THE PHYSICS OF BATING.

It is impossible in the space of a short chapter to give an adequate explanation of the physical changes taking place during the puering process. An outline only can be given, and perhaps a few signposts to indicate for those interested, in what direction further work can be usefully done. It is obvious that if more be attempted, a treatise on physics and physical chemistry would be required; such a work is beyond the scope of the present volume. It is to be hoped that the whole of the questions dealt with here, including the physical chemistry of skin and of the whole tanning process, will shortly be fully treated by the master hand of our greatest tanning chemist, Professor H.R. Procter.

Since tanning, in the earlier stages, is for the greater part a physical absorption, or colloidal co-precipitating action,44 the physical state of the skin fibre, or condition of the skin before it enters the tanning liquor, is of the greatest importance. In fact, the whole of the operations to which the skin is subjected previous to tanning are directed towards changing its physical state, and the chemical changes undergone are small and principally hydrolytic.

The two most important physical influences over which we have control are pressure and temperature.

The pressure during puering is practically constant, viz. the atmospheric pressure, but, as we shall see later, a diminution of pressure is favourable to falling or depletion. It would, therefore, be interesting to conduct experiments on puering in vacuo, or under reduced pressure.

Before considering the effect of temperature and the changes in the volume of the skin during puering, it will be well to consider the properties of the skin and the puer liquor.

In the skin prepared for tanning, practically all the keratin and epidermis and soluble matters are got rid of, and we have a mass of fibres which are composed of collagen, a proteoÏd, which by prolonged boiling with water is converted into gelatin.

Neither the chemical formula nor the molecular weight of collagen is known with certainty, but, from a series of ultimate analyses, the change of collagen into gelatin is represented by Hofmeister45 by the following equations:—

from which it will be seen that he assigns a molecular weight of 2416 to collagen, and 2434 to gelatin. Other considerations, however, make it probable that the molecular weight is double the above figures.

Gelatin is a typical colloid, and we may consider that skin is practically a colloid with a structure (a very important point, as we shall see later), but behaving in many ways as gelatin. The puer solution is to a large extent colloidal. Therefore, both the skin and the puer are in the colloidal state. In the typical crystalloid solution of an electrolyte, the dissolved body is separated into its molecules, and to a large extent into individual ions, while, in the colloid state, the units of distribution are either large and often conjugated molecules, or more frequently minute particles composed of many molecules united by cohesive attraction.46 In the case of the skin the molecules are not free to move, but are held in place by the structure of the skin, and the fibres thus act as semi-permeable membranes, with capillary spaces between them, in which water and other fluids are merely held by capillary attraction.

If filtered puer liquor is put into a vessel closed by a membrane of skin, and the whole immersed in clear lime water, the puer solution becomes turbid in a short time, but the outer solution remains clear, showing that the skin is permeable to the lime solution (crystalloid), but not to the puer solution (colloid). It was observed, however, that the acids contained in the puer diffused through the membrane; this was shown by the addition of a few drops of phenolphthalein to the lime solution, when the pink colour disappeared after a short time in the neighbourhood of the skin. These phenomena are due to osmosis. It is the fundamental property of all animal membranes, to allow some substances to pass through them more easily than others. In many cases, such membranes, while freely permeable to water, are practically impermeable to certain substances in solution, and play the part of sieves in directing and controlling the diffusion. In the case of skin the phenomena are complicated by the fact that the skin combines chemically with many substances in solution, and thus we do not always know what part to assign to chemical combination and what to the osmotic phenomena.

Procter has shown (Colloid. chem. Beihefte 1911, ii., pp. 243–284) that, while gelatin is very permeable as such to solutions of acids and salts, there may be formed in the presence of excess of acid a hydrolysable chemical complex of the nature of a salt, in which the gelatin functions as base, and which is probably less permeable to acids and their salts than the neutral gelatin. The conditions would then be similar to those which obtain when solutions of an acid and its salt are separated by a movable membrane, which is permeable for the acid and water but not for the salt solution. From the organized structure of the skin surface, it is unlikely that osmosis takes place between the skin itself and the outer solution, with the two surfaces of the skin as semi-permeable membrane. Osmotic action is most likely to occur in the interior of the skin, between the skin fibres themselves and the interfibrillar spaces. The colloids in the puer solution, which constitute a large proportion of its material, cannot, from their nature, penetrate the skin. This may be shown by the above-mentioned experiment. From this it is reasonable to assume that the lime is not actually dissolved from the interior of the skin by the puer acids, but that solution takes place for the most part after it has diffused out into the puer solution. It is probable, however, that part of the bodies of acidic character present in the puer are capable of penetrating the skin fibre, as has been explained above.

