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 



joining it with tjie 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 



