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.):-- In petroleum ether 1·2568 In carbon tetrachloride 1·2570 In alcohol 1·2580 Mean 1·2572 This is equivalent to a specific volume _v__{1} = 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. Procter[52] 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. [51] Th. Körner, 10 Jahres. Bericht, d. Deutsch. Gerberschule zu Freiberg, 1899, p. 32. [52] Procter, Ueber die Einwirkung Verdünnter Sauren und Salzlösungen auf Gelatine, Kolloid Chem. Beihefte, 1911, ii. p. 243. 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.)