ORGANIC MATTER. 19 



under a pressure of forty atmospheres, and ordinarily exists 

 as a liquid, only in combination with water. Again, " hydrate 

 of potash may be said to possess double the velocity of diffu- 

 sion of sulphate of potash, and sulphate of potash again double 

 the velocity of sugar, alcohol, and sulphate of magnesia," 

 differences which have a general correspondence with differ- 

 ences in the massiveness of the atoms. 



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

 tively small-atomed crystalloids have immensely greater 

 diffusive power than the relatively large-atomed 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 colloids as a class. Hydro- chloric 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. And 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 organic processes will be 

 obvious. Still more obvious will its bearing: be, on 



O ' 



joining it with the remarkable fact, that while crystalloids 

 can diffuse themselves through colloids nearly as rapidly as 

 through water, colloids can scarcelv diffuse themselves at all 



* ' v 



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 

 i i seven days ; while the spread of " caramel through the 

 jelly appeared scarcely to have begun after eight days had 



