo fermentation, suddenly staining the leather a dark-red colour, and is therefore not in extensive use. The imports of it are mainly from Maracaibo, Paraiba, and St. Domingo. Maracaibo, in 1880, exported 197,674 lb. of divi-divi, value 3222¹/₄ dol. (4s. 2d.), to New York. Our imports of divi-divi into Liverpool, according to figures kindly furnished by Haw & Co., were 2200 tons in 1877, 1740 in 1878, 2132 in 1879, and 780 in 1880. The approximate market value is 12-17l. a ton.

Galls.—The generic term "gall" is applied to those excrescences on plants which are produced by the punctures of insects, for the purpose of depositing their eggs. The excrescences are usually considered to be a diseased condition of vegetable tissue, resulting from the injection of some secretion of the insects. But this has been combated by A. S. Wilson, of Aberdeen, who considers that all insect galls are in reality leaf-buds, or fruit-buds, and not mere amorphous excrescences. The vascular lines which would form leaves can easily be followed up in the structure of the oak-leaf galls. And in cases where the egg has been deposited in the tissue of a young branch, the cap of the gall is sometimes surmounted by a leaf 2-3 in. long. But in the large blue Turkish galls, many lacunæ occur where the fleshified leaves have not filled up the spaces between them. If a dissection be made of one of the weevil-galls on the bulb of the turnip, the second or third slice will show the outer foliations, exactly similar to those of the root-buds. When the centre has been reached, where the maggot will be found, there will also be a vascular pencil running up from a medullary ray in the bulb, and bearing on its top a bud of the same description as that produced by a ray running out from a root. The insertion of the ovipositor brings a medullary ray into action, producing a tuberculated bud, and it is only the bud which the larva feeds upon. The growth of a bud is an intelligible cause of the growth of a gall, but nothing can be inferred from the injection of a fluid. The analogy to leaves is further shown by the fact that various microscopic fungi are matured in the interior of imperforate galls.

The principal commercial kinds of gall are oak-galls and Chinese galls.

Oak-galls, Nut-galls, Aleppo or Turkey-galls (Fr., Noix de Galle, Galle d'Alep; Ger., Levantische or Aleppische Gallen, Galläpfel).—These are formed by the punctures of Cynips [Diplolepis] Gallæ tinctoriæ on Quercus lusitanica var. infectoria [Q. infectoria], a shrubby tree of Greece, Cyprus, Asia Minor, and Syria, and probably other varieties and even species of oak. The female insect is furnished with a delicate ovipositor, by means of which she pierces the tender shoots of the tree, and lays her eggs therein. In the centre of the full-grown gall, the larva is hatched and undergoes its transformations, finally (in 5-6 months) becoming a winged insect, and boring for itself a cylindrical exit-hole. The best commercial galls are those which have been gathered while the insect is still in the larval state. Such have a dark olive-green colour, and are comparatively heavy; but after the fly has escaped, they become yellowish-brown in hue, and lighter. Hence they are distinguished in the London market as "blue" or "green," and "white." In Smyrna, they are classified as "white," "green," and "black," the first two sorts generally fetching nearly the same price, while the black obtain considerably more, the approximate quotations being: white and green, per Turkish oke (of 2·83 lb.), 8¹/₂-9 piastres (of 2d.); black, 13¹/₂-14 piastres. The "nuts" come mostly from Melemen, Cassaba, and Magnesia, also from the Syrian coasts, being plentiful on the east of the river Jordan, and are chiefly forwarded to France, England, and Salonica. The triennial yield is said to be invariably the best. They begin to reach Smyrna from the interior towards the end of July. The crop of 1880 was estimated at over 50,000 okes. The province of Aleppo, which used to afford 10,000-12,000 quintals (of 2 cwt.) annually, only exported 3000 in 1871. The galls collected in the Kurdistan mountains are marketed at Diarbekir, and sent thence to Trebizonde for shipment. Bussora, Bagdad, and Bushire also export considerable quantities.

Knoppern, a species of gall formed from the immature acorns of Quercus pedunculata and Q. sessiliflora, are largely used for tanning throughout Austria.

The exports from Aleppo (including yellow berries) in 1880 were:—60 tons, 3600l., to Great Britain; 322 tons, 19,320l., France; 15 tons, 900l., Italy; 44 tons, 2640l., Austria; 55 tons, 3300l., Turkey; 30 tons, 1800l., Egypt; total, 526 tons, 31,560l. In 1878, the figures were 673 tons, 38,400l. Alexandretta exported in 1879 (including yellow berries):—41 tons, 2460l., to England; 299 tons, 17,940l., France; 20 tons, 1200l., Italy; 25 tons, 1500l., Austria; 87 tons, 5220l., Turkey; 6 tons, 360l., Egypt; total 478 tons, 28,680l. The shipments from Trebizonde by steamer in 1880 were (from Turkey):—47 sacks (of 2 cwt.), 188l., to Turkey; 240 sacks, 960l., Great Britain; 264 sacks, 1056l., France; 103 sacks, 412l., Austria and Germany; 26 sacks, 104l., Greece; total, 680 sacks, 2720l.; (from Persia): 25 sacks, 100l., Great Britain; 31 sacks, 124l., France; 30 sacks, 120l., Austria and Germany; total, 86 sacks, 344l. Bushire despatched 5000r. worth to India in 1879. Syra sent 248l. worth to Great Britain in 1879. Venice exported 1745 tons of gall and bark, value 34,906l., in 1879.

The best oak-galls contain 60-70 per cent. of tannic or gallotannic acid, and 3 per cent. of gallic acid. "Rove" is a small crushed gall, containing 24-34 per cent. of gallotannic acid. There are many other varieties of non-commercial oak-gall.

Chinese or Japanese Galls.—These are vesicular protuberances formed on the leaf-stalks and branches of the Rhus semialata [Bucki-amela], a tree of 30-40 ft., common in North India, China, and Japan, ascending the outer Himálaya and the Khasia Hills to 2500-6000 ft., by punctures of the female of Aphis chinensis. The galls are collected when their green colour is changing to yellow, and are then scalded. They are light and hollow, 1-2¹/₂ in. long, and of very varied and irregular form. The Japanese are the smaller and paler, and usually more esteemed. The galls contain about 70 per cent. of tannic or gallotannic acid, and 4 per cent. of another tannin. They are consumed mainly in Germany, for the manufacture of tannic acid.

Hankow exported 30,949 piculs (of 133¹/₃ lb.) in 1872; and 21,611 piculs, value 136,214 taels (of about 6s.), in 1874. In 1877, the total Chinese export did not exceed 17,515 piculs. Hankow exported 24,742¹/₂ piculs in 1878, and 28,392 piculs, 59,614l., in 1879; Pakhoi, 62l. worth in 1879; Canton, 3155¹/₃ piculs in 1877, 1939 in 1878, 3163¹/₂ in 1879; Ichang, 100¹/₂ piculs, 132l., in 1878, 402¹/₂ piculs, 586l., in 1879; Shanghai, 27,659¹/₂ piculs in 1879.

In China trade returns, they are always miscalled "nut-galls" or "gall-nuts": correctly, they are wu-pei-tze. Oak-galls are exported from China resembling those of Western Asia. Japanese galls, kifushi, are sent in increasing quantities from Hiogo.

Our imports of galls in 1880 were:—24,590 cwt., 68,697l., from China; 17,311 cwt., 60,648l., from Turkey; 9182 cwt., 9013l., from other countries: total, 51,083 cwt., 138,358l. Our re-exports in the same year were:—6260 cwt., 18,479l., to Holland; 6022 cwt., 18,147l., to Germany; 3214 cwt., 11,002l., to France; 3045 cwt., 8598l., to Belgium; 2651 cwt., 11,004l., to the United States; 1625 cwt., 5205l., to other countries; total, 22,817 cwt., 72,435l. The approximate London market values of galls are:—Bussora, blue, 82-102s. a cwt.; do., white and in sorts, 50-90s.; China, 50-70s.; Japan, 55-56s.

Gambier, Pale Catechu, or Terra Japonica (Fr., Gambir, Cachou jaune; Ger., Gambir).—These names are conferred upon an extract from the leaves of Uncaria Gambier [Nauclea Gambir] and U. acida, containing 36-40 per cent. of a brown tannin, which rapidly penetrates leather, and tends to swell it, but alone gives a soft porous tannage; it is largely used in conjunction with other materials for tanning both dressing- and sole-leather. The plants are stout climbing shrubs, the first-named being a native of the countries bordering the Straits of Malacca, and especially the islands at the eastern end, though apparently not indigenous to any of the islands of the volcanic band, growing also in Ceylon, where no use is made of it; while the second, probably a mere variety, flourishes in the Malay islands.

The shrubs are cultivated in plantations, often formed in jungle clearings; the soil is very rapidly exhausted, and further injured by excessive growth of the ineradicable lalang-grass (Andropogon caricosus). It is found advantageous to combine pepper-culture with that of gambier, the boiled leaves of the latter forming excellent manure for the former. The gambier-plants are allowed to grow 8-10 ft. high, and as their foliage is always in season, each plant is stripped 3 or 4 times in the year. The tools and apparatus for the manufacture of the extract are of the most primitive description. A shallow cast-iron pan about 3 ft. across is built into an earthen fire-place. Water is poured into the pan, a fire is kindled, and the leaves and young shoots, freshly plucked, are scattered in, and boiled for about an hour. At the end of this time, they are thrown on to a capacious sloping trough, the lower end of which projects into the pan, and are squeezed with the hand so that the absorbed liquor may run back into the boiler. The decoction is then evaporated to the consistence of a thin syrup, and baled out into buckets. When sufficiently cool, it is subjected to curious treatment: instead of simply stirring it round, the workman pushes a stick of soft wood in a sloping direction into each bucket; and, placing two such buckets before him, he works a stick up and down in each. The liquid thickens round the stick, and, the thickened portion being constantly rubbed off, while at the same time the whole is in motion, it gradually sets into a mass, a result which, it is said, would never be produced by simple stirring: it is reasonable to suppose that this manner of treating the liquor favours the crystallisation of the catechin in a more concrete form than it might otherwise assume. The thickened mass, resembling soft yellowish clay, is now placed in shallow square boxes; when somewhat hardened, it is cut into cubes, and dried in the shade. The leaves are boiled a second time, and finally washed in water, which is saved for another operation.