The intensity of the osmotic action of puer upon skin must depend upon the quantity of substances contained in it, to which skin substance acts as an impermeable membrane, and which on that account induces an osmotic pressure between the outer puer solution and the solutions held in the skin fibre. The effect of puering does not necessarily imply the actual expulsion of water from the skin—in fact, well puered skin may quite possibly contain as much water as it did in the swelled condition. The difference consists in the manner in which the water is held in the skin, and its freedom to move from parts which are submitted to pressure. In the swollen skin, the fibres may be conceived as swollen by the water and holding it in the same manner as a gelatin jelly; after puering, the fibres are “fallen,” and the water, hitherto held by them, surrounds them in the liquid form.

The osmotic pressure47 of a solution of concentration c, temperature T, and pressure p, is the difference of pressure exerted on both sides of a semi-permeable membrane in thermodynamic equilibrium, having on the one side the solution under the above condition, and on the other side the pure solvent under the pressure p₀ of its own saturated vapour. On this definition the osmotic pressure of a normal solution is over 22 atmospheres, or 330lb. per sq. in.; and since a saturated lime solution is about 1/20 normal, its osmotic pressure is about 1·1 atmospheres, or 16lb. per sq. in.—this represents the force causing the lime to diffuse into water in which the skin is placed. The puer solution being of a colloidal nature, exerts practically no osmotic pressure, and since it contains substances capable of entering into combination with lime, the latter is removed from the surface very quickly. The curve representing the removal of lime by water has been given in Chapter I., p.6. That for puer is not of such a simple character (see Fig.7, p.38), but it will be seen that the greater part of the lime is removed during the first 10 minutes. The curve is plotted for percentage of ash, since the lime is no longer in a caustic condition but in the form of salts. It is remarkable that the percentage of ash, after reaching a minimum, increases considerably. This phenomenon still requires investigation.

Density of Skin.—Coming to the consideration of the volume of skin and its changes during puering, we know that the volume v is the reciprocal of the density, i.e.—

v = 1/d,

and therefore

d = 1/v.

Carini48 has carried out exhaustive experiments on the density of skin during tanning, but, so far as I am aware, little or no work has been done as to the effect of puering on the density of raw skin.

The usual methods of determining density are well known,49 and consist in weighing the body first in air, then in water or other liquid. If m be the weight of the body, and it loses the weight w when weighed in water—

d = m/w.

For experiments on skin, instead of weighing in water it has been found more convenient to use a simple volumenometer, which was devised by Mr. Douglas J. Law (see Coll. 1911, p.230.)

The apparatus (Fig.8) consists of the two vessels A and B, connected by means of thick rubber tubing to the burette C, of which the top is enlarged to a bulb. The bottle A, which was specially made for us by Messrs. Townson and Mercer, London, is of about 1 litre capacity, and the wide mouth is closed by a stopper, accurately ground to fit the neck, and extending down to the bottom of the neck. The upper part of the stopper is elongated to a tube, which is closed by the tap G. The vessel A is also fitted with a tap E. The vessel B serves as a reservoir, and is used to adjust the level of the liquid in the burette C by means of the tap at F. To find the volume of a piece of skin, the method of procedure is as follows. The bottle A is filled with water up to the neck, and the stopper D, carefully greased, is inserted. The tap E is then closed, and the burette is filled with water. Then, with the taps G and E open, the bottle A is filled with water up to D by raising the burette. G and E are then closed, and by opening the tap F the level of the water in the burette is adjusted to zero when F is again closed. G and E are then opened again, and, by lowering the burette, the water in A is allowed to fall below the level of the neck of the bottle. E is then shut, the stopper D removed, and the piece of skin is carefully introduced, avoiding air bubbles. The stopper D is then replaced with the tap open, and, by opening E and raising the burette, the water is allowed to come up to the stopper D again. The taps G and E are then closed, and the volume of the piece of skin read off directly from the burette scale. Volumes up to 50c.c. are measurable as described above, but larger volumes may be measured if a known volume of water is run from the burette into the reservoir before introducing the skin.

