ORGANIC MATTER. 19 



which have a general correspondence with differences in the 

 massiveness of their molecules. 



But the fact of chief interest to us here, is that the rela- 

 tively small-moleculed crystalloids have immensely greater 

 diffusive power than the relatively large-moleculed colloids. 

 Among the crystalloids themselves there are marked differ- 

 ences of diffusibility; and among the colloids themselves 

 there are parallel differences, though less marked ones. But 

 these differences are small compared with that between the 

 diffusibility of the crystalloids as a class, and the diffusibility 

 of the collloids as a class. Hydrochloric acid is seven times 

 as diffusible as sulphate of magnesia; but it is fifty times 

 as diffusible as albumen, and a hundred times as diffusible as 

 caramel. 



These differences of diffusibility manifest themselves with 

 nearly equal distinctness, when a permeable septum is placed 

 between the solution and the water. The result is that 

 when a solution contains substances of different diffusibilities, 

 the process of dialysis, as Professor Graham calls it, becomes 

 a means of separating the mixed substances : especially when 

 such mixed substances are partly crystalloids and partly col- 

 loids. The bearing of this fact on the interpretation of 

 organic processes will be obvious. Still more obvious 



will its bearing be, on joining with it the remarkable fact 

 that while crystalloids can diffuse themselves through colloids 

 nearly as rapidly as through water, colloids can scarcely 

 diffuse themselves at all through other colloids. From a 

 mass of jelly containing salt, into an adjoining mass of jelly 

 containing no salt, the salt spread more in eight days than it 

 spread through water in seven days; while the spread of 

 " caramel through the jelly appeared scarcely to have begun 

 after eight days had elapsed." So that we must regard the 

 colloidal compounds of which organisms are built, as having, 

 by their physical nature, the ability to separate colloids from 

 crystalloids, and to let the crystalloids pass through them 

 with scarcely any resistance. 



