CHAPTER IX.

_THE PHYSICAL CHEMISTRY OF THE HIDE-FIBRE._

The nature of the changes which take place in the conversion of raw hide into leather, and the causes of swelling and “falling” in the various stages of the wet-work and tannage are among the most difficult problems with which we have to deal, and no intelligible explanation can be given without taking into account facts which are among the most recent discoveries of physical chemistry; and of which even yet our knowledge is by no means complete.

We know from our study of the structure of hide, that it consists in its natural state of gelatinous fibres which are soft and swollen with water, and easy putrescible. When these are dried, they contract and adhere to each other, forming a hard and almost homogeneous mass, resembling in degree, a sheet of glue or gelatine. After the tanning process, the fibres are changed in character, though not in form; they no longer absorb water so freely, and in drying they do not adhere together, but remain detached and capable of independent movement. The leather is therefore porous, flexible, and opaque on account of the scattering of light from the surfaces of the fibres, although the individual fibres are translucent. At the same time, chemical changes have taken place which render the fibres incapable of ordinary putrefaction. Our first necessity, therefore, in the conversion of skin into leather is to dry the fibres without allowing them to adhere. This is accomplished in the most primitive mode of leather dressing, by mechanically working fatty substances into the skin as it slowly dries, so as to coat and isolate the fibres, which are loosened by kneading and stretching; while at the same time the fat forms a waterproof coating which prevents them from again absorbing the water which is necessary to putrefaction. Similar results may be produced by causing chemical changes in the fibres themselves, which render them insoluble in water, and consequently non-adhesive; and a sort of leather may even be made by merely replacing the water between the fibres with strong alcohol, in which they are insoluble, and which absorbs and withdraws the water from them, allowing them to shrink and harden, while preventing their adhesion. The merit of having first clearly seen and expressed these cardinal principles in leather production belongs to the now venerable Professor Knapp, who published in 1858 a short paper (_Natur und Wesen der Gerberei und des Leders_) which is a model of clear explanation and practical experiment. Knapp, however, deals mainly with the changes in the condition of the fibre which are necessary to convert it into leather, and not with their physical causes; and before we can explain the means by which these changes are brought about, we must be acquainted with certain facts and theories about solutions which have become much clearer since he wrote.

The particles (molecules) of all substances are drawn together by attractive forces somewhat of the same character as the attraction of gravitation which holds together the solar system, and which is the cause of weight. It is indeed even possible that these forces are identical. Like gravitation, these molecular attractions increase rapidly in intensity as the distance of the attracting bodies diminishes, so that in solids and liquids, where the molecules are near together, they are immensely powerful, while in gases and vapours they are barely perceptible. These attractions are opposed by the motion of heat, which takes the same part in molecular physics which the energy of planetary motion does in the solar system. In solids, the attractive forces hold the molecules rigidly in position, the motion of heat being limited to short vibrations round a fixed point, the effects of which are visible in the expansion caused by rising temperature. If the temperature is increased, most substances become liquid, a condition in which the particles can roll round each other, but are still held together by their mutual attractions, as the sun holds the earth from flying off into space. If the temperature goes on rising, the orbits of the molecules become greater, the liquid expands, and finally molecules fly off at a tangent out of reach of the attractions of the mass of liquid, and are only diverted from their course by colliding with solids or with other flying molecules, from which they rebound. This constitutes the state of vapour or gas.

The molecules usually consist of _groups_ of atoms. Thus in the vapour of water, each molecule contains one atom of oxygen combined with two of hydrogen, and it is only at immense temperatures that this inner grouping is broken up. Naturally, the more complicated and heavier the molecular group, the more easily it is broken up by outside causes into simpler groupings, and molecules may exist in liquids or solids, which break up before they reach the gaseous form. Of such substances the chemist says that they “cannot be volatilised without decomposition.” In very rare instances does the gaseous molecule consist of a single atom; even those of the most perfect gases, such as hydrogen, oxygen and nitrogen consist of pairs which are not broken up at any known temperature. The pressure of a gas, and its tendency to expand is due simply to the motion and impact of its flying molecules, and it may be noted that at the same temperature and pressure equal volumes of all gases have the same number of molecules, the lighter molecules making up for their want of weight by their greater velocity. The average velocity of a molecule of oxygen (O₂) at freezing point is 461 meters per second or about that of a rifle-bullet. It must not be taken however, that in any given solid, liquid, or gas, all the molecules at any temperature move at a uniform velocity, but that each individual molecule may vary from moment to moment from rest up to a very high speed, while the temperature of the mass only represents the average. Thus it happens that in all liquids, and even in solids, a certain proportion of the molecules at any temperature will have a speed sufficient to enable them to leave the surface, and take the form of vapour, while a certain proportion will fall back and be caught and retained. Thus every liquid, and theoretically every solid, has a “vapour-pressure,” rising with the temperature, and depending on it only, and at the boiling temperature of the liquid equal to that of the atmosphere, or about 15 lb. per square inch, and therefore able to form bubbles in the interior of the liquid. If a little of a liquid is confined in a flask, the flask will become filled with its vapour, and so long as any of the liquid is present, the pressure of the vapour will depend only on the temperature and not at all on the respective quantities of liquid or vapour. Neither will it be affected by the pressure of other vapours or gases present in the flask, the total pressure in which will be the sum of the “partial” pressures of all the gases and vapours present.[52]

[52] Cp. p. 421.

