16 THE DATA OF BIOLOGY. 



* * * &quot; 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 soft 

 ness of the gelatinous colloid partakes of fluidity, and enables 

 the colloid to become a medium of liquid diffusion, like water 

 itself.&quot; * * * &quot; Hence a wide sensibility on the part of 

 colloids to external agents. Another and eminently charac 

 teristic quality of colloids is their mutability/ * * * &quot; The 

 solution of hydrated silicic acid, for instance, is easily ob 

 tained in a state of purity, but it cannot be preserved. It may 

 rcrnain v 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 crys 

 talline condition (H. Rose). The colloid is, in fact, a dyna 

 mical 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.&quot; 



The class of colloids includes not only all those most com 

 plex nitrogenous compounds characteristic of organic tissues, 

 and sundry of the carbo-hydrates found along with them; 

 but, significantly enough, it includes several of those sub 

 stances classed as inorganic, which enter into organized 

 structures. Thus silica, which is a component of many 

 plants, and constitutes the spicules of sponges as well as the 

 shells of many foraminifera and infusoria, has a colloid, as 

 well as a crystalloid, condition. A solution of hydrated silicic 

 acid passes in the course of a few days into a solid jelly that 