By using various solutions in the apparatus instead of pure water, the real swelling or contracting effect of these upon the skin may be observed. The skin is introduced into the bottle A, and the solution adjusted to G, which is then closed; then, by leaving the tap E open, the real swelling or contraction of the skin is measured by the rise or fall of the liquid in the burette.

Petroleum or other liquid may be used instead of water; in some cases, the use of petroleum is more advantageous.

The density of dry gelatin as determined by LÜdeking is 1·412, which is not greatly different from that of skin. Carini gives the following figures for ox-hide:—

but does not give details how these figures were obtained, and if corrected for ash and fat.

The density of limed and puered sheepskin, determined by drying the skin over sulphuric acid until the weight was constant, then determining the volume in petroleum, gave the following results:—

By determining the volume of the wet skin in the volumenometer, and the per cent. of water on drying the skin, the calculated densities were—

Correcting for ash and fat, the dry ash-free skins had the densities—

The density of the wet limed skin was 1·063, but the density calculated from the results (I.) above is 1·0475; from this, it is evident the fibres of the swollen limed skin undergo compression on swelling, or that the water contained in them is in a state of compression, in the same way as gelatin swollen with water occupies a less volume than the sum of the volumes of gelatin and water. LÜdeking50 found for 10 per cent. gelatin jelly d = 1·069; d calculated was 1·041. He attributed the whole of the compression to the water, so that 1c.c. of water in a 10 per cent. gelatin jelly occupied a volume of 0·96069c.c. See also par. 4 p.68.

It may be of interest to give the figures used in the preceding lines in tabular form.

In addition to the diminution of the density of the skin by puering, the increase of fat is very marked.

The percentage of water referred to the wet skin may be calculated from the formula

v₂ – v₁/1 – v₁ × 100

where v₁ = specific volume of the dry skin.
v₂ = specific volume of the wet skin.

The specific volume of the dry skin (v₁) may be calculated from the specific volume of the wet skin (v₂), the percentage of water (w) being known by the following formula:—

and by comparing the volume thus obtained with the volume determined by direct experiment, the amount of contraction is ascertained. In the case we have been considering (viz. limed skin with 80 per cent. water)

The specific volume of the dry skin

The difference for the dry skin is surprising, and considerably greater than anticipated, but has been verified by a large number of experiments. It may be remarked that the exact determination of the density of skin by drying out pieces is liable to error on account of the non-homogeneity of the pieces. There appears to be some chemical combination between the water and the skin very similar to that between water and alcohol, and although the figures obtained in the above experiments by this method are accurate, it is not certain that they represent the true density. It is possible that the interior of the skin may still contain some moisture, and, in order to arrive at absolute certainty, it would be necessary to powder the skin, and ascertain its density in a finely divided condition.

When this is done, the results obtained are very much more concordant than when pieces of skin are used. The following results were obtained for the density of dry hide powder (as used for analysis by the I.A.L.T.C.):—

This is equivalent to a specific volume v₁ = 0·7954.

The density of the powdered skin may also be determined in the air volumenometer (Say; Kopp. Kohlrausch Physical Measurements, p.53).

Swelling and Falling.—The skin, in its living condition on the healthy animal, is the most supple and perfect of coverings, and in producing soft leathers it is the object of the tanner to retain this supple condition. To do this, the swollen fibres must, as we have seen, be got back to their natural state. The phenomena of swelling and depletion of skin have been discussed by KÖrner,51 and recently Prof. H.R. Procter52 has published a paper which goes into the whole theory of colloidal swelling. In order to understand depletion, which is the opposite of swelling, it is necessary to consider what takes place when a body like skin is swollen. I am indebted to Dr. Th. KÖrner for the following summary of the phenomena of swelling. There are three types of swelling: 1. Capillary attraction; 2. Endosmose; 3. Molecular imbibition.

