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 pressure[47] 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 330 lb. per sq. in.; and since a saturated lime solution is about 1/20 normal, its osmotic pressure is about 1·1 atmospheres, or 16 lb. 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/δ,
and therefore
δ = 1/v.
Carini[48] 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.