6o8 SCIENCE PROGRESS 



have for water is the cause of diffusion — that the diffusing sub- 

 stance is pushed out from the solution by the entry of water. 

 One of the few passages in Graham's account of his experiments 

 in which he departs from his practice of merely recording results 

 and makes it clear that his mind was at work endeavouring to 

 unravel the process of diffusion has a bearing on this point. 

 After dwelling on the retardation of the diffusion of chloride of 

 copper by the presence of a large excess of hydrogen chloride 

 in solution and on the similar retardation of the diffusion of 

 common salt by hydrogen chloride, he remarks : 



" These remarks are interesting in a very different point of 

 view. I have always watched for the appearance of some 

 absorbent or imbibing power on the part of the acids more 

 analogous to an endosmotic attraction for water as usually con- 

 ceived. If such an attraction existed, it would complicate the 

 phenomena of diffusion, for the volume of water absorbed by 

 the acid would displace and project a portion of the latter into 

 the reservoir, the phial not being extensible. The high diffusi- 

 bility of hydrochloric and nitric acids would be thus explained. 

 But by such a mechanical displacement the chloride of sodium 

 would be thrown out in the preceding experiment, as well as 

 the hydrochloric acid, which is not the case." 



It would seem probable therefore that diffusion is not a 

 simple displacement effect but a process of direct interchange of 

 water and other molecules and the outcome of some attractive 

 influence reciprocally exerted by the dissolved molecules of the 

 substance and those of the surrounding water— not a mere 

 kinetic phenomenon but one that has to be considered also, if 

 not solely, from the point of view of the chemist. 



The fact that very soluble substances which are highly 

 attractive of water, such as the inorganic acids and bases, are 

 the most diffusive is in accordance with this view. 



It is well known that very finely divided particles suspended 

 in a liquid exhibit so-called Brownian movements. The character 

 of the movement, it has been said, irresistibly impresses the 

 observer with the idea that the particles are hurled hither and 

 thither by the action of forces resident in the solution and that 

 these can only arise from the continuous movement of the 

 invisible molecules of which the fluid is composed. The invisible 

 molecules of a divided substance must equally be thought of as 

 similarly exposed to forces resident within the solution — but it 

 may be questioned whether, in the case both of the Brownian 