The last named is of the greatest importance in tanning. KÖrner (loc. cit.) enumerates certain principles governing molecular swelling.

1. A body capable of swelling, when put into water, absorbs a definite quantity of the water up to a maximum, which cannot be exceeded. (C. Ludwig.)

2. The maximum of swelling depends upon the chemical composition of the body, on its cohesion and elasticity, and on the temperature and interior pressure of the liquid. (C.Ludwig.)

3. Power of resistance to swelling increases from the exterior to the interior, according to a parabolic law; i.e. the external layers of the body attain the maximum swelling sooner than the internal portions. (L. Mathiessen and A. Schwarz.)

4. The volume of the swollen body is smaller than its original volume, plus that of the liquid absorbed. (Quincke.)

5. Swelling is accompanied by development of heat.53

The production of heat is simply due to the contraction, and not to any chemical phenomenon, such as hydration.54 This explains a fact well known to tanners, viz. that skins swell in cold water and “fall” in warm water. Riecke55 concludes that the degree of swelling, m₂/M (where m₂ = mass of water absorbed, M = mass of the body swollen), in a space filled with aqueous vapour, unsaturated, is a function of the pressure and temperature.

The velocity of swelling (Pascheles) may be expressed by the formula—

dQ/dt = (M – Q) K

where M = maximum of swelling, Q = amount of swelling in the time t, and K = a constant.

The differential quotient dQ/dt gives the velocity for each moment, and it will be seen that the swelling becomes slower and slower as the maximum is approached. Thus, the law of the velocity of swelling is identical with that of the velocity of inversion of cane sugar, itself an application of the law of masses.56

For every process of swelling, the constant K must be determined experimentally from observation of M, and it may be shown that—

K = 1/t log M/M – Q

whence the value of K may be calculated for each series of determinations.

On the manner in which water is held in swollen colloid bodies, three hypotheses have been put forward.57

1. The hypothesis that colloids have a structure in the form of a honeycomb. (BÜtschli.)

2. The water is absorbed at the surface of colloids in a specially condensed form.58

The water forms, with the swollen body, a “solid solution.” (NÄgeli.)

Swelling, and its opposite, contraction, are connected with the surface tension between the swelling or contracting bodies and the surrounding solution. With diminished surface tension, the surfaces of contact between the two become greater, i.e. swelling takes place, with simultaneous diminution of the volume of the whole system, and vice versa.

For the absorption phenomena which occur, the following relation holds good: When a substance in solution diminishes the surface tension at the dividing surface, its concentration is increased; it is absorbed. When a substance in solution increases the surface tension at the dividing surface, its concentration is diminished.

In the system water-hide substance the researches of Wiedemann and LÜdkeing have shown that swelling is accompanied by evolution of heat. Since a rise of temperature is favourable to a system formed by absorption of heat, it therefore hinders swelling and vice versa. This is confirmed by practical experience, and most bating operations are conducted at temperatures between 35° and 40°C.

With regard to the influence of pressure on swelling, a similar law holds good as for temperature, which may be expressed as follows. When a chemical system is compressed at constant temperature, its equilibrium is shifted in that direction by which the reaction is accompanied by diminution of volume. Quincke, however, has shown what appears to be a paradox, namely, that swelling is accompanied by diminution of volume, i.e. the swelling substance, plus the water taken up, occupies a smaller space than the sum of the two constituents taken together; therefore, an increase of pressure must be favourable to swelling, and conversely a diminution of pressure is favourable to “falling.”

We must clearly bear in mind that in all cases of swelling, it is the entry of water into the system which is the cause of the swelling, and since puering is a process which acts in a contrary direction, i.e. the skin falls, this means that water is expelled from the system, skin plus water. By direct determination, from 3 to 8 per cent. of water is expelled during puering. Calculations made from density determinations are masked by the fact, above referred to, that the density of the skin changes during puering.