The behaviour of gases and vapours has been described in some detail because it possesses very close analogy to that of substances in solution. The molecules of liquids are held together by attractions which are very powerful over the short distances which separate them, amounting in most cases to many tons per square centimeter of sectional area, but the range over which they act is very small. In the interior of the liquid the attractions on one side of a molecule are of course exactly balanced by those on the opposite side, so that it is free to move within the liquid without hindrance, but at the surface a very small part of the force due to the attractions of the surface-layer is unbalanced and acts as a sort of elastic skin holding the liquid together, and is called “surface-tension.” Familiar examples of this are found in the force which supports a drop on the end of a tube, the possibility of laying a slightly oily needle on the surface of water without sinking, and the ability of some flies to walk on water as if it were covered with a sheet of india-rubber. Many liquids will mix or dissolve in each other in any proportions, e.g. water and alcohol; the attraction of the alcohol for the water-molecule being as great or greater than that of alcohol for alcohol, or water for water. In other cases, such as water and oil, or water and petroleum spirit, practically no mixture takes place, their mutual attraction being small; and each retains a considerable surface-tension at the points of contact, though less than that of the free surfaces, since each exerts an attraction on the other. There are also many intermediate cases, such as water with chloroform, carbolic acid, or ether, in which each solvent dissolves a portion of the other, but the two solutions do not mix, but form separate layers. In these cases an equilibrium is attained, in which there is just as much tendency for either of the liquids to pass into as out of the other layer. In this there is an extraordinary resemblance to what has been said of vapour-pressures; and the tendency to pass into solution is often called solution-pressure; and it may be noted that when equilibrium has been reached, not only is the solution-pressure, but the vapour-pressure of each constituent equal in both solutions. Like vapour-pressures, the solution-pressures usually increase with rise of temperature, more of each constituent passing into the other, till at last the composition of the two layers becomes identical, their surface-tensions disappear, and complete mixture takes place. With phenol (carbolic acid) and water this takes place at about 70° C.

Most of what has been said of the mutual solution of liquids is also true of the solution of solids, but the latter may be divided into two very distinct classes, colloids and crystalloids (which, however, shade off into each other). The colloid or gluey bodies are mostly miscible in any proportion with liquids in which they dissolve, and there is no such thing as a definite point of saturation. There are however some which form _jellies_ which have great analogy to the partially miscible liquids; there is a mutual solubility, a portion of the solid dissolving to a liquid solution, while the remainder of the liquid dissolves in the solid, increasing its volume, but still retaining the characteristics of the solid state. As the temperature is raised, this mutual solubility generally increases, till at a given point the jelly melts, and complete solution takes place, as in the case of partially miscible liquids. These phenomena are of prime importance in the theory of tanning, but their further consideration must be deferred till a few words have been said about the crystalloids. These are characterised by regular crystalline form, indicating that the attractive forces of their molecules are exerted in definite directions, giving them a tendency to attach themselves together in definite geometrical arrangements. They dissolve in themselves no part of the solvent, but are dissolved by it till an equilibrium is reached in which the tendency of further particles of the solid to pass into the solvent is balanced by that of those already dissolved to attach themselves to the remaining solid, or “crystallise out.” Such a solution is “saturated” with respect to the solid residue, but the word has no meaning unless solid crystals are present, and where a body has, as sometimes happens, more than one crystalline form, a solution may be saturated with regard to one of them, and more or less than saturated with regard to another. In “supersaturated” solutions, crystallisation is at once started by the addition of a “seed” crystal of the proper form.

If a crystalloid substance, such, for instance, as copper sulphate, be placed in a solvent (e. g. water), the dissolved salt will gradually spread itself through the whole body of the solvent, though in the complete absence of currents in the liquid, the motion is extremely slow, and years may be taken for the diffusion to rise through a few feet. In many cases salts diffuse through aqueous jellies at the same speed as they would through still water. Colloid substances on the other hand have little or no power of diffusion and mostly cannot pass through jellies at all. This is the reason why tannage with mineral salts is so much more rapid than with vegetable tannins which are of colloidal character, and diffuse through the gelatinous fibres of the hide with extreme slowness.

