294 PHYSICAL PROPERTIES 



that the velocity of swelling is at a maximum at the instant of 

 immersion and therefore decreases with time. This corresponds 

 to the experimental facts. 



As we shall see in the next chapter, the proteins exert a small, 

 but definite, osmotic pressure. They are at the same time not 

 diffusible through colloids, or only very slightly so. Any crystal- 

 loids which may be present in the external fluid which bathes 

 the gelatin can penetrate the gelatin albeit, possibly, more slowly 

 than the water. The gelatin plate, therefore, acts like an osmom- 

 eter which provides its own membrane which is permeable to 

 crystalloids and not to colloids. Hence osmotic forces must 

 play a part in the taking up of water by protein jellies. A phe- 

 nomenon in the domain of crystalloids which presents some 

 analogies to this aspect of the swelling of colloids is the following:* 

 If we place at the bottom of a column of distilled water a layer 

 of phenol and introduce below this a layer of a saturated solution 

 of KCl in water and now allow the system to stand at constant 

 temperature the layer of phenol gradually moves up the column 

 of water; in other words the layer of solution below the phenol 

 "swells." The solvent, water, being soluble in phenol, the phenol 

 is permeable to it, while the KCl being insoluble in phenol, cannot 

 pass through the layer of phenol. 



Not only osmotic, but also chemical phenomena must, however, 

 play a part in the swelling of protein jellies. As we have seen in 

 Chaps. VI and XI the passage of a protein into solution involves 

 the addition of the elements of water to terminal — NH2 and 

 -COOH groups and also, possibly, to internal -N.HOC- 

 groups, resulting in the depolymerization of the protein. Not 

 only osmotic phenomena but hydration of the gelatin must there- 

 fore occur in the process of swelling. In evidence of the correctness 

 of this view Pauli points to the fact that, according to Weidemann 

 and Liideking Goc. cit.), the swelling of gelatin is accompanied 

 by a disengagement of heat, while the solution of gelatin is ac- 

 companied by an absorption of heat. Evidently the processes of 

 solution and swelling are each composed of two factors, one 

 leading to a disengagement and the other to the absorption of 

 heat. The former process is, Pauli believes, the chemical binding 

 of water by the protein, the latter the passage of the hydrated 



* To which my attention was drawn, in this connection, by my colleague 

 Dr. F. G. Cottrell. 