A second plan is as follows:—The leaves are boiled, and bruised in a wooden mortar (lesong), from which they are put into a kind of basket of rattan open-work, which is pressed by a long piece of wood acting as a lever; the liquid is received into a trough, and there allowed to settle. When the sediment has acquired sufficient substance, it is put into a kulit-kayo, formed like a tub without a bottom, which lets the superfluous water drain off; when that is done, it is taken out, made into small cakes, and dried for use. A plantation employing 5 labourers contains 70,000-80,000 shrubs, and yields 40-50 catties (of 1¹/₃ lb.) of gambier daily.

Plantations were commenced in Singapore in 1829, and once numbered 800; but owing to scarcity of fuel, abundance of which is essential to the manufacture, and dearness of labour, the culture was fast declining in 1866. In 1872, it had much recovered. It is largely pursued on the mainland (Johore), and in the Rhio-Lingga Archipelago, S.-E. of Singapore; on Bintang, the most northerly of the group, there were 1250 plantations of it in 1854. None is cultivated in Sarawak, though found wild in many parts; the foreign export from Sarawak in 1879 had a total value of 88,148 dol. The best kind is brought largely from Sumatra, but is often adulterated with sago. The Rhio product is also thus sophisticated, and rendered heavier by the Chinese purposely packing it in baskets lined with wet cajangs, occasioning a loss to the purchaser of about 30 per cent.

Singapore is the great emporium for gambier, and exported 34,248 tons in 1871, 19,550 tons having been imported, chiefly from Rhio and the Malay Peninsula. In 1876, the export increased to over 50,000 tons of pressed block, and 2700 tons of cubes. In 1877, it fell to 39,117 tons, owing to differences with the Chinese dealers concerning adulteration; of this quantity, 21,607 tons were for London, 7572 for Liverpool, and 2345 for Marseilles. The United Kingdom imports in 1872 were 21,155 tons, 451,737l., almost all from the Straits Settlements; in 1880, they were 26,061 tons, 461,781l., from the Straits, and 352 tons, 6468l., from other countries; total, 26,413 tons, 468,249l. Our re-exports in 1880 were:—2487 tons, 48,507l., to Holland; 1591 tons, 31,542l. to Germany; 1137 tons, 23,694l., to Russia; 594 tons, 12,026l., to other countries; total, 5809 tons, 115,769l. The approximate London market values are 15s. 6d.-21s. 6d. a cwt. for block, 18-24s. for pressed cubes, and 23-27s. for free cubes.

Hemlock.—The bark of the hemlock or hemlock spruce (Abies canadensis), of Canada and the United States, contains nearly 14 per cent. of tannin. The stripping of the bark commences in the southern parts of the United States in spring, and lasts during April-May; in New York, Michigan, and Wisconsin, the season is June-July; and farther north, it is still later. It is said that the best product is obtained farthest south. The destruction of the hemlock forests is fast approaching. Within the last 25 years, the preparation of an extract from the bark, containing 18-25 per cent. of a deep-red tannin, giving considerable weight and firmness to leather, has superseded the export of crude bark. One mode of preparing the extract is as follows:—The bark in pieces ¹/₂-1 in. thick, and several inches long, is soaked for about 15 minutes in water at 200° F. (93° C.); it is then fed into a hopper, which conducts it to a 3-roller machine, something like a sugar-cane mill, through which it passes, coming out lacerated and compressed; it next falls into a vat of hot water, where it is agitated by a wheel, that the tannin from the crushed cells may be dissolved in the water; hence it is raised by a series of buckets on an endless chain, somewhat in the manner of a grain-elevator, to another hopper, whence it is fed to another 3-roller mill; here it receives its final compression, and comes out in flakes or sheets, like coarse paper, and almost free from tannin. The buckets are made of coarse wire, that the water may drip through during the elevation. In order to avoid the blackening action of iron, wherever this metal will come into contact with the solutions it is thickly coated with zinc. The solution is evaporated to a solid consistency, generally by vacuum-pans. About 2 tons of bark are represented by 1 bar. (of less than 500 lb.) of extract. The chief makers are A. S. Thomas, Elmira, N.Y.; S. Brown & Co., New York; Canada Tanning Extract Co., St. Leonard and Bulstrode; J. Miller & Co., Millerton, New Brunswick. The total production is probably over 10,000 tons annually, ranging in value between 14l. and 20l. a ton. Our imports are included in barks and extracts.

Kino (Fr., Kino; Ger., Kino).—The term "gum kino" is applied to a class of astringent extracts of varied origin, none of which can accurately be called either resins or gums.

Pl. II.

E. & F. N. Spon, London & New York.

"INK-PHOTO." SPRAGUE & CO. LONDON.

REMOVING THE HAIR; SCRAPING AND CLEANING THE SKINS.

(1) East Indian or Amboyna Kino.—This is obtained from Pterocarpus Marsupium, a common tree in the central and southern parts of the Indian peninsula, and in Ceylon; and a liquid kind from P. indicus, of South India, Burma, Malacca, Penang, the Andamans, and Malaysia. The collection of the juice is effected in the following manner. A perpendicular incision, with lateral offshoots, is made in the stem of the tree when blossoming has set in, and a receptacle is placed at the foot of the incision. The exuding juice appears like red-currant jelly, but it soon thickens by exposure to the air, and when sufficiently dried, is packed into wooden boxes for exportation. It is one of the reserved timber-trees of the Government forests in Madras, and its juice is collected by natives, who pay a small fee for the permission. The hardened juice consists of blackish-red, angular, pea-like grains, partially soluble in water, almost entirely in spirit of wine of sp. gr. 0·838, readily in caustic alkaline solutions, and largely in a saturated solution of sugar. The liquid kino produces a very inferior article on drying. The annual collection of kino in Madras probably does not exceed 1-2 tons. Its approximate London market value is 60-150s. a cwt. It is employed medicinally, and in the manufacture of wines, and might be employed as a source of tannin in dyeing and tanning, if sufficiently cheap.

(2) Butea, Bengal, Palas or Dhak Kino.—This variety is afforded by the palas or dhak tree (Butea frondosa), common throughout India and Burma, and affording a dyestuff, and a fibre, as well as by B. superba and B. parviflora. During the hot season, there issues from natural fissures and from wounds made in the bark of the stem, a red juice, which quickly hardens to a ruby-coloured, brittle, astringent mass. It occurs in small drops or tears, and in flat pieces which have been dried on leaves, and is almost always mixed with bark-fragments. It is transparent, freely soluble in cold water, and does not soften in the mouth. It is unknown in European commerce, but is employed in India as a substitute for the kind first described.

(3) African or Gambia Kino.—This is derived from Pterocarpus erinaceus, a native of Tropical West Africa, from Senegambia to Angola. The juice exudes naturally from fissures in the bark, but more abundantly from incisions, and soon coagulates to a blood-red and very brittle mass, known to the Portuguese of Angola as sangue del drago ("dragon's-blood"). It is practically undistinguishable from the officinal kind first described, but is not a regular article of commerce.

(4) Australian, Botany Bay, or Eucalyptus Kino.—Several species of Eucalyptus afford astringent extracts, those from the "red," "white," or "flooded" gum (E. rostrata), the "blood-wood" (E. corymbosa), and E. citriodora, being quite suitable for replacing the officinal kind. It is chiefly obtained by woodcutters, being found in a viscid state in flattened cavities in the wood, and soon becoming inspissated, hard, and brittle. Minor quantities are procured in a liquid state by incising the bark of living trees, forming a treacly fluid yielding 35 per cent. of solid kino on evaporation. It is imported from Australia, but there are no statistics to show in what quantity.

Mimosa- or Wattle-bark.—The bark of numerous species of Acacia, natives of Australia, contains considerable percentages of deep-red mimo-tannic acid, which forms a hard and heavy tannage if used strong, though soft upper-leathers may be tanned with it in weak liquors. The chief kinds are as follows:—The common wattle (Acacia decurrens), including its variety A. mollissima, is known also under the names of green, black, and feathery, but must not be confounded with the silver wattle (A. dealbata), though but doubtfully a distinct species. The bark is obtainable in vast abundance, and is much used by tanners. The trees are stripped in September and the 2 or 3 months following, and the bark, being allowed to dry, is then in a marketable condition. This tree, which grows in the uplands, affords a larger percentage of tannin than the silver wattle.

Blackwood or lightwood (A. melanoxylon) yields tanners' bark, which, is inferior, however, to that from A. decurrens. The bark of A. penninervis yields of tannic acid 17·9 per cent., and of gallic acid 3·8 per cent. The bark of the native hickory (A. suppurosa) yields of tannic acid 6·6 per cent., and of gallic acid 1·2 per cent.

The bark of A. saligna, of South-Western Australia, is much used by tanners, as it contains nearly 30 per cent. of mimo-tannin. A. harpophylla, of South Queensland, furnishes a considerable share of the mercantile wattle-bark for tanning purposes. The bark of A. lophantha contains only about 8 per cent. of tannin.

The broad-leaved or golden wattle (A. pycnantha), of Victoria and South Australia, deserves extensive cultivation. It is of rapid growth, will succeed even in sandy tracts, and yields seed copiously, which germinates with the greatest ease. The perfectly-dried bark contains about 25 per cent. of tannin. The aqueous infusion of the bark can be reduced by boiling to a dry extract, which in medicinal and other respects is equal to the best Indian cutch. It yields approximately 30 per cent. of tannin, about half of which, or more, is mimo-tannic acid. Probably no other tanning plants give so quick a return in cultivation as the A. pycnantha and A. decurrens of Australia. The latter varies in its proportions of tannin from 8 to 33 per cent. In the mercantile bark, the percentage is somewhat less, according to the state of its dryness, it retaining about 10 per cent. of moisture. The bark of the silver wattle (A. dealbata) is of less value, often even fetching only half the price of that of the black wattle. The bark improves by age and desiccation, and yields 40 per cent. of tannin, rather more than half of which is tannic acid.

Amongst all the kinds, the bark of the broad-leaved wattle is considered the most valuable, containing the greatest quantity of tannin; that of the silver wattle is not so valuable, being deficient in tannin; the black wattle is considered the most productive species; it can be barked at 8 years of age, and will produce 40-60 lb. dried bark, and full-grown trees will yield 100-150 lb. per tree.