All dissolved crystalloids do not pass through gelatinous membranes with equal ease, and substances are known, mostly gelatinous precipitates, which do not permit the diffusion of dissolved salts, though they allow water to pass freely. Thin layers of such precipitates form what are called “semipermeable membranes.” The existence of such membranes affords us the possibility of direct measurement of the tendency to diffusion, or as it is generally called the “osmotic”[53] pressure of dissolved bodies. Thus a porous earthenware battery-cell may be immersed in a solution of copper sulphate, and filled with one of potassium ferrocyanide. In this way its pores will be filled with a gelatinous precipitate of copper ferrocyanide, which is pervious to water, but impervious to most dissolved substances. If now the cell be filled with a dilute solution of some crystalloid, say sugar, and its top closed by a perforated cork fitted with a vertical tube, and the cell be plunged in water, the latter will pass into the cell, and the dilute solution will rise in the tube to a height of many feet above the water outside. By substituting a mercury pressure gauge for the vertical tube, exact measures of the pressure in the cell can be made, which is the osmotic pressure of the dissolved substance. At first sight it is paradoxical that the water should flow into the solution, apparently against a heavy pressure, but the explanation is simple. Mention has already been made of the enormous internal pressures of liquids produced by the attractions of their molecules. In the solution a portion of this is borne by the dissolved substance, and the water flows in from the outside till an internal mechanical pressure is produced, equal in amount to the osmotic pressure of the dissolved substance. The resemblance of the phenomena of solution to those of vapour-pressure has already been mentioned, and it is found to be even quantitative, since the measured osmotic pressures are exactly equal in amount to those which the dissolved body would produce if it were in the state of vapour at the same temperature and occupying the same volume as the solution. It acts, in fact, precisely as the “partial pressure” of a vapour. There are several indirect ways of measuring the osmotic pressure of dissolved bodies, as for instance, from the lowering of the freezing point, or the raising of the boiling point of the solution as compared to those of the pure solvent, all of which confirm the direct measurements, and show that in a given volume at the same temperature, the same number of molecules will produce the same osmotic pressure whatever their nature, or conversely, that at the same osmotic pressure and temperature equal volumes of any solution must contain the same number of molecules. The use of these facts in determining molecular weight is obvious.

[53] Solution-pressure and osmotic pressure are really two names for the same force; the former being employed to signify the tendency of a solid to dissolve, and the latter the pressure produced by the dissolved body which tends to prevent further solution. Thus, in a saturated solution in contact with its solid, the two pressures are always equal, but exerted in opposite directions.

A curious apparent deviation from this law is however noticed in solutions of salts, acids, and alkalies, and indeed of electrolytes generally; thus a dilute solution of sodium chloride produces an osmotic pressure nearly double that corresponding to the number of molecules of NaCl present; and in fact behaves as if it were a solution of Na and Cl existing separately. Such a solution conducts a current of electricity very readily, while at the same time the chlorine is carried to the positive, and the sodium to the negative pole, where they separate as Na₂ and Cl₂ (the Na decomposing the water present and forming NaOH). In fact, the modern theory of electrolysis asserts that these dissociated atoms are not separated from each other by electricity, but that they exist already separated in the solution of the electrolyte, and merely act as carriers for the electricity, and that the work done by the latter is not that of breaking up the salt-molecule, but of giving its dissociated atoms fresh charges of electricity which enable them to combine as new molecules, and escape from the electrolyte. Complex salts do not always break up into single atoms, thus calcium sulphate dissociates into Ca and SO₄, hydrogen sulphate (sulphuric acid) into 2H and SO₄, and so on. These dissociated atoms and atom-groups are called “ions,” and may be monovalent, divalent, and so on; the divalent ion carrying double the electrical quantity or charge of the monovalent. Without discussing the ultimate nature of electricity itself, the matter is most easily pictured by assuming that the molecule of the undissolved salt is made up of an ion with a + charge (“kation,” e.g. Na), and an ion with a - charge (“anion,” e.g. Cl), by the electrical attraction of which charges they are held together. In the solution these attractions are balanced by those of other ions, so that they can wander freely within the liquid, but in order to take the molecular form of free elements and escape, say as Na₂ and Cl₂, the pair of kations must go to the - pole and give up one + charge, and at the same time a pair of anions must go to the + pole and receive a + charge. Thus the Na and all other kations separate at the - pole, and the Cl and all other anions at the + pole.