A fallen skin contains less water than when in the swollen condition, but the difference between the percentages of water in the skin in the two states is only relatively small, and obviously insufficient in itself to account for the great difference in the physical properties of the skin in the two conditions. It is evident, therefore, that the state in which the water is held in the two stages must be different. It is reasonable to suppose that, in the case of swollen skin substance, the whole of the capillary cavities of the organized structure of the skin are completely filled with water, which, owing to its incompressible nature, confers upon the skin elastic and unyielding properties which it always possesses in this condition. The water, entering the organized cell structure in the first place by osmotic pressure, is fixed there and confers upon the skin, in part, its own property of incompressibility. It is a well-recognized fact that, during the puering process, large quantities of the skin substance are dissolved by the action of the enzymes in the bate, and it is probable that the finer organized structure is first attacked. The walls of the cells which hold the water are partly destroyed, leaving the skin with, quite possibly, the same amount of water contained in it as a whole, but with the water dispersed at large throughout its interior structure instead of being held in the fibres. The result of this is, that when a small area of the puered skin is subjected to local pressure the water in the portion undergoing compression is free to move into the adjacent portions of the skin, and there is no force acting upon it, except that of capillarity, to cause it to return to that area when the pressure is removed from it. When a portion of a puered skin is compressed, the surrounding part is always swollen to an equivalent amount. That this view is, in the main, correct is supported by an observation of Abt (Paris), that, while cell nuclei may be demonstrated in the layers near the epidermis of an unpuered skin, they are entirely absent from a puered skin, showing that the puer has completely dissolved the nuclear structure of the cells.

The swelling caused by alkalis is of a different nature to the swelling caused by acids. Procter has pointed out59 that alkaline swelling is not repressed by sodium chloride, even when caused by sodium hydrate, but is repressed by sufficient concentration of the hydroxyl ion in the outer solution.

MÜller60 points out that jellies containing certain non-electrolytes (glucose, glycerine, alcohol) render the diffusion of soluble matter much slower than pure jellies of the same concentration, but the presence of urea favours the permeability of gelatin and agar jellies. The differences in the velocity of diffusion of various salt solutions through jellies of the same strength, are thus not due altogether to the greater diffusibility of the particular salt solution, but are to be attributed to the influence of the salt on the permeability of the colloidal medium. Limed skin treated with a 1 per cent. solution of urea at 43°C. is depleted or “falls,” but both the solution and the skin remain alkaline.

Pribram61 has recently shown that the swelling and contraction of muscle are to be ascribed to absorption and expulsion of water from the cells of the muscle; the rapid changes taking place, are brought about by changes in the concentration and composition of the liquid components of the muscle. Lactic and phosphoric acids are produced with enormous rapidity, and of relatively high concentration; during a short period these cause instantaneous changes in the swelling of the muscle cells, followed by a return to their original state. For the evidence of this the original paper must be consulted, but it goes to show that the equilibrium of the system—colloids, electrolytes, water—depends on the proportions and quantities of each of these constituents. The phenomena we are considering thus conform to the general law of mass action (Guldberg and Waage).

Influence of Solid Matter in the Bate.—It was first pointed out by Wood (J.S.C.I. 1899, p.991) that a filtered puer liquor had less action on the skin than an unfiltered liquor containing much finely suspended matter.

On adding an inert solid (kaolin) to the filtered bate, the action was hastened. No doubt much of the colloidal matter, and with it some of the enzymes, are removed by filtration, but nevertheless, the suspended particles in the bate have some effect on the process.

In a most interesting paper, Perrin62 has recently shown, that the granules in suspension in a colloidal liquid function like the invisible molecules of a perfect gas with a molecular weight of 3·3 x 10^-9. Such granules are endowed with the molecular (Brownian) movement, and therefore may exert a mechanical effect on the fibres of the skin. Although the mass of the particles is large63 compared with molecular dimensions, they are small enough to penetrate the pores of the skin, and where puering is carried too far, may become deposited beneath the fine hyaline layer, and thus render it cloudy and unsuitable for fine colours.

Hydrogen ion Concentration of Puer Liquors.—It has already been shown, in Chapter II., that fresh puer liquors have a certain acidity (7c.c. N/10 per 100c.c.) at the commencement of the operation, but that at the end they are alkaline (3c.c. N/10 per 100c.c.). If an artificially acid liquor be made by diluting hydrochloric acid until it shows the same number of c.c. by titration, it will be found far too “strong,” and will swell the skins. This brings us to the consideration of what is meant by the strength of acids.

