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.