COLLOID CHEMI«TkY. 210 



wliicli concentration the swelling reaches a maximum, falling gradually 

 to below twenty times at 04 grm.-mol. per litre, much beyond which the 

 experiment cannot be carried because of solution of the jelly in the acid. 

 Similar effects occur with all other acids which the writer has investigated 

 in which a sufficient concentration can be employed without causing solu- 

 tion. The concentration of the ac d in the swollen jelly is always greater 

 than in the external solution, increasing (in the case of gelatine at least) 

 somewhat rapidly at first, before the maximum of swelling is reached, and 

 later remaining nearly constant at 0002 or t<-003 mol. per litre above 

 the outside solution ; while the total quantity of absorbed acid per grm. 

 of gelatine increases by a law very approximately exponential. The 

 whole of the hydrochloric acid can be estimated by titration with caustic 

 alkali in presence of phenolphthalein, but only a portion with methyl 

 orange, showing that one part of the acid exists in a less ionised state 

 than the remainder. The swollen jelly can no longer be dehydrated by 

 absolute alcohol, showing that the osmotic pressure of water into the jelly 

 is now greater than that into the alcohol, but a part of the acid, approxi- 

 mately corresponding to that estimated by methyl orange, can be removed 

 by repeated treatment with alcohol. The jelly is still pervious to acid and 

 salt solutions, though probably to a less extent than neutral jelly, and it 

 is obvious that one portion of the acid exists in some sort of combination 

 with the jelly, while another is merely osmotically absorbed. It is clear 

 that no definite conclusions can be drawn until in some way these portions 

 can be distinguished ; and as a first approximation (obviously not quite 

 correct) it was assumed that the acid absorbed in swelling was of the same 

 concentration as that of the outer solution, the less ionised and more con- 

 stant portion, which may be called 'fixed acid,' being ascertained by sub- 

 tracting the calculated value of the acid in the absorbed liquid from the 

 total found in the jelly. The result showed that the ' fixed acid ' rapidly 

 reached a maximum coincident with the concentration of maximum 

 swelling, after which it remained approximately constant, a slight down- 

 ward curve being probably due to the inaccuracy of the assumption that 

 the strength of the absorbed solution was equal to that outside, or, what 

 amounts to the same thing, that the whole of the water in the jelly was 

 present as absorbed outer solution. With a small correction in this sense 

 the amount of ' fixed acid ' obtained in this way closely corresponds to 

 that not estimated by phenolphthalein, and incapable of being washed 

 out by alcohol. From inspection of the curves it is obvious that that 

 of total acid per grm, of gelatine is of an exponential type, but it is com- 

 pounded of that of ' fixed acid ' the character of which strongly suggests 

 chemical combination somewhat complicated by dissociation or hydrolysis ; 

 and of the curve of swelling, which after its maximum is reached, is 

 of a hyperbolic character. If, as is probable from the empirical formula, 

 the molecular weight of gelatine is about ISOO, or its multiple, it 

 would roughly correspond to the fixation of two mols. of HCl by one of 

 gelatine. 



The action of salts on unacidified gelatine has been mentioned, and 

 common salt causes a small increase of swelling as compared with water, 

 and even saturated solutions have no dehydrating eflFect. If, however, 

 salt is added to jelly swollen with hydrochloric acid, swelling is imme- 

 diately reduced, and very powerful dehydration comparable with that of 

 absolute alcohol is obtained by saturated salt solutions in presence of the 

 amount of acid necessary to the maximum swelling effect. At the same 



