KATES OF DIFFUSION. 223 



but if the second solution contains some of the salt, like that in 

 the first solution, the rate of diffusion is retarded. 



605. The rate with which a salt passes from a stronger into 

 a more dilute solution is nearly proportional to the degree of 

 concentration. The approximate times required for the diffu- 

 sion of equal weights of various substances into water are given 

 in the following table : 



Hydrochloric acid 1. 



Sodic chloride 2.33 



Magnesic sulphate 7. 



Cane-sugar 7. 



Albumin 49. 



Caramel 98. 



606. Of the colloids, Graham says: 1 "Low diffusibility is 

 not the only propert}' which the bodies last enumerated possess 

 in common. . . . Although often largely soluble in water, they 



1 Philosophical Transactions, 1861. 



Graham says further : "Although chemically inert in the ordinary sense, 

 colloids possess a compensating activity of their own arising out of their 

 physical properties. While the rigidity of the crystalline structure shuts out 

 external impressions, the softness of the gelatinous colloid partakes of fluidity, 

 and enables the colloid to become a medium for liquid diffusion, like water 

 itself. The same penetrability appears to take the form of cementation in such 

 colloids as can exist at a high temperature. Hence a wide sensibility on the 

 part of colloids to external agents. Another and eminently characteristic qual- 

 ity of colloids is their mutability. Their existence is a continued metastasis. 

 A colloid may be compared in this respect to water while existing liquid at a 

 temperature under its usual freezing point, or to a supersaturated saline solu- 

 tion. Fluid colloids appear to have always a pectous modification (TTTJKTOS, 

 curdled), as fibrin, casein, albumin. But certain liquid colloid substances are 

 capable of forming a jelly, and yet still remain liquefiable by heat and soluble 

 in water. Such is gelatin itself, which is not pectous in the condition of ani- 

 mal jelly, but may be so as it exists in the gelatiferous tissues. Colloids 

 often pass under the slightest influences from the first into the second condi- 

 tion. The solution of hydrated silicic acid, for instance, is easily obtained in 

 a state of purity, but it cannot be preserved. It may remain fluid for days or 

 weeks in a sealed tube, but is sure to gelatinize and become insoluble at last. 

 Xor does the change of this colloid appear to stop at that point. For the 

 mineral forms of silicic acid, deposited from water, such as flint, are often 

 found to have passed during the geological ages of their existence, from the 

 vitreous or colloidal into the crystalline condition (H. Eose). The colloidal 

 is, in fact, a dynamical state of matter ; the crystalloidal being the statical 

 condition. The colloid possesses energia. It may be looked upon as the 

 probable primary source of the force appearing in the phenomena of vitality. 

 To the gradual manner in which colloidal changes take place (for they always 

 demand time as an element), may the characteristic protraction of chemico- 

 organic changes also be referred." 