The cultivation of wattles for commercial purposes has till now remained undeveloped; but no doubt, as soon as it is understood, the utilisation of many acres of land lying waste, or which have already been exhausted and rendered unfit for the growth of cereals, will be effected by the cultivation of the wattle. It requires so little attention as to make it very profitable, and wattle-growing and grazing can be combined satisfactorily. After the first year, when the young trees in the plantation have reached the height of 3-4 ft., sheep can be turned in.

Wattles grow in almost any soil, even the poorest, but their growth is most rapid on loose sandy patches, or where the surface has been broken for agricultural purposes. When the soil is hard and firm, plough furrows should be made at a regular distance of 6-8 ft. apart, into which the seeds are dropped. The seed should be sown in May, having been previously soaked in hot water, a little below boiling temperature, in which they may be allowed to remain for a few hours. The seed should be dropped at an average distance of 1 ft. apart along the furrow, in which case, about 7200 seeds would suffice for one acre of land. The seed should not be covered with more than about ¹/₄ in. of soil.

On loose sandy soil, it might even be unnecessary to break up the soil in any way; the furrows may be dispensed with, and the seed sown broadcast after the land is harrowed. After the plants have come up, they should be thinned so that they stand 6-8 ft. apart. When the young trees have attained the height of 3-4 ft., the lower branches should be pruned off, and every effort afterwards made to keep the stem straight and clear, in order to facilitate the stripping, and induce an increased yield of bark. It is advisable that the black and broad-leaved should be grown separately, as the black wattle, being of much larger and quicker growth, would oppress the slower-growing broad-leaved one. Care should be taken to replace every tree stripped by re-sowing, in order that there should be as little variation in the yield as possible. The months of September-December, in Victoria, are those in which the sap rises without intermission, and the bark is charged with tannin. Analysis proves that the bark from trees growing on limestone is greatly inferior in tannin to that obtained from other formations, differing 10-25 per cent.

The estimated expenditure on a wattle-bark plantation of 100 acres during 8 years is:—

The receipts derivable from a wattle plantation of 100 acres, planted in the manner proposed, would be:—

The exports of mimosa-bark in 1876 were 11,899 tons from Victoria, 4758 from South Australia, and 1735 from Tasmania. Later returns are included in barks, [p. 39]. Shanghai imported 7038 piculs (of 133¹/₃ lb.) in 1879. The approximate London market values of mimosa-bark are:—Ground, 6-13l. a ton; chopped, 5-12l.; long, 5l.-9l. 10s. A very superior extract has been made from this bark.

Myrobalans or Myrabolams.—The fruits of several species of Terminalia constitute the myrobalans of commerce; they are chiefly T. Chebula and T. Bellerica, natives of India, the former being a tree 40-50 ft. high, and esteemed for its timber also. The fruits contain 30-35 per cent. of gallotannic and ellagitannic acids, producing a soft and porous tannage, and good samples giving a bright-yellow colour. The tannin exists in the pulp, and is absent from the very hard "stone." The dried fruits are known locally as har, harra, or bahera, and are used commonly for dyeing, but not for tanning.

Our imports of myrobalans in 1880 were:—238,151 cwt., 121,465l., from Bombay and Sind; 115,670 cwt., 51,339l., from Madras; 11,020 cwt., 4717l., from Bengal and Burma; 3520 cwt., 1402l., from other countries; total, 368,361 cwt., 178,923l. Our re-exports in 1880 were 8015 cwt., 4328l., to Germany; 16,127 cwt., 8515l., to other countries; total, 24,142 cwt., 12,843l. The approximate London market values of myrobalans are 7-14s. a cwt. for good, and 5-10s. for common. Shanghai imported 4403 piculs (of 133¹/₃ lb.) in 1879.

Oak-barks (Fr., Écorces de Chêne; Ger., Eichenrinden).—The barks of several species of oak have valuable tanning properties. They are chiefly:—The common oak (Quercus Robur, varieties: sessiliflora, Ger. Traubeneiche; pedunculata, Ger. Stieleiche), which is of even greater importance as a timber-tree; the cork-oak (Q. Suber); the evergreen oak (Q. Ilex); and the American chestnut-oak (Q. Castanea). These barks are among the most esteemed tannins as regards quality of leather, but are incapable of giving much weight, and from their bulk are costly to handle, containing only 10-12 per cent. of tannin (quercitannic acid). They give a reddish fawn-coloured leather, and deposit a good deal of bloom, but yield little or no gallic acid. The barks of the cork-oak and evergreen oak from Southern Europe, are stronger and darker-coloured than English bark. The American chestnut-oak contains a peculiar fluorescent principle like æsculin.

Our imports of unspecified barks for tanners' and dyers' use in 1880 were:—189,399 cwt., 101,108l., from Australia; 123,302 cwt., 32,974l., Belgium; 57,232 cwt., 20,988l., United States; 22,100 cwt., 6030l., Holland; 18,648 cwt., 3676l., Italy; 16,151 cwt., 6972l., Algeria; 22,669 cwt., 8838l., other countries; total, 449,501 cwt., 180,586l. Our imports of unenumerated bark-extracts in the same year were valued at:—516,578l. from Holland, 92,654l. France, 30,187l. United States, 16,315l. British North America, 12,796l. Belgium, 13,769l. other countries; total, 682,299l. Our re-exports of barks in 1880 were:—19,548 cwt., 10,348l., to Germany; 14,627 cwt., 7425l., France; 4555 cwt., 3041l., Holland; 10,304 cwt., 6080l., other countries; total, 49,034 cwt., 26,894l.

With regard to cork-tree bark, James Gordon & Co., Liverpool, obligingly write that very little comes to England, the great bulk going direct to Ireland, where the consumption is large. The imports at Liverpool in 1880 were 186 tons, average value 8l. per ton. Of oak-bark, Hungary, in 1877, produced 25,000 tons, of which, 20,000 were exported to Germany for tanning purposes. The approximate London market values of oak-bark are:—English, 12-16l. per load of 45 cwt.; Foreign, tree, 5-8l. a ton; ditto, coppice, 6-8l. In 1879, Algiers exported 12,660,047 kilo. (of 2·2 lb.) of tanning bark.

Quebracho.—The local name quebracho, contracted from quebra-hacho ("axe-breaker"), is applied to several South American trees possessing hard wood, belonging to distinct genera. They are chiefly as follows:—(1) Aspidosperma Quebracho, the quebracho blanco, a tree growing in the province of Catamarca, Argentine Republic; (2) Loxopterygium [Quebrachia] Lorentzii, the quebracho colorado, most prevalent in the province of Corrientes, the wood and bark of which come largely into commerce as tanning materials; (3) Iodina rhombifolia, the quebracho flojo, whose wood and bark are mixed with those of No. 2; (4) Machærium fertile [Tipuana speciosa], the tipa, which affords both wood and bark of less tanning value than No. 2. It would seem that the wood and bark of No. 2 are by far the most largely employed, containing 15-23 per cent. of a bright-red tannin. The wood and an extract from it are imported into Europe.

From information kindly furnished by James Gordon & Co., and Haw & Co., of Liverpool, it appears that the imports of quebracho-wood into Liverpool in 1880 were 200 tons, value about 4l. 10s. a ton; and of quebracho-bark, about 20 tons, none of which had been sold.

Sumach or Shumac (Fr., Sumac; Ger., Gerbersumach, Schmack).—The commercial term "sumach" is applied to the dried leaves of a number of South European and American tannin-yielding plants. These are chiefly as follows:—In Sicily, the European or tanning-sumach (Rhus Coriaria); in Tuscany, R. Coriaria, often adulterated with leaves of Pistacia lentiscus; in Spain, several Rhus spp., the products being divided into 3 kinds—Malaga or Priego, Malina, and Valladolid; in the Tyrol, the smoke-tree or fragrant or Venetian sumach (R. Cotinus); in France, Coriaria myrtifolia, divided into 4 sorts—fauvis, douzère, redoul or redon, and pudis; in Algeria, Tezera sumach (R. pentaphylla), used by the Arabs for making morocco-leather; in North America, the smooth or white sumach (R. glabra), the Canadian sumach (R. canadensis), the staghorn sumach (R. typhina), and the dwarf or black sumach (R. copallina). These are found growing wild in the countries indicated, and are further subjected to cultivation in some districts, notably in Sicily. R. glabra and R. copallina are recommended chiefly for extended cultivation in the United States.

The soil usually chosen for cultivation of the plants is poor and light; but a much larger crop of leaves can be secured from strong, rich, deep soils, and it is generally admitted that the product in the latter case is also better. In Italy, limestone soils are considered to be especially suited to this culture, but the American varieties appear to be well adapted to sandy and clay soils as well. The primary requisite in a soil is that it shall be well drained, the presence of stagnant water about the roots being exceedingly prejudicial. To prepare the soil for planting, it is ploughed as deeply as possible, and laid out in rows about 2 ft. apart. In Italy, small holes are made about 2 ft. long, 7 in. wide, and 5 in. deep, and a plant is inserted at each end. A more convenient method would consist in marking the field in shallow furrows in one direction 2 ft. apart, and then, with a heavy plough, tolerably deep furrows the same distance apart as, and at right angles to, the first. A plant may then be placed in the deep furrows at each intersection, the furrow again filled with the plough, and the earth pressed about the plant with the foot. If this were done in early spring-time, as soon as the earth is sufficiently dry to be conveniently worked, there can be no doubt that it would be successful, while it would certainly involve little cost. Plants are generally propagated from the young shoots which form each year about the base of an older plant, but may also be produced from cuttings made from young well-ripened wood, rooted by setting in a nursery or in frames, as in the propagation of grape-vines from cuttings. This latter method is scarcely ever required, however, when the cultivation has been started. Plants are also raised from seed, and seedlings are always found to be strong, vigorous, and thoroughly hardy; but on account of the greater time and labour involved in their production, this method of propagation has not received extended application. The first-mentioned generally gives the quickest, and probably most satisfactory results.

In selecting plants from any source, there are certain points to be observed:—(1) The shoots should come from young vigorous plants; (2) they should be over 1 ft. long; (3) those with large roots and few rootlets should be rejected; (4) those having white roots, covered with a fibrous, white, silky down, are also to be rejected, this being an indication of the presence of a very injurious subterranean parasitic fungus, capable of destroying the entire crop; (5) a good shoot is straight, at least ¹/₂ in. diam., 18 in. long, furnished with numerous buds close to each other, root short, but covered with rootlets. Shoots for planting may be collected in autumn, after the leaves have fallen, and be preserved in a nursery until spring; or this may be done in early spring, when the ground is very moist and soft. In either case, care should be observed that the rootlets are not injured by drying, or from any other cause.