From what has been said, it will be obvious that free ions can only exist in solution, and can neither evaporate, nor separate as solids; but that in the liquid they act much like other dissolved molecules, exerting their own osmotic pressure independently of each other or of the dissolved salt, but with the limitation that a solution must always contain at the same time equal numbers of + and - ions. As a solution is diluted, more ions are liberated; as it is concentrated, more recombine to form undissociated salt. This will be made clearer by an example. In a saturated solution of sodium chloride with solid salt present, we have dissolved salt at the solution-pressure of the crystallised salt, and Na and Cl ions at the dissociation-pressure of the saturated salt solution, and neither affect the others. If we now add hydrochloric acid, it has no effect directly on the solubility of the _salt_, but as HCl dissociates largely into H and Cl, it increases the pressure of the Cl ions, and so compels the salt to recombine till the Cl pressure is reduced to its normal amount. This increases the concentration of the undissociated salt-solution, and thus salt is precipitated or crystallises out till the solution is no longer super-saturated with respect to the salt-crystals.

Most chemical reactions, and especially those between acids and bases, are really reactions of the ions. Thus NaOH in dilute solution is mostly ionised into Na and OH, while HCl is similarly ionised into H and Cl. On the other hand, water ionises only very slightly. Hence, on mixture, the H and OH combine and form water, with evolution of heat, while no actual combination occurs between the Na and Cl, so long as they remain in _dilute_ solution. For this reason, the heat of neutralisation of all strong acids and bases is the same, independent of their nature, since strong acids, bases and salts are almost completely ionised. The rapidity of action, and consequently what we call the “strength” or “avidity” of an acid or base depends on the number of its free ions in solution; very weak acids and bases are very little ionised, though their salts ionise almost completely in dilute solution. On this depends the explanation of a fact of great practical importance. Hydrochloric acid, a strong acid, is almost completely ionised in solution; acetic, a weak one, very little; while sodium acetate and sodium chloride as salts are both almost completely ionised. If we add hydrochloric acid to a solution of sodium acetate, we shall have sodium-ions, acet-ions, chlorine-ions and hydrogen-ions in the solution. As the pressure of the acet-ions and the hydrogen-ions will be greater than the dissociation-pressure of acetic acid, they will combine to form it, till the pressure is equalised, and we shall have in the solution, free acetic acid slightly ionised, the sodium- and chlorine-ions of sodium chloride, and the sodium- and acet-ions of any excess of sodium acetate left. If the hydrochloric acid were just sufficient to combine with the whole of the sodium, we should have an equilibrium containing much (ionised) sodium chloride and little sodium acetate, together with much free acetic acid, and little hydrochloric. Thus the “strong” acid would displace the weak one.

Taking another example, we add sodium acetate to a solution of acetic acid. As the ionisation-pressure of the acetic acid is much less than that of sodium acetate, and both have a common acet-ion, the ionisation of the acetic acid will diminish, and more undissociated acetic acid will form, till by its concentration the two pressures are equalised. The total quantity of free acetic acid will be unchanged, but a less proportion of it will be ionised, and it will act like a weaker acid. This reduction of the activity of a weak acid by the addition of its neutral salt is often made use of by chemists. Instances in tanning practice are the use of excess of potassium dichromate with chromic acid in the chrome tanning process, the effect of neutral salts in “mellowing” the action of tanning liquors, and the use of salt in “pickling.”

Let us now try to apply these facts to the physics of tanning, taking first the simplest cases, where electrolytic dissociation does not take place. We may consider the wet hide as made up of a mass of fibres of gelatine-jelly, with interspaces which are filled with water. In fact, for many purposes of experiment we may substitute for hide, mere sheets of swollen gelatine, so as to avoid the complications introduced by the water or solution mechanically retained between the fibres.

If we place a sheet of dry gelatine in water, it swells, absorbing perhaps seven or eight times its weight of water, but does not appreciably dissolve. A condition of equilibrium is reached when the attraction of the water-molecules for the gelatine is equal to the sum of the cohesive attraction of the gelatine for itself and the internal attraction of the water outside. An increase of the cohesion of the gelatine would tend to make it contract and expel part of the water, and this contraction would tend further to increase both the cohesion of the gelatine, and its attraction on the diminished number of water molecules it contained, and clearly these causes would act in opposing directions. The equilibrium is therefore a very unstable one, and slight causes might be expected to produce great changes in the degree of swelling, which is indeed the case. If we increase the temperature we diminish the cohesion of the gelatine, till at a point it becomes less than its attraction for the water, and the jelly suddenly loses its solid condition and dissolves.

The absorption of water by colloids (including gelatine) is accompanied by contraction of volume (compression) of the water absorbed, and by evolution of heat, and, as has been pointed out by Koerner,[54] it is opposed (and swelling decreased) by increase of temperature. Solution, on the other hand, absorbs heat, and is therefore favoured by rise of temperature.

[54] Beiträge zur wissenschaftlichen Grundlage der Gerberei, Freiberg, 1899.