Procter and Jones64 have drawn attention to the point in their paper on “Acids in Tan Liquors.” As is well known, the ionic theory affirms that degree of acidity depends on the concentration of hydrogen ions, a strongly acid solution being one in which the hydrion concentration is great, an alkaline solution one in which it is extraordinarily minute, and if we adopt pure water as our standard of neutrality, a neutral solution is one in which the hydrion concentration is approximately 10^-7 normal. A normal solution of hydrogen ions would contain 1 gram of hydrogen ions per litre; in the case of hydrochloric acid this would equal 1·35 N/1 HCl.

Sand and Law,65 and Wood, Sand and Law,66 have described the mode of estimating the hydrogen ion concentration in tan liquors directly by means of the electrometric method, and this method is especially applicable to the estimation of the hydrogen ion concentration in puer liquors. It can also be used to titrate the liquors, and we have already given some of the results in Chapter II.

The method is based on the theory of Nernst, that the difference of potential between a metal plate and the solution of one of its salts into which the metal is dipping, depends on the osmotic pressure of the free ions of that metal in the solution (in other words, on the concentration of the solution). In the case of hydrogen ions, we use a plate of platinum coated with platinum black and saturated with hydrogen, and the difference of potential depends, therefore, on the concentration of the hydrogen ions in solution.

The accompanying figure (Fig.9) shows the hydrogen electrode I, and the auxiliary electrode II, drawn to scale,67 on the right, whereas the electrical apparatus is explained diagrammatically on the left.

The principle of the method of measurement consists in connecting the two ends P and Q, of a sliding rheostat to the terminals of a dry cell, D, and balancing the potential-difference to be measured against the potential-difference between one end, P, and the slider, S, by means of a special form of enclosed capillary electrometer, E. The value of this potential-difference is read directly on a delicate voltmeter, V. The connexions, which are found ready-made in the box, have been drawn out, whereas, those to be made by the operator are shown by dotted lines. The steps to be taken by the latter, consist first in taking off the capillary electrometer and manipulating it in such a manner, that on returning it into position the capillary may be partly filled with a thread of mercury and partly with the acid. The terminals, X and Y, marked battery + and -, are connected to a dry cell, and the terminals, Z and U, marked cathode and auxiliary respectively, to the hydrogen and calomel electrode. Very careful insulation of the connexion between the terminal marked auxiliary and the calomel electrode is necessary. The hydrogen is passed through the hydrogen electrode until a constant P.D. between it and the calomel electrode is obtained. This P.D. is measured by moving the slider up and down until no movement of the mercury in the capillary electrometer is observed on depressing the key K marked electrometer.

Fig.10 is a view of the apparatus as set up for the titration of a puer liquor. H is a cylinder of compressed hydrogen;68 I, the hydrogen electrode, dipping into the beaker C containing the liquor for titration; b, the burette, containing N/10 acid or alkali; II, the auxiliary electrode, the capillary of which is also seen dipping into the beaker C; P, the potentiometer box containing the sliding rheostat S and electrometer E; D, a dry battery. The acid or alkali is run in from the burette until the voltmeter shows 0·69 volts, indicating that the liquid is neutral or has a hydrogen ion concentration of 10^-7.

The following table shows some results obtained on puer liquors before and after skins have been put through. p+H has been called by Sorensen the exponent of the hydrogen ion concentration C, and is defined by the equation

p+H = log 1/C

i.e. it is equivalent to the logarithm of the reciprocal of the factor of normality of the solution with respect to the hydrogen ions.69 p+H for pure water or a neutral solution is 7, corresponding to 0·69 volts. The measurements were made on filtered puer liquors, using a N/1 KCl calomel electrode as auxiliary, the capillary of the electrode being filled with 3·5N potassium chloride:—

No. 6 was a very old “spent” liquor. The mean hydrogen ion concentration before goods was 0·588 volt, i.e. the concentration was 10^-5·32 normal equivalent to 0·00000479grm. per litre of hydrogen ions. Therefore the value of p+H was 5·32. A solution of hydrochloric acid of the same strength by titration consumed 0·7c.c. N/1 alkali per 100c.c. Measured by the electrometric apparatus, it showed 0·410 volt, corresponding to p+H = 2·1, or a hydrion concentration of ·0079N. In other words, the HCl solution has an acidity or strength 1600 times that of the puer liquor.

