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 _p__{0} 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. [47] M. Planck, Zeit. f. Phys. Chem. xlii. p. 584 (1903). *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. [48] Sull’ appliccazione della bilancia idrostatica per il controllo della concia delle pelli, Milan, 1903. 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-- δ = _m_/_w_. [49] _See_ Kohlrausch, An Introduction to Physical Measurements, 1894, p. 43. _See also_, Weighing Hides in Water, C. E. Parker and G. H. Russell, Tanners’ Year-Book 1905, p. 45. 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.) [Illustration: Fig. 8.--Volumenometer for Raw Skin.]