The culture to be given the plant is somewhat similar to that required by Indian corn: the earth about it should be kept tolerably mellow and free from weeds, and such conditions can probably be maintained to a degree sufficient for sumach, by working several times during the growing season with a cultivator, and passing through the rows occasionally with a plough. All this work is not absolutely necessary to the life of the plant, but its vigour, and consequently its yield in leaves, may be considerably increased and strengthened thereby. After the first year, the number of operations may be diminished, but they should always be sufficient to keep the ground free from weeds and grass.

Shortly after planting, and when the plant is well set, the stock is pruned to a length of 6-8 in., when the plant is left to assume any form, and is no further pruned except by the process of collecting the leaves, unless hand-picking is resorted to; in such case, after the 2nd year, pruning takes place each year in the fall or winter, the plant being reduced to a height of 6-10 in. After the 3rd year, the plant begins to produce the shoots from about its base, already mentioned; these, if not needed for new plantations, should be removed each year, for if left to develop, they weaken the plant. If not removed during the summer, the operation should without fail be effected during the fall or winter.

The 1st crop of leaves may be secured during the year following that of planting. This develops and matures somewhat later than that from older plants, and in Italy it is not collected until the end of August or the 1st of September; but there are reasons for believing that in the United States, especially in the Northern States, the collection of leaves from native varieties should be made much earlier, because the summer is much shorter, and the habits of the varieties grown differ from the Sicilian. Macagno has shown (Chem. Soc. Journ., xxxviii., p. 733) that the leaves from the upper side of the branches contain much more tannin than those below, and that especially in the lower leaves the percentage of tannin is much higher in June than in August. All the leaves, except the young and tender ones of the extremities of the branches, are stripped off and placed in baskets, in which they are carried to a threshing-floor, where they are spread out in thin layers to dry. Here they must be frequently stirred and turned over, for which purpose a fork with wooden prongs is employed. In the fall, when growth is finished, and before the leaves have had time to become red, those remaining on the extremities are collected. To this end, the branches are broken just below the tuft of leaves, and the latter are allowed to remain suspended from the branch by a piece of bark not detached, and left in this condition until nearly or quite dry. They are then collected and treated in the same manner as other leaves, but the product obtained in this way is always of inferior quality.

After the 2nd year, crops of larger quantity and superior quality are obtained, and the collection is made in a different way, and much more frequently. The two methods followed in Sicily are (1) pruning, and (2) defoliation. The first, which is the more ancient, but much less costly, requires less care, and is simple and rapid; but it is injurious to the future condition of the plant, and the quantity of subsequent crops. The second, though slower, serves to better maintain the vigour of the plant, and the uniform quantity of the crop from year to year; in consequence, it reduces the necessity for frequent renewal of stocks.

Harvest by pruning is carried on in Italy as follows. During May, the lower leaves, which, from greater age, appear to have attained full maturity, and may be in danger of loss from falling, are removed in the same manner as described for collecting the leaves from yearling plants. Toward the end of June, and during the course of July, all branches bearing leaves are cut away, reducing the plant to the principal stock: by this means, the crop is harvested and the plant is pruned at the same time. But even in Sicily, the time for this operation is limited to no absolute period, and varies with the development of the leaf, as indicated by cessation of growth and increase in size. In this condition, also, the leaves will have acquired their deepest green colour, and attained their maximum weight and best quality. It is further stated that while this time varies according to locality, about Palermo it is never earlier than June nor later than July. The harvest by pruning must always be made by men accustomed to the work, and equal to the exertion required. Provided with a pruning-bill, they cut off all leaf-bearing branches, collecting them an the left arm, until each has cut as much as he can conveniently carry, when he places the armful on the ground with the butts in the direction of the prevailing wind, which, if tolerably strong, might carry away some of the leaves if turned in the opposite direction; finally, he presses down the branches with his foot, to make the heap more compact, and leave less surface exposed to the wind and sun. Another labourer deposits a second armful in the same place, presses it with his foot in like manner, and the two deposits constitute a bundle. At the close of the operation, there remain the young shoots which are formed about the base of the plant, the leaves of which are not fully developed, and consequently not fit for collection until at least 20 days later. After this time, they are removed by hand, care being observed not to injure the buds, especially if the shoots are to be used for stocks in the formation of plantations in the following year.

Defoliation, or collection by hand, is carried on whenever the leaf may be fully developed and ripe, beginning at first with the lower leaves, and continuing eventually to the ends of the branches. It takes place at 3 different times during the season: the 1st in May, the 2nd late in July or early August, and the 3rd in September. At the last collection, the extremities of the branches are broken down, and the leaves are allowed to dry before removal from the plant, as described under collections of the 2nd year. In the application of this method, the regular pruning is effected during the fall or winter, when the plant is dormant, and under such conditions the operation becomes a regenerative one, giving in this particular an advantage over the other method, in which the pruning is effected in the summer when the plant is in full vegetative activity, and so has a strongly deteriorating influence. In both methods of pruning, care should be observed to leave a long slanting section, upon which water will be less likely to settle and promote decay.

The leaves collected by either method are dried in the open field where they have grown, and when dried, are carried to a threshing-floor to be beaten, or at once to the threshing-floor and dried there. In the former, the operation is rather more rapid, but there is greater danger of injury by rain, the effect of which is very deleterious, especially if it fall upon the leaves when they are partially dried. The damage resulting from this cause is less if the leaves are not lying upon the ground, and are so arranged that the air may circulate freely about and under them. In the pruning method, the leaves are dried upon the branches and in the heaps where they are first deposited. Sometimes they are turned, but generally it is considered better not to disturb them until completely dried, and ready for transportation to the threshing-floor. In this way, they are protected to a greater extent from the action of direct sunlight, which is said to be injurious to the quality of the product. When the leaves are collected by hand, they are dried upon the threshing-floor, where they are spread in thin layers, and stirred 3-4 times a day. They are then beaten with a flail to separate the leaves from the branches and stems. If this be done during the middle of the day, when the leaves are most thoroughly dry and consequently brittle, they are reduced to small particles, producing what is called "sumach for grinding." But if it be done in the morning, or on damp days, when the air is charged with moisture and the leaves are tough, they are separated from the stems more nearly entire and less broken, and the product obtained is called "sumach for baling." The stems remaining after the separation of sumach for baling still retain small particles of leaves attached to them, and they are therefore again beaten when perfectly dry for the production of a low-grade sumach, called by the Italians gammuzza. The products are classed as follows:—

To prepare these different grades for ultimate consumption, they are ground in mills similar to those employed for crushing olives, that is, in which two large stone wheels follow each other, revolving upon a circular bed, the whole construction being about the same as the Spanish or Mexican arrastre. The sumach thus pulverised is passed through bolting-screens, to separate the finer from the coarser particles.

In Virginia, the leaves are collected and cured by the country people, and sold and delivered to owners of mills for grinding. Their particular object being to secure the largest possible quantity of product at the lowest cost, little attention is given to the quality obtained, or the manner of collecting. The most intelligent dealers in the raw material urge upon collectors to observe the following particulars:—The leaf should be taken when full of sap, before it has turned red, has begun to wither, or has been affected by frost, to ensure a maximum value for tanning purposes. Either the leaf-bearing stems may be stripped off, or the entire stalk may be cut away, and the leaves upon it allowed to wither before being carried to the drying-shed; but care must be observed that they are neither scorched nor bleached by the sun. When wilted, they are carried to a covered place, and spread upon open shelving or racks to dry, avoiding the deposit in any one place of a quantity so great as to endanger the quality of the product by overheating and fermentation. Sumach should be allowed to remain within the drying-house at least one month before sending to the market; in case of bad weather, a longer period may be required. When ready for packing for shipment, it should be perfectly dry and very brittle, otherwise it is likely to suffer injury in warehouses from heating and fermentation.

Buyers of sumach leaves for grinding depend largely upon colour for the determination of the value; the leaves should, therefore, when ready for market, present a bright-green colour, which is evidence that they have suffered neither from rain after being gathered, nor from heating during the process of drying. Leaves having a mouldy odour or appearance are rejected. The Virginian crop reaches 7000-8000 tons, and is collected at any time between July 1 and the appearance of frost.

There is an important difference in the value of the European and American products. The proportion of tannic acid in the latter exceeds that found in the former by 6-8 per cent., yet the former is much preferred by tanners and dyers. By using Sicilian sumach it is possible to make the finer white leathers, in great demand for gloves and fancy shoes; while by the employment of the American product, the leather has a disagreeable yellow or dark colour, apparently due to a colouring matter, which, according to Loewe, consists of quercitrin and quercetin, and exists in larger quantity in the American than in the Sicilian.

The experimental results obtained by collecting sumach at different seasons were:—

It is evident, therefore, that in order to secure the maximum amount of tannic acid, the sumach should be collected in July, but the colouring matter of the leaves has an important influence upon the value of the product. The leaves of the upper extremities of the stalks are always richer in tannic acid than those of the base; and the increase of age of the plant is accompanied by a general diminution of this acid. Yet the collection of the crop should be delayed as long as possible, because the diminution of tannin in the leaves will be abundantly compensated for by the quality of the product.

Experiments upon the presence of colouring matters were made by treating gelatine solutions, and gave the following results:—

It is therefore advised that for the purpose of tanning white and delicately-coloured leathers, the collection should be made in June; while for tanning dark-coloured leathers, and for dyeing and calico-printing in dark colours, where the slightly yellow colour will have no injurious effect, the collection be made in July. It appears that for all purposes, the sumach collected after the 1st of August is inferior in quality.

Fig. 8.

[Fig. 8] shows a mill for grinding sumach-leaves; it consists of a heavy solid circular wooden bed a, 15 ft. diam., with a depression around the edge b, a few inches deep and 1 ft. wide, for the reception of the ground sumach from the bed, and 2 edge-rollers c, weighing about 2500 lb. each, 5-6 ft. diam., and provided with numerous teeth of iron or wood, thickly inserted. Most mills have to be stopped to allow the unloading of the bed, but this delay is obviated by an apparatus consisting of an angular arm d, attached to a scraper e, and worked by a lever f, which passes through the hollow shaft g and extends to the room above, where it terminates in a handle h. The scraper carries the ground sumach to the opening i, whence it is taken by an elevator to a revolving sieve or screen in a room above. After screening, the sumach is packed in bags, 15 to the ton, being always sold by that weight. The chasers and beds are inclosed in a case or drum, and the grinding is done by the application of power to the upright shaft g. The mills are fed from above. The packing is sometimes done by machinery alone. The best mills cost about 600l. In Europe, and in some parts of the Southern States, sumach is still ground by stones revolving on a stone bed, and the sifting is often done by hand.

