no COLLOIDS. PARTI. 



incumbent external water more freely than the sulphate 

 of alumina can do. 



Common salt diffuses itself in a solid mass of jelly 

 almost as easily and extensively as in the same bulk of 

 free water. Thus colloid bodies do not interfere with the 

 diffusion of crystalloids such as salts, but they almost 

 entirely arrest the diffusion of one another. Solutions 

 of salts, sugars, and other crystalloids pass freely 

 through colloid substances, such as parchment-paper, 

 vellum, and membrane into water, although they 

 have no pores, because the particles of the crystals unite 

 diffusively with the water combined in these substances, 

 which solutions of gum, caramel, and .other colloids 

 cannot do. These colloid substances are permeable 

 to solutions of crystalloids, impermeable to solutions 

 of colloids. This constitutes Dialysis. 



The instrument used by Mr. Graham was a little 

 tray formed of vellum or membrane stretched tightly 

 over a hoop of gutta-percha and capable of holding a 

 liquid and floating on water. When a mixed solution 

 of equal parts of salt and gum is put into the tray, 

 after a time all the salt will have passed into the water 

 below, leaving nothing in the tray but an aqueous 

 solution of gum. 



The following is one of the most extraordinary re- 

 sults of dialysis. Mr. Graham took a silicate of soda, a 

 soluble crystalline salt formed by fusing quartz with 

 carbonate of soda at a red heat, which diffuses readily. 

 He acidulated the aqueous solution of the salt with 

 hydrochloric acid, which changes the constituent silica 

 from being a crystalloid substance into a colloid form. 

 When the liquid was poured into the tray floating on 

 water, after, four days, the whole of the acid and the 

 chloride of sodium had been diffused in the water and 

 nothing remained in the tray but an aqueous solution 

 of quartz. There remained in fact, a solution of sand 