If we place the swollen jelly in alcohol, it parts with water and contracts. The gelatine and alcohol are not mutually soluble, the sum of the attraction of water for alcohol, and the cohesive attraction of the gelatine is greater than the attraction of the latter for water, and as the alcohol cannot pass into the gelatine, the water passes out, and the jelly contracts. The greater the concentration of the alcohol, the more completely is the jelly dehydrated, and in strong alcohol it may become quite hard and solid. If we like to express the same facts in language more familiar to the modern chemist, but perhaps less clear to the non-chemical reader, we may say that the alcohol exerts an osmotic pressure outside the gelatine, but little or none inside it, and therefore the water is squeezed out. It would be equally true to say that the water passes out of the jelly till its osmotic pressure is equal in both the jelly and the alcohol. The jelly is a true “solid solution” of water in gelatine, and in a solution we may regard either of the two constituents as the solvent. Exact parallels may be found in the distribution of a third substance between two immiscible solvents (see p. 76), say alcohol between water and ether.

The osmotic pressure of water into alcohol may be demonstrated in a very simple way, taking advantage of the fact that a film of jelly is permeable for water but not for alcohol. If the experiment described on p. 78 be made by placing alcohol in a cell previously washed out with a gelatine solution, and the cell be placed in water, the water will pass into the cell, and the alcoholic solution will rise many feet in the vertical tube. The insolubility of gelatine in alcohol may be made use of for its estimation. If three times its volume of absolute alcohol be added to a solution containing gelatine, the latter will separate as a solid mass on a stirring rod, or on the sides of the beaker, and may be washed with further portions of alcohol. The method is useful in the analysis of gelatine lozenges and “jelly squares,” roller compositions, hectograph masses, and the like; and for the determination of true unaltered gelatine in glues, and commercial gelatines (see page 60). Many other colloids are however also precipitated by alcohol.

If hide be treated with alcohol, as in Knapp’s experiment (p. 74), the action is precisely the same as has been described with gelatine-jelly. The water is withdrawn, first from the spaces between the fibres, and then from the fibres themselves, and the skin dries with the fibres isolated and non-adherent, and is in fact converted into a sort of leather, which, however, returns to raw pelt on soaking in water.

The action of solutions of sugars, glycerine, and the like is in principle similar to that of alcohol, but more complex, since in general these bodies are soluble not only in the water, but in the gelatine or hide-fibre, so that their effect cannot be foretold, though usually it tends towards contraction rather than swelling. In general terms the equilibrium is a balance of the attraction of the water and the sugar for the gelatine, against the sum of their mutual attraction in the solution outside and the resisting cohesive force of the gelatine; and will depend not only on the nature of the substances, but on temperature and concentration.

The action of acids, alkalies and salts on gelatinous fibre is yet more complex, since not only electrolytic dissociation, but most probably actual chemical combination comes into the question. The chemical constitution of gelatine is as yet quite uncertain, but it is known that the molecule contains both amido-groups capable of linking with acids, and carboxyls which will combine with bases (see p. 58). Hence hide-fibre absorbs both acids and bases with great avidity, so much so that the sulphuric acid of a decinormal solution may be completely removed by hide, leaving only water without a trace of acid recognisable by litmus. Alkalies are absorbed in a similar way, and in both cases the gelatine or gelatinous fibre acquires a greatly increased power of absorbing water, and consequently of swelling. Familiar cases of this are the swelling of hide by acid, and by lime, and in neither case can the added substance be removed in any reasonable time by mere washing with water. Hence to free hides from lime or acids it is necessary to neutralise the alkali with acids (see p. 153) or the acid with chalk or alkalies (p. 91). No accurate determination has yet been made of the amount of acid or alkali with which gelatine or hide-fibre will combine, since the matter is complicated by the volume of acid or alkaline solution which is absorbed mechanically, and by the tendency of the compound to partially decompose on washing with water. Experiments made by the author lead to the conclusion that 1 grm. of air-dried gelatine will combine with about 0·025 grm. of actual hydrochloric acid (HCl) when placed in a very dilute solution of the latter, and this compound will absorb 40 or 45 grm. of water while still retaining the jelly state. The maximum swelling, with both acids and alkalies, is obtained with dilute solutions; and with the stronger acids, the outside solution must be almost neutral when equilibrium is attained, increasing quantities of acid diminishing the amount of water absorbed. The same statement is true of the strong alkalies. Thus in both cases, where swelling is desired, the object is defeated by the use of too strong solutions, and the quantity of acid or alkali should be rigidly adjusted to the weight of pelt, and not to the volume of solution.

As regards a physical explanation of the effect of acid and alkaline solutions upon gelatine, anything which can yet be said must be regarded rather as speculation than as actual scientific knowledge. It must also be admitted that while the view that actual _chemical_ combination takes place between the gelatine and acids (or alkalies) seems much the most probable, difficulties arise from the fact that different acids apparently do not always combine in proportion to their equivalents, though it is probable that these will prove only apparent anomalies when more accurate means are known of determining how much acid is really combined, and how much merely mechanically absorbed.