The mean hydrogen ion concentration of the liquors after goods was 0·000000076grm. per litre, corresponding to 0·715 volt and p+H = 7·12, i.e. the liquor was alkaline to a slight extent. For comparison saturated lime-water gave a reading of 1·01 volt, corresponding to p+H = 12·5.

The hydrogen ion concentration is of the greatest importance for the proper action of the enzymes in the bate;70 we shall, however, treat of this in Chapter VII.

Conductivity of Puer Liquors.—It was thought of interest to examine the electrical conductivity of puer liquors in actual use, in the hope that the numbers obtained might give some useful indications. It was found that the conductivity increased, as might be expected from the lime going into solution, but the difficulties of the method render it of less use than ordinary chemical analysis. The results of a typical liquor are given here as a record—

Conductivity (K) of liquor before goods

0·00316 1/ohm × cm.

Conductivity (K) of liquor after goods

0·00423 1/ohm × cm.

The difficulty of expressing the complex reactions of puering numerically is, we have seen, very great, for, as Minot71 says, “with human minds constituted as they actually are, we cannot anticipate that there will ever be a mathematical expression for any organ or even a simple cell, although formulÆ will continue to be useful for dealing now and then with isolated details. Nevertheless, the value of graphic methods to every student of science has been immense.”

It has long been my endeavour to express quantitatively the degree to which a skin has fallen. My friend Dr. Sand has suggested that this may be done by subjecting a piece of the skin successively to increasing and then decreasing pressures, and measuring the thickness under each load. Experiments carried out with the apparatus described below show that a limed skin treated in this way is first compressed, and then on releasing the pressure recovers more or less of its former thickness, according to the amount of plumping it has received, i.e. it shows a certain amount of resilience. A well-puered sheep-skin, on the other hand, shows no resilience at all, i.e. on releasing the pressure the whole of the compression persists. In the case of an ox-hide subjected to a bate of hen-dung, a slight recovery takes place on releasing the pressure. This accords with the fact that it will never be possible to puer a thick ox-hide so effectively as a thin sheep-skin. A piece of india-rubber, on the other hand, is completely resilient, i.e. it wholly recovers its thickness on releasing the pressure. The relative thickness of the same skin in the limed and puered conditions under varying loads is also of interest. The process of puering may, as a rule, be taken to reduce a limed skin to between two-thirds and one-half of its thickness in the swollen condition. If both limed and puered skin be then subjected to the same load, the puered skin will at first be compressed very much more than the limed one. This is probably due to the expulsion from it of water, held simply by capillary attraction. On further increasing the load, however, the compression decreases greatly in the case of the puered skin; with both limed and puered skin increase of compression ultimately becomes practically proportional to increase of pressure, and is slightly greater with the former than with the latter.

The table gives representative results obtained on the same sheep-skin (roan) in the limed and in the puered condition. These results are expressed graphically in Fig.12.

? = difference in thickness of the skin—i.e. compression under the same load.

Fig.11 shows the apparatus72 that was employed to obtain these results. It consists essentially of a commercial form of micrometer for measuring the thickness of leather. To one of its jaws a pan for weights is attached, by means of the frame f f, in such a manner as to secure a perfectly straight pull. The weights of the frame and pan are counterbalanced in the manner shown by a counterpoise b. The delicacy of measurement may be increased by inserting larger jaws in the form of suitably fashioned disks, but even when this is done the results are to a certain extent vitiated by the rather considerable friction of the micrometer.

An apparatus free from this fault is shown in Fig. 13. It consists essentially of a counterbalanced lever A, to which the upper jaw J is rigidly attached. By means of a sliding weight W, any desired load, from zero upwards, may be put on this jaw. The lever carries a very delicate spirit level, which allows it to be set accurately horizontal in every experiment. The lower jaw is movable vertically between parallel guides, and its position is controlled by the screw-wheel S which bears a divided circle on its circumference. The position of this wheel, and therefore of the lower jaw, may be accurately read on the vernier v. In every experiment it is adjusted so as to make the upper lever accurately horizontal.

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