16 THE DATA OF BIOLOGY. 



instances as " hydrated silicic acid, liydrated alumina, and 

 other metallic peroxides of the aluminous class, when they exist 

 in the soluble form ; with starch, dextrine and the gums, cara- 

 mel, tannin, albumen, gelatine, vegetable and animal extractive 

 matters." Describing the properties of colloids. Professor 

 Graham says : — " Although often largely soluble in water, 

 they are held in solution by a most feeble force. They ap- 

 pear singularly inert in the capacity of acids and bases, and 

 in all the ordinary chemical relations." ^ "^ ^ "Al- 

 though 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 crystal- 

 line structure shuts out external impressions, the softness of 

 the gelatinous colloid partakes of fluidity, and enables the 

 colloid to be^me a medium of liquid diffusion, like water 

 itself." ^ ^ ^ " Hence a wide sensibility on the part of 

 colloids to external agents. Another and eminently charac- 

 teristic quality of colloids is their mutability." ^ ^ ^ " The 

 solution of hydrated silicic acid, for instance, is easily obtain- 

 ed 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. Nor 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 crystal- 

 line condition (H. Rose). The colloid is, in fact, a dynami- 

 cal state of matter, the crystalloidal being the statical 

 condition. The colloid possesses energia. It may be looked 

 upon as the 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." 



The class of colloids includes not only all those most com- 

 plex nitrogeneous compounds characteristic of organic tissue, 