Leaving out of account for the moment the question of swelling, a few words must be said about a property of these acid- (and alkali-) gelatine compounds, a knowledge of which is essential to understanding the swelling process. If a mass of acid-gelatine be suspended in pure water, a certain portion of it will be decomposed into neutral gelatine and free acid, and the latter will diffuse into the water. Thus acid-gelatine can only exist in presence of a certain amount of free acid. This dissociation by water is a common property of all salts, and necessarily follows from what has been said of ionisation; but it is only where the combining affinity of the constituents is weak, that it becomes practically perceptible. Water to a _very_ small extent ionises to H and OH. If we imagine a salt dissolved in it, such as NaCl, which ionises almost completely to Na and Cl, we see that a certain proportion of NaOH and HCl must be formed by combination with the water-ions. In the case named the quantity is absolutely negligible, since both sodium hydrate and hydrochloric acid are almost completely ionised themselves, but if either the acid or the base is weak (that is little ionised), the process of combination must go on till the acid or basic solution is strong enough to have an ionisation-pressure equal to that of the salt. As this acid or base is no longer in an ionised condition, it may be removed from the solution by volatilisation or diffusion. For instance, if a solution of ferric chloride be confined in a tray of parchment paper, through which it has little power of diffusion, and this tray floated upon water which is frequently changed, the dissociated acid will diffuse through the membrane into the water, and in this way the whole of it may be ultimately removed, leaving nothing but a colloid solution of hydrated ferric oxide in the tray. Actions of this sort, in which the gelatinous fibre of the hide plays the same part as the parchment-paper membrane, have an important share in many of the phenomena of tanning.[55] Thus, in the case of hide swollen with acid, the acid compound with the fibre is somewhat dissociated, and if the hide be hung in water which is constantly changed, the acid diffuses into it, and the whole may be ultimately, though slowly removed. A similar effect is produced in the familiar operation of removing acid from pelt or chromed leathers by paddling with “whitening” (calcium carbonate). The latter is insoluble in water, and therefore cannot penetrate into the hide, but as it instantly combines with any acid which diffuses out, the acid-gelatine compound is rapidly decomposed, since it is only permanent in a solution containing enough free acid to have an ionisation-pressure equal to that of the compound. Similar statements are true of the alkali-gelatine and lime-gelatine compounds.

[55] Probably chrome, aluminium and iron salts are decomposed in this way in mineral tanning, and thus fixed in the hide as insoluble basic salts. Cp. pp. 186, 215.

It will be easier to follow the results of what has been said if we take a concrete case which has been carefully investigated by the author and others; that of the action of hydrochloric acid solutions on gelatine. If a weighed sheet of gelatine be placed in a very dilute solution of the acid, it swells much more considerably than it does in water, a maximum swelling being attained with a concentration of the outer solution of 0·1 to 0·2 grm. of HCl per litre. The swollen jelly has then a volume of about 45 c.c. per gram of the air-dried gelatine, and a concentration equal to about 0·75 grm. of HCl per litre of swollen jelly, or at least about five times that of the outer solution. As the concentration of the latter is increased, the concentration in the jelly also increases, but in a much smaller ratio, while the volume of the jelly diminishes, till, with a concentration of 5 grm. of HCl per litre in the outer solution, the volume of the jelly is only about 18·5 c.c., and its concentration not quite 6 grm. per litre. These facts cannot be accounted for by any theory of simple solution of the HCl in the jelly, since the law of such solutions is that the concentration in each maintains a constant ratio, unless chemical change takes place. It is possible that they might be explained by adsorption (surface attraction), but as it is known that gelatine contains both amido-groups capable of combining with acids, and carboxyl-groups which can combine with bases, it is much more likely that actual chemical combination takes place, and that the apparent irregularities in the amount of acid fixed are due to partial hydrolysis of the compound.[56]

[56] Cp., however, Walker and Appleyard on the ‘Absorption of Acids by Silk,’ Chem. Soc. Trans. 1896, p. 1334.