E. Coez & Co., St. Denis, near Paris, make a sumach extract. It is concentrated to a syrupy consistence in a vacuum-pan, and keeps well, exhibiting none of the acidity which is manifested by a simple decoction of sumach leaves. Sumach contains 16-24 per cent. of gallotannic acid, and is somewhat similar in tanning properties to myrobalans, but paler in colour. It is principally used for tanning morocco and other fancy leathers.

The district of Ancona yields 200 tons per annum of sumach, said to be equal to and cheaper than the Sicilian, but mostly consumed locally. Palermo exported of "ventilated" sumach to the United States 120,043 bags (14 = 1 ton) in 1877, and 50,085 in 1878, the average value being 14l. a ton. Trieste exported 7800 cwt. by land in 1877; in 1878, the shipments to England were 16,600 kilo. (of 2·2 lb.), value 1328 fl. (of 2s.), and in 1880, 91,800 kilo. 7344 fl. Rustchuk in 1880 exported 1400 tons, chiefly to Roumania and Austria. Our imports in 1880 were 10,573 tons, 133,249l. from Italy, and 1047 tons, 12,416l., from other countries; total, 11,620 tons, 145,665l. The approximate London market value is 15s.-16s. 6d. a cwt. for Sicilian, 10-11s. for Spanish.

Valonia (Fr., Vélanèdes; Ger., Valonia). This is the commercial name for the large pericarps or acorn-cups of several species or varieties of oak, chiefly Quercus Ægilops and Q. macrolepis. The former is found growing in the highlands of the Morea, Roumelia, the Greek Archipelago, Asia Minor, and Palestine; the latter constitutes vast forests in many parts of Greece, and especially on the lower slopes of Taygetos, towards Ætylon and Mani (Laconia). Prof. Orphanides, of Athens, alludes to a third species or variety called porto galussa, which yields a superior kind of valonia, and named by him Q. stenophylla. The chief localities of production in Asia Minor are Ushak, Borlo, Demirdji, Ghiördes, Adala, Nazlü, Buldur, Sokia, Balat, Troja, Aivalik, and Mytilene. The annual exports, mainly from Smyrna, reach 600,000 quintals (of 2 cwt.), value about 400,000l. In Greece, the production is chiefly centred in the following districts: (1) The province of Lacedemonia, which afforded 10,000 cwt. in 1872; (2) the province of Gythium, in the lower part of Mount Taygetos, which gave 60,000 cwt. in 1872; (3) the island of Zea, which formerly yielded 30,000-40,000 cwt., lately reduced to 15,000 cwt. yearly; (4) Attica, especially the neighbourhood of Cacossalessi, grows 3000-5000 cwt., shipped from Oropos, in the Strait of Chalcis; (5) the island of Eubœa, whence about 1000 cwt. are shipped annually at Bouffalo; (6) the province of Triphyllia raises 3000 cwt., which go to Trieste, viâ Cyparissie; (7) the province of Pulos, especially the commune of Ligudista, grows over 2000 cwt., despatched from Navarino to Trieste; (8) the province of Achaia has a yearly crop of 30,000-40,000 cwt., shipped to Trieste from Courupeli and Caravostassi, between Patras and Cape Papa; (9) the small towns of Anatolico and Astakos (Dragomestre) collect the valonia of the eastern parts of Ætylon, Acarnania, and Cravassaras (a port in the Gulf of Arta), and of all the other western parts, to be sent to Trieste for shipment to England and Italy. Ætolia and Acarnania furnish abundant crops, that of 1872 exceeding 100,000 cwt. The total area of the Greek valonia-yielding forests is said to be about 13,000 stremme (of 119¹/₂ sq. yd.). The total production in 1877 was estimated at 2,601,000 quintals (of 2 cwt.); the greater part is exported, about ²/₃ going to Austria, and the rest to Italy and England. The proportions of tannic acid in the valonia from different districts of Greece are said to vary as follows: Patras, 19-28¹/₂ per cent.; Gythium, 27¹/₄-35¹/₂; Zea, 12¹/₄-25¹/₄; Vonitza, 18-20.

In Turkey, the fruit ripens in July-August, when the trees are beaten, and the fallen acorns left on the ground to dry. The natives afterwards gather them, and transport them on camel-back to stores in the towns, whence they go by camel and train to Smyrna, and are there placed in heaps 5-6 ft. deep in large airy stores for some weeks, during which the mass heats, and the acorn itself, which contains but little tannin, and is used for feeding pigs, contracts and falls from the cup. This incipient fermentation is attended with considerable risk; if carried too far, a large proportion of the valonia becomes dark-coloured and otherwise damaged. When ready for shipment, the heaps are hand-picked, the best being reserved for the Austrian market (Trieste), and the rest going to England. In some cases, the rubbish having been removed, the remainder is known as "natural," and is thus exported to England.

In Greek commerce, three qualities are distinguished, chamada, rhabdisto, and charcala. The chamada (camata and camatina of Asia Minor) is the best; it is collected in April, before the acorn is matured, hence the cup which encloses the acorn is small and incompletely developed. The rhabdisto is the second quality; it is collected in September-October, and is distinguished by the fruit being larger and riper; the name means "beaten," the fruits being beaten down from the trees with sticks. After mid-October the collection ceases, because the first rains cause the fallen fruit to ferment or turn black, and they then take the name of charchala. They are distinguished by the cups being completely open, and containing no acorns. They are considered much inferior, possessing little tannin.

Pl. IV.

E. & F. N. Spon, London & New York.

"INK-PHOTO." SPRAGUE & CO. LONDON.

REVOLVING, WASHING AND PRESSING MACHINES.

Sometimes the acorn cup is attacked by a kind of honey-dew, which deposits on the cup, and makes it very liable to heat when gathered, the cup becoming very dark and deficient in tannin. The Turkish crop of 1875 was much damaged from this cause, many parcels reaching England in an unsaleable condition. The cause of the disease is yet unknown; it seems specially prevalent when the crop is large and the acorn fully developed. A good sample of valonia should be composed of medium-sized cups, with the rim or wall very thick, and the exterior spines small and uniform. The cut or broken cup should show a bright-drab fractured surface. Valonia contains 25-35 per cent. of a tannin somewhat resembling that of oak-bark, but giving a browner colour and heavier bloom. It makes a hard and heavy leather, and is generally used in admixture with oak-bark, myrobalans, or mimosa-bark.

The Greek crop in 1880 was much damaged by the cold spring: It gave 600 tons in Acarnania and Ætolia, 650 in Cape Papa, and 1400 in Mania; total, 2650 tons. Calamata and Messenia produced 115 tons, 1700l. Syra exported in 1879, 1174l. worth to Great Britain, 348l. Austria, 259l. Russia, 250l. Turkey, 178l. Egypt. Hungary exported 942 tons in 1880. Adana shipped 9450l. worth in 1878; and Dedeagatch, in the same year, 1,500,000 lb., 9000l. Musyna [Mersineh] sent 670 tons, 3350l., to Italy, and 450 tons, 2250l. to Austria, in 1879; and 480 tons, 2240l., to Italy, and 128 tons, 640l., to Greece, in 1880. Our imports in 1880 were:—From Turkey, 30,391 tons, 471,637l.; Greece, 2916 tons, 41,312l.; other countries, 466 tons, 7105l.; total, 33,773 tons, 520,054l. The approximate London market values are:—Smyrna, 12s. 6d.-20s. 6d. a cwt.; Camata, 15s.-19s.; Morea, 10s. 6d.-18s.

Miscellaneous.—Besides the foregoing tannins, which already occupy prominent places in European and American commerce, there are many others as yet of minor importance, but possessing qualities which may bring them into note in the near future. They are as follows:—

Abies Larix bark, the larch, contains 6-8 per cent. of a red tannin.

Acacia albicans fruits, the hiusache of Mexico, are used as substitutes for gall-nuts, costing locally about 5d. a lb. A. arabica, the babul of India, yields a tannin which gives a nearly pure-white precipitate with gelatine: the proportions are 12·55 per cent, in trunk-bark, 18·95 in branch-bark, 15·45 in twig-bark. The supply is unlimited. It works well with myrobalans. A. Cebil, the red cebil of the Argentine Republic, contains 10-15 per cent. of tannin in the bark, and 6-7 per cent. in the leaves; another variety, the white cebil, contains 8-12 per cent. in the bark, and 7-8 per cent. in the leaves. A. Cavenia, the espinillo of the Argentine Republic, has 33-34 per cent. of tannin in the fruit-husks. A. penninervis bark, the "hardy" acacia of Australia, contains 18 per cent. of tannic acid and 3-4 of gallic.

Alnus glutinosa bark, the common alder, contains about 16 per cent. of tannin.

Cœsalpinia Cacalaco fruits, the cascalote of Mexico, are very rich in tannic and gallic acids, and are locally used for tanning.

Comptonia asplenifolia leaves, the sweet-fern of the United States, contain 9-10 per cent. of tannin.

Coriaria ruscifolia bark, the tutu of New Zealand, contains 16-17 per cent. of tannin.

Elæocarpus dentatus bark, the kiri-hinau of New Zealand, contains 21-22 per cent. of tannin. E. Hookerianus bark, the pokako of New Zealand, contains 9-10 per cent. of tannin.

Ephedra antisyphilitica, on the tablelands of Arizona and Utah, gives 11-12 per cent. of tannin.

Eucalyptus longifolia bark, the "woolly-butt" of Australia, contains 8·3 per cent. of tannic acid, and 2·8 of gallic. The "peppermint"-tree contains 20 per cent. of tannic acid in its bark. The "stringy-bark" (E. obliqua) gives 13¹/₂ per cent. of kinotannic acid. The Victorian "iron-bark" (E. leucoxylon) contains 22 per cent. of kinotannic acid, but is available only for inferior leather.

Eugenia Maire bark, the whawhako of New Zealand, contains 16-17 per cent. of tannin. E. Smithii bark, the "myrtle"-tree of Australia, contains 17 per cent. of tannic acid and 3-4 of gallic.

