SWELLING 295 



protein into solution (or, in swelling, the osmotic inhibition of 

 water). In swelling the chemical heat-effect predominates; in 

 the dissolving of the gelatin, the heat-effect of solution. 



That a part, at least, of the water in swollen gelatin is chemi- 

 cally bound by the protein is shown by the following experiment 

 (74). If two gelatin plates be brought to the same degree of 

 swelling, the one in neutral and the other in acid water, and if 

 they be then immersed in absolute alcohol, the alcohol will ab- 

 stract all of the water from the plate which has been swollen in 

 neutral water, but not from the plate which has been swollen 

 in acid water. Now, as we have seen in Chap. V, the fact that 

 a constituent of a system can be completely removed from it by 

 washing out with an appropriate solvent affords no valid proof 

 that the constituent in question did not exist, within the system, 

 in a state of chemical combination. But if, on the contrary, it 

 should prove impossible or exceptionally difficult to remove the 

 constituent by washing with a fluid in which it is very soluble, 

 then the prima facie evidence that it exists in the system in a 

 condition of chemical union is very strong. We may conclude, 

 therefore, that when gelatin is swollen in acid water a part of 

 the water which is taken up by the gelatin is chemically bound 

 by it; and we have no valid reason for supposing that the water 

 taken up from neutral solutions is not similarly bound although 

 less firmly. 



The equilibria attained in. the swelling of gelatin in solutions 

 of acids have recently been very thoroughly investigated by 

 Procter. This investigator has found that gelatin absorbs both 

 acid and water from acid solutions, but absorbs the acid in excess, 

 so that the proportion of acid in the surrounding fluid diminishes. 

 If the initial concentration of acid in the external fluid lies be- 

 tween 0.01 and 0.25 N, then assuming that at the end of the process 

 (attainment of maximal swelling) the concentration of free acid 

 is the same within and without the jelly, the amount of acid 

 which is "bound" by the gelatin is 0.7 to 0.8 X 10~ 3 (= 70 to 

 80 X 10~ 5 ) equivalents per gram. The equivalence, at the 

 attainment of maximal swelling, is the same for all strong acids 

 but falls below this value for weak acids. While the proportion 

 of acid which is "bound" by the gelatin varies but slightly with 

 the concentration of the acid in the surrounding fluid, this is not 

 true of the degree of swelling attained, which in strongly acid 