The following may be suggested as a working hypothesis. As both water and hydrochloric acid can pass freely in and out of the jelly, it must be in osmotic equilibrium with the outer solution in every respect, and neither the un-ionised hydrochloric acid of the solution, nor the small amount which may be formed by hydrolysis of the gelatine compound can have any effect on the swelling. So long as the outer solution is very dilute, by far the greater part of the acid present is absorbed and fixed by the gelatine, and almost the whole of the outer acid will be ionised, as well as a portion of that in combination with the gelatine. In the latter case, however, the ions will be unable to pass out of the jelly, and will therefore cause an internal osmotic pressure, and the gelatine will swell till the Cl-ions are in osmotic equilibrium with those of the outer solution. At the same time, this internal pressure of Cl-ions will oppose the entry of the Cl-ions (and therefore also of their associated H-ions) from the outer solution, and the acid solution absorbed mechanically will be somewhat less concentrated than that outside. As the concentration of the outer solution is increased, the pressure of the outer Cl-ions will repress the ionisation of the gelatine-chloride, and at the same time its tendency to hydrolyse. Thus the acid actually combined with the gelatine should somewhat increase, but the swelling should diminish, as is actually the case.[57] It is impossible to carry the concentration of the hydrochloric acid much above 5 grm. per liter without causing solution of the gelatine, but the addition of common salt to the outer solution should equally increase the pressure of its Cl-ions, and cause further diminution of swelling, the Na-ions in this case increasing the outside pressure in the same way as the hydrogen ions. In fact the addition of salt in sufficient quantity will reduce the swelling till the gelatine becomes quite solid, and retains only about its own weight of water, while at the same time the apparently combined acid largely increases. This cannot be attributed to any direct dehydrating action of the salt, since concentrated sodium chloride solutions have no dehydrating, but rather a swelling effect on gelatine in the absence of acid, and the concentration of the salt in the outer solution and in the jelly proves precisely the same within the limits of experimental error. Several other facts may be noted, tending to support the explanation which has been given. The tendency to swell gelatine is common to all acids of appreciable strength, and in all cases where the concentration of the acid could be increased to a moderate extent without causing solution of the jelly, the effect of a maximum swelling, diminishing as the concentration of the acid increased, has been observed. Other salts also produce similar effects to sodium chloride; thus the swelling caused by sulphuric acid is repressed by sodium sulphate. Sodium chloride seems to diminish the swelling caused by all acids, but in presence of large excess of sodium chloride, most of the acid in combination with the gelatine will probably be hydrochloric, whatever the acid used to originally produce the swelling. A curious fact observed by the author, is that absolute alcohol, which so effectually dehydrates neutral gelatine, is almost powerless to remove either water or acid from gelatine swollen by hydrochloric acid. HCl is freely soluble even in absolute alcohol, but H- and Cl-ions can only exist in it to a very small extent, so that we may conclude that the acid which causes the swelling and retains the water of the jelly exists either in actual combination with the gelatine, or in an ionised condition.

[57] The acid retained by the gelatine, as measured by deducting from the total contained in the jelly, a quantity equivalent to the volume of solution absorbed, at first rises rapidly to a maximum, then slightly diminishes and remains practically constant. On the theory suggested, it is evident, however, that the absorbed solution must be more dilute than that outside, and the actual combined acid greater than that shown by the above calculation. The “combined” acid, as determined by indicators, shows slight but continuous increase. It is acid to phenolphthalein, but neutral to methyl orange.

Solutions of caustic alkalies are in most respects analogous in their swelling action to those of strong acids. A portion of the alkali is in some way fixed by the gelatine, while another portion is simply absorbed as solution. A maximum swelling effect is also noticed with dilute solutions, which is diminished as the concentration increases. Swelling by alkalies is not diminished by chlorides so far as has been observed, and especially it may be noted that the swelling produced by caustic soda is not diminished by sodium chloride. On the theory which has been suggested there is no reason why alkaline swelling should be reduced by chlorides, since the swelling agent has no Cl-ion, but it is somewhat singular that the sodium salt, having a common Na-ion should produce no repression of the swelling by caustic soda. In the present state of our knowledge no definite explanation can be given, but it is quite possible that the swelling in this case is not produced by the sodium-ion but by some more complex one, or even by the hydroxyl-ion, like most of the characteristic reactions of alkalies. Apparently the gelatin-alkali compound is still strongly alkaline, affecting phenolphthalein indicator like uncombined alkali--an effect which is known to be due to the presence of free HO-ions.