Fuchsia macrostemma root-bark is thin, brittle, and easily exhausted; it contains about 25 per cent. of a bright-red tannin, which has been successfully tried. It is the churco bark of Chili, which, however, is attributed by the Kew authorities to Oxalis gigantea.

Inga Feuillei pods, the pay-pay of Peru, contain 24 per cent. of an almost colourless tannin.

Laurus Peumo rind is used in Chili for tanning uppers.

Malpighia punicifolia bark, the naucite, or manquitta bark of Nicaragua, contains 20-30 per cent. of a very light-coloured tannin.

Persea Lingue bark is red-brown, soft, and easily exhausted by water; it contains 20-24 per cent. of tannin, and much slimy matter which promotes the swelling of the hides. It serves in South America, especially in the Chilian province of Valdivia, for tanning Valdivia leather. In Southern Chili are enormous forests of the tree. The imported bark has given good results with heavy leathers.

Phyllocladus tricomanoides bark, the kiri-toa-toa of New Zealand, contains 23 per cent. of tannin.

Polygonum amphibium leaves, an annual plant abundant in the Missouri Valley, contain 18 per cent. of tannin, and can be mown and stacked like hay. It is largely used in Chicago tanneries, and said to give a leather which is tougher, more durable, of finer texture, and capable of higher polish, than that tanned with oak-bark.

Punica Granatum fruit-rind, the pomegranate, contains about 13·6 per cent. of a tannin like myrobalans, and a considerable quantity of starch; the tannin is greatest in the bitter kind, which is used for preparing morocco leather; the root-bark also is rich in tannin.

Rhizophora Mangle bark, the mangrove, of Venezuela, contains 24-30 per cent. of deep-red tannin, if obtained from young stems; samples from the West Indies have given 11·94 per cent., probably by the gelatine process; two samples from Shanghai, by Löwenthal's improved method, gave respectively 9·8 and 9·5 per cent. calculated as oak tannin, and 71·96 and 78·52 of woody fibre. Guayaquil exported 9328 cwt. of the bark to Peru in 1879.

Tecoma pentaphylla bark, the roble colorado of Venezuela, contains 27 per cent. of tannin, accompanied by a soluble orange-red colouring matter.

Wagatea spicata pods contain 15 per cent. of tannic acid. The plant, a scrambling shrub, is a native of the Concans.

Weinmannia racemosa bark, the tawhero towai, or kamai of New Zealand, contains 12-13 per cent. of tannin.

[CHAPTER IV.]

THE CHEMISTRY OF TANNINS.

The essential constituents of tanning materials are various members of a large group of organic compounds called tannins or tannic acids.[D]

[D] Ger. gerbsäure; in German the word tannin denotes usually gallotannic acid only.

These bodies often differ widely both in chemical constitution and reaction, but have the common property of precipitating gelatin from solution, and forming insoluble compounds with gelatin-yielding tissues. By virtue of this power, they convert animal hide into the insoluble and imputrescible material called "leather." They are mostly uncrystallisable; and all form blackish-blue or blackish-green compounds with ferric salts, and in common with many other organic substances are precipitated by lead and copper acetates, stannous chloride, and many other metallic salts, and those of organic bases, such as quinine. In some cases, the tannin combines with the base only, liberating the acid; but frequently the salt as a whole enters into combination. This is the case with the precipitates formed with lead and copper acetates. With alkalies, the tannins and many of their derivatives give solutions which oxidise and darken rapidly, usually becoming successively orange, brown, and black. A. H. Allen has shown that these bodies also give instantaneously a deep-red coloration with a solution of potassium ferricyanide and ammonia. The reaction is one of considerable delicacy.

Tannins are more or less soluble in water; and freely so in alcohol, mixtures of alcohol and ether, and ethyl acetate, but scarcely in dry ether alone, nor in dilute sulphuric acid; and insoluble in carbon disulphide, petroleum spirit, benzene, and chloroform.

From their amorphous character, tannins are extremely difficult to purify; and when, as is frequently the case, two or more tannins occur in the same plant, it is often quite impossible completely to separate them. Owing to their considerable differences in character, no general method of purification can be given, but the following processes will be found in many cases to give good results. For the special methods adopted by different investigators, the original memoirs must be consulted, references to many of which will be found in the following pages.

Preparation and Purification of Tannins.

The oldest method of separating tannins from other constituents is that applied by Pelouze to the preparation of commercial gallotannic acid from gall-nuts. The finely pulverised material is placed in a percolator and exhausted with commercial ether containing water and alcohol. The liquid separates, on standing, into 2 layers of which the lower contains most of the tannin in a tolerably pure form, dissolved in water and alcohol with a little ether, while the upper mainly ethereous layer contains the gallic acid. Gall-nuts thus treated yield 35-40 per cent. of tannin. If equal parts of ether and 90 per cent. alcohol are used, a larger yield is obtained, but the liquid does not separate into 2 layers, and it is questionable if the product is so pure. For Chinese galls, washed ether acts better than ether alcohol. The tannin may be still further purified by dissolving in a mixture of 1 part water with 2 of ether, when 3 layers are formed, of which the lowest contains nearly pure tannin.

These methods are applicable to the dried or highly concentrated extracts of many tanning materials. Many tannins may be separated from their strong aqueous solution in a state of considerable purity by first agitating with ether to remove gallic acid, and then saturating with common salt, and shaking well with acetic ether, which takes up the tannin. Another method is to extract with alcohol, evaporate to a small bulk at as low a temperature as possible, and treat at once with a considerable quantity of cold water. The infusion is then precipitated with successive small quantities of lead acetate; the first and last portions of the precipitate are filtered off and rejected as contaminated with colouring matters and other impurities, while the remainder, after rapid washing, is suspended in water and decomposed with sulphuretted hydrogen. The filtrate is shaken with ether to remove gallic acid, and the aqueous portion is evaporated at a low temperature in a partial vacuum to a thin syrup, and the drying completed over sulphuric acid in vacuo.

General Chemistry.

The natural tannins are all compounds of carbon, hydrogen and oxygen only. They all contain the benzene group of carbon atoms, but their ultimate structure is, except in the case of gallotannic acid, very imperfectly understood, and probably differs considerably in type in different members of the family.

In order to make clear to those readers who have not studied modern organic chemistry, what we do know on the subject, a few words of introduction will be necessary. All organic compounds contain carbon, in combination with hydrogen, and very frequently also with oxygen, nitrogen, and other elements. A single atom of carbon is able to combine with 4 atoms of hydrogen, as it does to form marsh gas, or methyl hydride, CH₄. Other elements may be substituted for the hydrogen; for instance, if we replace 3 of the hydrogen atoms with chlorine, we obtain chloroform, CHCl₃. Again an atom of oxygen may be inserted between the carbon atom and one of the hydrogen atoms, producing methyl hydroxide or wood spirit. The group CH₃ is called methyl, and we may substitute in wood spirit this entire methyl group for one of the atoms of hydrogen, when we shall have ordinary alcohol, C₂H₅OH. This building-up process may be repeated almost ad infinitum, producing a whole series of alcohols of higher and higher boiling point as the atoms of carbon become more numerous. Again, if in wood spirit we substitute an atom of oxygen for 2 of the remaining atoms of hydrogen we obtain formic acid, CHO.OH, the first of a long series of acids, of which the second, corresponding to ordinary alcohol, is acetic acid, and the highest members, such as stearic acid, C₁₈H₃₅O.OH, are solid fats. Hence the whole series are commonly called the fatty acids. A few structural formulæ will serve to make these points clearer, but it may be well to say that such formulæ must be taken simply as indicating the order in which the different atoms are united, and in no sense their actual position in space. The atoms in a molecule are held together by attractions and are in continual motion, so that they are more comparable to the planets of the solar system than to a rigid shape.

In benzene, C₆H₆ we have a compound of another type. There is reason to think that the carbon atoms in this case are united in a ring, as shown,

H H H
| | |
C──C══C
║ |
C──C══C
| | |
H H H

This benzene group forms the foundation of an immense number of bodies known as the aromatic series, to which belong aniline, carbolic acid, picric acid, gallic acid, and a host of other compounds important alike in a scientific and commercial sense, and among which we may pretty safely group the whole of the tannins. Commencing with benzene, we may, by inserting atoms of oxygen, produce a series of alcohols or phenols, of which common phenol (usually but incorrectly called carbolic acid) is the first.

The following table gives a general view of some of these, so far as they are known, with their corresponding acids:—

C6H6	C6H5OH	C6H4(OH)2	C6H3(OH)3
Benzene.	Phenol.	Pyrocatechol (or catechol), Hydroquinol, Resorcinol.	Pyrogallol, Phloroglucol.
C6 H5 CO.OH	C6 H4 OH CO.OH	C6 H3 (OH)2 CO.OH	C6 H2 (OH)3 CO.OH
Benzoic acid.	Salycylic acid, Oxybenzoic acid.	Protocatechuic acid (and 5 other isomeric acids).	Gallic acid, &c.

It will be noticed that a large proportion of the formulæ given above represent several compounds identical in composition, but frequently very distinct in their properties. The explanation of these differences lies in the different relative position of the OH and CO.OH groups round the benzene ring. Thus the following diagram represents the relative positions of the pyrocatechol series. It may be noted that each phenol yields two isomeric[E] acids. Miller (C. S. Jour., xli. 398), who has investigated these acids, remarks, "Of the 3 phenols C₆H₄OH₂, catechol alone gives a precipitate with lead acetate, and of the 6 acids, C₆N₅OH₂, CO.OH, none yields precipitates with lead acetate, except the 2 which are obtained from catechol."

[E] Isomeric, of similar composition but different structure and properties.

All the natural tannins with which we are acquainted, are derived from, and yield on decomposition either catechol, phloroglucol, or pyrogallol, and sometimes more than one of these. Artificial products, however, with many of the reactions of tannins have been obtained from other members of the group, and most phenols and their derived acids give either purplish or greenish black with ferric salts.

Several classifications of the tannins have been suggested. The division most obvious to the tanner is into those tannins which yield the whitish deposit in the surface of the leather, called "bloom," and those which do not. Stenhouse, some years since, divided tannins into 2 classes, one of which gives a bluish, and the other a greenish-black with ferric salts. In the main these 2 classes correspond to the 2 former, as most tannins which yield a blue-black with iron acetate also give bloom to the leather. In some cases, however, the difference of tint is due to accidental impurities, and even gallotannic acid will give a decided green with strongly acid ferric chloride. These classifications both correspond to well-defined differences of constitution, and it is obviously more scientific to arrange tannins according to the products which they yield on decomposition, and which indicate their ultimate structure, rather than on any less essential point.

