OF ARTS AND SCIENCES : MAY 24, 1870. 237 



He also swept away the arbitrary distinctions made by previous experi- 

 menters, showed that this wbole class of phenomena are essentially 

 similar, and called this manifestation of power simply " osmose." 



While studying osmotic action, Graham was led to one of his 

 most important generalizations, — the recognition of the crystalline 

 and amorphous states as fundamental distinctions in chemistry. 

 Bodies in the first state he called crystalloids ; those in the last 

 state, colloids (resembling glue). That there is a difference in struct- 

 ure between crystalloids, like sugar or felspar, and colloids, like 

 barley candy or glass, has of course always been evident to the most 

 superficial observer ; but Graham was the first to recognize in these 

 external differences two fundamentally distinct conditions of matter not 

 peculiar to certain substances, but underlying all chemical differences, 

 and appearing to a greater or less degree in every substance. He 

 showed that the power of diffusion through liquids depends very much 

 on these fundamental differences of condition, — sugar, one of the least 

 diffusible of the crystalloids, diffusing fourteen times more rapidly than 

 caromel, the corresponding colloid. He also showed that, in accord- 

 ance with the general chemical rule, while colloids readily combine 

 with crystalloids, bodies in the same condition manifest little or no 

 tendency to chemical union. Hence in osmose, where the membranes 

 employed are invariably colloidal, the osmotic action is confined almost 

 entirely to crystalloids, since they alone are capable of entering into 

 that combination with the material of the septum on which the whole 

 action depends. 



On the above principles Graham based a simple method of sepa- 

 rating crystalloids from colloids, which he calls " dialysis," and which 

 was a most valuable addition to the means of chemical analysis. A 

 shallow tray, prepared by stretching parchment paper (an insoluble 

 colloid) over a gutta-percha hoop, is the only apparatus required. 

 The solution to be " dialyzed " is poured into this tray, which is then 

 floated on pure water, whose volume should be eight or ten times 

 greater than that of the solution. Under these conditions the crystal- 

 loids will diffuse through the porous septum into the water, leaving the 

 colloids on the tray, and in the course of a few days a more or less 

 complete separation of the two classes of bodies will have taken place. 

 In this way arsenious acid and similar crystalloids may be separated 

 from the colloidal materials with which, in the case of poisoning, they 

 are usually found mixed in the animal juices or tissues. 