The effect of acids and alkalies has been studied by Procter and others on actual pelt as well as on gelatine, and has been found to be qualitatively, if not quantitatively quite similar to that on gelatine, though from the acid retained mechanically in the interfibrous spaces, exact quantitative determination is more difficult. The amount of swelling produced is not proportional to the strength of the acid, some weak and little ionised acids such as lactic producing larger swelling than stronger acids such as hydrochloric and sulphuric, of which the ionic pressure in the external solution is greater. Dilute solutions generally produce greater swelling than more concentrated, so that where swelling is required without destructive effect on the fibre, dilute solutions of such weak acids are to be preferred, and the presence of neutral salts is to be avoided. On the other hand, where it is desired to remove lime, or to bring the pelt into an acid condition without swelling, the addition of neutral salts, and especially of chlorides is advantageous. A very important application of this principle is the “pickling” of sheep-skins, and especially of sheep-grains, in order to preserve them for export. The principle of this operation is that the skins are first swollen slightly with sulphuric acid, and the swelling is then reduced by salt, either added, or used in a subsequent bath. In practice, salt is now generally also added to the first bath to moderate the swelling. A suitable strength for the “rising solution” is about 80 grm. common salt, and 7·5 grm. sulphuric acid per litre. 100 c.c. of this solution will therefore require about 15 c.c. of N/1 alkali to neutralise it, and it should be tested after each lot of skins, and maintained at the same strength by suitable additions of acid. The acid absorbed by the skins is mainly hydrochloric, sodium sulphate accumulating in the bath. The salt is not absorbed by the skins in the same way as the acid, but will be continually diluted by the water they bring in, and occasional additions of salt must therefore be made, the density being maintained at about 65° Bkr. (1·065 sp. gr.) After paddling or being stirred in this bath for about ¹⁄₂ or ³⁄₄ hour the skins are transferred to saturated brine, and stirred in it till fully fallen in thickness, the density of the liquid being maintained by excess of salt. They may be allowed to remain some hours in the saturated brine with advantage.

Within moderate limits, the strength of the rising liquor is not of great importance, since the skins will only absorb a certain amount of acid (increasing with the concentration of salt). In the second or falling liquor the large excess of salt forces all the acid present into the skins, none diffusing into the bath. Skins may be effectively pickled with very much smaller quantities of acid than those prescribed above, or ordinarily used, and are much easier to tan satisfactorily; but it is said that they are more liable to suffer from mildew. Pickling may also be done by placing the skins in a concentrated brine-bath, and adding a calculated quantity of acid, not exceeding 0·1 grm.-molecule of sulphuric acid per kilo. of dry hide substance, but the method is not economical in practice from the dilution of the bath produced by the water brought in by the skins and the necessity of constant large additions of salt.

Pickled skins must not be brought in contact with water, which by diluting the brine they contain, allows the excess of acid to act upon and destroy the fibre. Even drops of water, accidentally sprinkled on the skins produce this effect, and it is said that it spreads to parts which have not been wet. For similar reasons, it is necessary in tanning pickled skins, at least to begin the process in liquors to which salt has been added, the quantity required being dependent on the amount of acid used in pickling the skins, and where this is reduced to a minimum, it is even possible to tan without further addition of salt than that contained in the skins.[58] The pickling process converts the skins into a species of white leather, and skins tanned in salted liquors after pickling, or by addition of both acid and salt to sumach liquors give good colour, and tough leather with a much diminished consumption of sumach. The permanency of such leather is somewhat doubtful, but the writer was unable to detect free sulphuric acid in a sample which he examined, and it may be that when no acid is added to the later liquors, that derived from the pickling is expelled by the tannin; but this is very doubtful.

[58] Instead of using salted liquors, the skins maybe “depickled” by a bath of whitening and water, borax, or some other mildly alkaline solution before tanning.

The facts which have been discussed in the preceding pages offer a sufficient explanation of the causes which operate in those deliming processes which depend on the simple neutralisation of the alkaline matters present in the hide, and of the swelling by means of acid which forms a step in the manufacture of many sorts of sole-leather, but they by no means fully elucidate the causes of the much more complete depletion of the pelt brought about by the bacterial products of bates and puers. It has been pointed out (p. 82) that gelatine and hide-fibre in a neutral condition are swollen by water, but that the equilibrium so reached is an unstable one, easily influenced by slight causes. Among these, as has been pointed out by Koerner,[59] the surface-tension between the water and the swollen fibre holds a place; and surface-tensions of this sort are greatly influenced by many substances of the class to which bacterial ferments belong. Many salts also alter the water-absorption of gelatinous fibres, sometimes causing swelling, and sometimes contraction, according to temperature, concentration, and the nature of the salt. Though most salts do not seem to be absorbed by hide-fibre, it is possible, as suggested by Koerner (_loc. cit._), that in some instances the base may combine with the acid-groups, and the acid of the salt with the basic groups of the gelatine-molecule, while other cases are known in which salts are actually dissociated, and their acid fixed by the affinities of the hide-fibre. An interesting case of this sort was recently proved by Paessler and Appelius,[60] who showed that sulphuric acid was absorbed from a solution of hydric sodic sulphate, and the neutral sulphate left in the solution. Similar reactions undoubtedly occur with some salts of strong acids and weak bases, but this point must be more fully discussed in connection with the theory of mineral tannages.

[59] Beiträge zur wissenschaftlichen Grundlage der Gerberei, Jahresberichte der deutschen Gerberschule zu Freiberg, 1898-9 and 1899-1900.

[60] Wissenschaftliche Beilage des Ledermarkt, 1901, ii. p. 106.