If those tannins which give bloom to leather are cautiously heated to about 392° F. (200° C.), they are decomposed, and a substance is volatilised which condenses in feathery crystals, and which on examination turns out to be pyrogallol. Those tannins, on the other hand, which yield no bloom, but red deposits, produce a somewhat similar sublimate of catechol. From oak-bark and valonia, which yield both bloom and red colouring matters, both catechol and pyrogallol have been obtained. We may, therefore, divide tannins broadly into derivatives of catechol, which yield no bloom, and usually give greenish-blacks with iron acetate, and which include hemlock, mimosa, cutch, gambier, quebracho, &c; derivatives of pyrogallol, which give bluish-blacks with iron, deposit bloom in leather, and embrace galls, sumach, divi-divi, myrobalans, pomegranate rind, &c., and tannins which contain both pyrogallol and catechol, such as oak-bark and valonia, and which, as is well known, yield bloom, and give blue-blacks with iron.

If tannins are boiled with dilute sulphuric or hydrochloric acids, and allowed to ferment under the influence of pectose and other natural ferments, which are always present in vegetable tanning materials, a different series of decompositions takes place. Many tannins yield glucose, or starch sugar, as one of their products, or as that of closely associated impurities. Of this more must be said later. In addition it will be found that the catechol tannins invariably yield insoluble reddish-brown bodies which have been called phlobaphenes, and which differ from the original tannins in containing one or more molecules less water, and which, in chemical language, are anhydrides of their respective tannic acids. The pyrogallol tannins, on the other hand, yield gallic acid, or ellagic acid (the deposit forming bloom) either alone or in mixture. Oak-bark and valonia give both bloom and insoluble reds, and by digestion with acids in sealed tubes also gallic acid.

If the red anhydrides, which are produced from the catechol tannins, be fused with caustic potash, or in many cases, if they be simply boiled with concentrated potash solution, they are broken up still further, and from the fused mass, protocatechuic acid (which bears the same relation to catechol that gallic acid does to pyrogallol) may always be obtained. This is in many cases accompanied by phloroglucol, a phenol isomeric with pyrogallol, as may be seen by the table on [p. 61], but which tastes sweet like a sugar. Cutch, gambier, mimosa, quebracho, and probably many others, are phloroglucide tannins. The tannins which do not yield phloroglucol frequently give acetic acid, and other acids of the "fatty" group, along with protocatechuic acid. We may summarise this classification in the following table:—

Tannins boiled with dilute sulphuric acid yield (frequently glucose, and) Insoluble Reds, which fused with potash yield protocatechuic acid, and,── Phloroglucol, as chestnut, gambier, kino, cutch, quebracho, rhatany, fustic, horse-chestnut, tormentil. Acetic acid, coffee, Peruvian bark, male-fern.
Reds and gallic and ellagic acids; no glucose		Oak-bark and valonia tannins.
No reds, but gallic and ellagic acids		Galls, myrobalans, sumach, divi-divi, pomegranate rind. These are probably mixtures of two tannins which yield
Gallic acid only		Digallic, or pure gallotannic acid.
Ellagic acid only		Pure ellagitannic acid.

This classification is as yet very incomplete, and there are many tannins of which the decomposition products have not been examined, while our knowledge of the differences between the tannins which are classed together is extremely limited. In order to make the information which has been given practically available for further research, the characteristics and mode of recognition of the different products will be given, and as simple a scheme as possible of treatment of the tannin to be examined will be described; but the recognition of such products in a state of mixture presents great practical difficulties, and the tanner will usually be compelled to confine his attention to simpler, though less conclusive tests, based on the work of chemical specialists. Such tests will be described later ([p. 111]).

Pl. III.

E. & F. N. Spon, London & New York.

"INK-PHOTO." SPRAGUE & CO. LONDON.

ARTIFICIAL FERMENTATION.

General Methods of Examination of Tannins.

Decomposition by Heat.—The ordinary method is to distil the tannin or dried extract in a small retort, and examine the distillate for catechol and pyrogallol. Unless the heat be very carefully regulated, much loss is caused by the destruction of the catechol and pyrogallol with formation of metagallic acid, &c., and their detection is greatly complicated by the presence of secondary products. This difficulty is somewhat lessened by passing a stream of carbon dioxide through the retort, which carries the products quickly out of the heated portion. A better method is to heat the tannin in glycerin (Thorpe, Chem. Soc. Abstr., 1881, 663; Allen, 'Commercial Organic Analysis,' 2nd ed.). About 1 grm. of the sample is heated with 5 c.c. of pure glycerin to 392°-410° F. (200°-210° C.) for 20 minutes. After cooling, about 20 c.c. of water is added, and the liquid is shaken with an equal volume of ether, without previous filtration. The ethereous layer, which contains the pyrogallol and catechol, is separated from the aqueous portion, evaporated to dryness, and dissolved in 50 c.c. of water. The filtered solution is divided into several portions and tested with lime-water, ferric chloride, and ferric acetate (see [pp. 66-7]); by these means it is easy to distinguish between catechol and pyrogallol; and either may be detected in presence of a small portion of the other; but if in nearly equal quantities, their recognition is difficult. Catechol may be derived from catechin, &c., and pyrogallol from gallic acid, and it is therefore necessary in some cases to remove these bodies from the tannin before treatment. As a general rule, however, catechins and catechol derivatives are only present in any quantity with catechol-tannins, and the same is true of gallic acid with regard to pyrogallol. (For methods of separation see pp. [69], [71], [80]). Catechol has been formed by long continued heating of cellulose, starch, and other carbohydrates with water under pressure (see [p. 67]).

Products of the Decomposition of Tannins by Heat.—Pyrogallol, pyrogallic acid, C₆H₆O₃, has a bitter, but not sour taste, and feebly reddens litmus, but the addition of the smallest trace of alkali gives it an alkaline reaction. It is poisonous, 2 gr. having killed a dog. It is soluble in less than 3 parts of cold water, and still more freely in hot. It is also soluble in alcohol, ether, acetone, ethyl acetate, and glycerin, but not in absolute chloroform, or petroleum spirit. It fuses at 268° F. (131° C.) (Etti), and sublimes at about 410° F. (210° C.).

With pure ferrous sulphate it gives a white precipitate, which redissolves to a fine blue liquid in presence of the least trace of ferric salt. Mineral acids change this to red, and the blue tint is restored by cautious neutralisation with ammonia, and is not destroyed, but sometimes rendered greenish by excess of acetic, and other organic acids. Any excess of ammonia produces an amethyst-red, and acetic acid restores the blue. Its solution is turned brown by traces of nitrous acid. With lime-water it produces a beautiful but evanescent purple, rapidly turning brown. In presence of alkalies it absorbs oxygen from the air with great avidity, turning orange, brown, and black. Pyrogallol does not precipitate gelatin. Its solution rapidly reduces permanganate, Fehling's solution, and salts of gold, silver,[F] mercury, and platinum. It precipitates copper and lead acetates, and with ammoniacal cupric sulphate it gives an intense purple-brown coloration. Gum arabic, saliva, and various other organic matters cause solutions of pyrogallol exposed to the air to absorb oxygen, by which purpurogallin is formed and separates in small yellow capillary crystals. If 0·2 per cent. of pyrogallol be added to a 1 per cent. solution of gum arabic, it becomes yellow in a few hours, and purpurogallin separates in hairlike crystals, which continue to increase for some months. It these crystals are freed from pyrogallol by washing with water, and a trace of alkali is added, they dissolve with an intense blue colour. Purpurogallol is also formed by oxidation with silver nitrate, potash permanganate, and many other reagents. Pyrogallol forms compounds with aldehydes; with formaldehyde, a body which reacts like a tannin and precipitates gelatin is produced. If pyrogallol be heated with hydrochloric acid, and aldehyde, chloral, or acetone, a red substance is produced. The less volatile portions of crude beech-tar creasote contains ethers of pyrogallol, methyl-pyrogallol, and propyl-pyrogallol, from which these bodies may be obtained by the action of hydrochloric acid under pressure. Methyl- and propyl-pyrogallols differ from ordinary pyrogallol in having an atom of hydrogen replaced by the groups CH₃ or C₃H₇ respectively; and it is very probable that some tannins are derivatives of such modified pyrogallols.

[F] Hence its use as a "developer" in photography.

If pyrogallol be heated rapidly to 482° F. (250° C.) it parts with the elements of water, and is converted into metagallic acid, C₆H₄O₂, a black amorphous body, insoluble in water, soluble in alkalies. When pyrogallol is made in the ordinary way by heating gallic or tannic acids to 410° F. (210° C.), much of this body is formed, even if the process be conducted in a stream of carbonic acid, and the yield of pyrogallol usually amounts to only about 5 per cent. of the gallic acid employed (see [p. 65]).

Catechol.—Pyrocatechol, pyrocatechin, oxyphenic acid, C₆H₄(OH)₂.

Sources.—Beside that of the decomposition of certain tannins by heat (see [p. 63]), catechol is produced by the dry distillation of catechin and some allied bodies which frequently accompany the tannins. It is also formed together with pyrogallol and its homologues (see above) by the dry distillation of wood, wood tar creasote consisting largely of ethers of pyrocatechol and its homologues, methyl and pyrocatechol, &c., and hence it is also found in crude pyroligneous acid. It has also been produced by heating carbohydrates with water under pressure, and is found ready formed in Virginia-creeper (Ampelopsis hæderacea), and probably in other plants. It has also been formed synthetically.

Reactions.—Catechol melts at 232° F. (111° C.) and sublimes at about the same temperature, condensing in brilliant laminæ like benzoic acid. It is readily soluble in water, alcohol, and ether, and is extracted from its aqueous solution when shaken with the latter. Its aqueous solution precipitates lead acetate but not gelatin or alkaloids. With lime-water or caustic soda solution it becomes reddish, but remains clear for some time. It does not colour ferrous salts, but gives a dark green with ferric (avoiding excess); and after some time a black precipitate. The green is changed to a fine violet-red by alkalies and hydric sodic carbonate, and restored by acids. To fir-wood moistened with hydrochloric acid it gives, like phloroglucol, a violet coloration by combination with the trace of vanillin which this wood contains. This reaction does not seem to be given by pyrogallol or by common phenol. Catechol gives a red coloration with citric acid, and after standing, ceases to react with iron.

Decomposition of Tannins by Dilute Acids.—It has been stated that tannins when heated with dilute sulphuric or hydrochloric acids are decomposed, yielding frequently glucose, and either gallic or ellagic acids, or red anhydrides. To determine whether glucose is produced, the tannin must first be carefully purified from glucose, gums, or other bodies likely to interfere, by the methods mentioned on [p. 58]. Either the tannin itself or its washed lead-salt may be used, and must be heated to 212° F. (100° C.) for some hours in a sealed tube, or tightly closed bottle with dilute hydrochloric acid. After cooling, the mixture must be allowed to stand for some time to separate any sparingly soluble products, which must be filtered off. The filtrate must be shaken with ether and acetic ether to remove gallic acid ([p. 59]), the aqueous solution must be boiled, neutralised with soda, precipitated with basic lead acetate to remove any traces of tannin or colouring matters, the liquid again filtered, and excess of lead removed with dilute sulphuric acid, the mixture again neutralised with soda, and heated to boiling with Fehling's copper-solution, when a yellow or red precipitate of cuprous oxide will prove the formation of glucose. The precipitate produced by cooling may consist (of lead chloride, if the lead salt has been used,) of ellagic acid, or of red anhydrides or phlobaphenes of the tannin. The lead chloride may be removed by washing with boiling water. If the remaining precipitate has a pale yellow or fawn colour it probably consists of ellagic acid (see [p. 71]), soluble in ammonia and hot alcohol and dissolving freely in strong nitric acid, forming an intense crimson liquid.

The ethereous layer will contain the gallic acid, if any has been formed, and must be evaporated to dryness, and the residue taken up with cold water, and filtered. Addition of a few drops of solution of potassium cyanide will produce a fine red coloration if gallic acid be present, which rapidly fades, but is restored by shaking. A solution of picric acid, to which excess of ammonia has been added, gives a red coloration rapidly changing to a fine green, even in very dilute solutions of gallic acid.

It is not, however, generally necessary to resort to so elaborate a process merely to distinguish the class to which tannins belong. The tannin, or its infusion, may be simply boiled with dilute hydrochloric acid for some time, replacing the acid lost by evaporation. The solution is diluted to 50 c.c. and allowed to cool. Ellagic acid and phlobaphenes may separate, and must be filtered off. If the precipitate is pale, it is probably ellagic acid, and maybe recognised by the nitric acid test. If red, it probably consists of phlobaphenes, and may be treated with cold alcohol, in which phlobaphenes are freely soluble, but ellagic acid very little. The ellagic acid will therefore be left on the filter if present in any quantity, while the alcoholic solution may be precipitated by the addition of water, and the phlobaphenes further examined by treatment with potash.

Gallic Acid.—Dioxysalicylic acid, C₆H₂(OH)₃CO.OH, exists ready formed in some plants, and is a product of the fermentation of gallotannic acid under the influence of the nitrogenous ferment, pectase, or of its decomposition by boiling with acids or alkalies. It crystallises in white, or yellowish white needles, containing 1 mol. (9·5 per cent.) of water, which it loses at 212° F. (100° C.). It is soluble in 100 parts of cold or 3 of boiling water, in alcohol or glycerin, and slightly so in ether, by agitation with which it may however be removed from its aqueous solution. Gallic acid fuses at a temperature of about 449° F. (232° C.) (Etti, Chem. Soc. Jour., xxxvi. 160), but at about 410° F. (210° C.) begins to lose carbonic dioxide, and yields a crystalline sublimate of pyrogallol (see [p. 66]). If the heat be raised suddenly to 482° F. (250° C.) a considerable quantity of black shining metagallic acid is formed.

Aqueous solution of gallic acid gives the following reactions:—Solution of ferric chloride gives a deep blue coloration which is destroyed by boiling. Ferrous sulphate, if free from ferric salt, gives no reaction in dilute solutions, but a white precipitate in strong ones. The mixture rapidly darkens by oxidation. In alkaline solution gallic acid absorbs oxygen from the air and darkens from the formation of tannomelanic acid. Lime-water produces a white precipitate which rapidly becomes blue from oxidation. The same reaction is produced by baryta-water, or by the chlorides of barium or calcium on addition of ammonia (distinction from pyrogallol). It is distinguished from gallotannic acid by the following:—It does not precipitate gelatin, except in the presence of gum. It does not precipitate tartar emetic in presence of ammonic chloride, though both tannin and gallic acid are precipitated by tartar emetic alone. It precipitates lead acetate but not lead nitrate, while tannin precipitates both. A dilute solution of potassium cyanide gives a red coloration which disappears on standing, but is restored by shaking with air. If to even a very dilute solution of gallic acid, sodic arsenate, or some other faintly alkaline salts be added, the mixture absorbs oxygen and becomes a deep green. Aqueous solution of picric acid to which excess of ammonia has previously been added gives a red coloration, changing to green. Tannic and pyrogallic acid produce no reaction with cyanide, and with ammonic picrate a reddish coloration only. Gallic acid reduces silver nitrate and gold chloride rapidly when hot, but not Fehling's solution, and decolorises acidified potassic permanganate. If tannin and other oxidisable bodies be removed from its solution it may be estimated quantitatively by titration with permanganate in presence of indigo (see [p. 118]). It may be separated from tannin by gelatin or hide raspings (see pp. [121], [124]). Gallotannic and quercitannic acids may also be removed by precipitation with ammoniacal solution of cupric sulphate, or by cupric acetate, in presence of excess of ammonic carbonate (see also [p. 125]). Many other tannins, however, give precipitates with cupric salts which are soluble in ammonia and ammonic carbonate. In absence of such tannins it may be estimated gravimetrically by precipitation with cupric acetate. The precipitate is rapidly washed with water and digested with a solution of ammonic carbonate, in which it dissolves; any insoluble cupric tannate is filtered off, the solution is evaporated to dryness and the residue moistened with nitric acid and ignited. The weight of the remaining cupric oxide multiplied by 0·9 gives the weight of the gallic acid plus a little tannin dissolved by the ammonic solution.

Gallic acid may also be separated from tannin by lead acetate strongly acidified with acetic acid, by which tannic acid is precipitated, while lead gallate is dissolved.

Ellagic acid C₁₄H₈O₉, when pure, is a sulphur-yellow crystalline body almost insoluble even in boiling water, and only slightly so in alcohol and ether, though by agitation with the latter, small quantities may be completely removed from aqueous solution. In hot alcohol it dissolves with a yellow colour, and crystallises on cooling. Solid ellagic acid gives with ferric chloride at first a greenish, and then a black coloration. In strong nitric acid it is soluble with a deep crimson coloration: that from divi-divi gives a crimson liquid on dilution with water, but from other sources it is rather orange.

Ellagic acid may be obtained in considerable quantity by pouring a concentrated alcoholic extract of divi-divi into water, when it separates and may be filtered off and recrystallised from hot alcohol. It may also be obtained by boiling the aqueous extracts of divi, myrabolans, pomegranate rind, &c., with dilute hydrochloric acid, and purified by the same means. It may be prepared from gallic acid by heating the latter with dry arsenic acid to 320° F. (160° C.), but is difficult to purify from traces of arsenic. Ellagic acid has not been reconverted into gallic acid. Its constitutional formula is, according to Schiff,

C6H2		CO.OH
		OH
		O───
		O
C6H2		CO
		O───
		OH
		OH

differing from gallotannic acid only by the loss of two atoms of hydrogen.

Air-dried ellagic acid, C₁₄H₈O₉ + OH₂, contains 1 mol. of water, which it loses at 212° F. (100° C.) but reabsorbs in moist air. When heated to 392°-410° F. (200°-210° C.) it forms an anhydride, C₁₄H₆O₈, losing another molecule of water, which it does not recover from moist air, but is slowly reconverted to ellagic acid by boiling with water.

The phlobaphenes or reds are chemically the anhydrides of the different tannic acids from which they are derived, or in other words they are formed from the tannins by the loss of one or more molecules of water. It is in this way that they are produced by the action of acids, and similarly they are often formed when alcoholic or highly concentrated aqueous extracts are poured into cold water, under which circumstances a part of the tannin seems unable to take up water again, and separates as a red precipitate. They exist ready formed in most tanning materials capable of producing them. They are soluble in alcohol, by which they may be extracted from tanning materials or dried residues containing them. They are also dissolved by dilute alkalies and alkaline carbonates, and by borax, which is said to be used in the preparation of some extracts, and was suggested by Sadlon as a means of making phlobaphenes available for tanning. Many of them are scarcely soluble in water even at a boiling temperature, though they become more so in presence of sugar, tannic acid, and some other substances. Their solubility in water depends on their degree of hydration, many tannins giving a series of anhydrides of which those containing only one molecule of water less than the original tannin are quite soluble in water, while the higher members of the series become less and less soluble as they lose water. Those which are soluble form the colouring matters of tanning materials, and generally are practically tannins, precipitating gelatin and combining with hide to form leather. Hemlock bark yields a series of such bodies, of which the lower members are deep red soluble tannins, while the higher form the red sediment, so well known to extract-tanners. Thus it is chemically impossible to decolorise hemlock extract without at the same time greatly lessening its tanning power, though by careful manufacturing and concentration at low temperature, the proportion of the higher anhydrides formed may be kept at a minimum. In many cases it is known, as in gambier, and in others it is probable that the tannin itself is merely the first anhydride of the series, and derived from a catechin which itself is a white crystalline body destitute of tanning properties (see [p. 79]).

Decomposition of the Phlobaphenes by Fusion with Caustic Alkalies.—It has been mentioned ([p. 64]) that the reds of different tannins yielded, in addition to protocatechuic acid, either phloroglucol, or acetic acid, or some other member of the fatty acid series. Some tannins, as those of alder and hop, give both phloroglucol and acetic acid, but it is very possible that this arises from the presence of two distinct tannins in these materials. It is stated that all those tannins which yield acetic acid on fusion with potash, also yield considerable quantities of glucose to dilute acids, while the phloroglucide tannins do not do so. Gallic acid fused with caustic soda has been found by Barth and Schroeder to produce a small quantity of phloroglucol, and it is similarly formed by resorcinol and common phenol (Chem. Soc. Jour., xliv. 60). It is therefore possible that in some cases where phloroglucol is detected, it may have been formed by the action of the alkali, and not have been originally a constituent of the tannin.