284 COLLOIDS 



ing the ordinarily insoluble silicic acid in colloidal solution ; 

 this solution, which he termed a hydrosol, was stable at first, 

 but after a few days it deposited silicic acid ; the precipitated 

 form of the colloid he termed a hydrogel, and since both dis- 

 solved and precipitated forms of colloid exist for many differ- 

 ent solvents, the terms hydrosol, hydrogel, alcosol, alcogel, 

 etc., were used to indicate the nature of the solvent. 



The great difference in diffusibility between crystalloids 

 and colloids is one of the most striking differences between 

 these two classes of substances ; it must not, however, be 

 concluded that colloids are absolutely indiffusible, but rather 

 that their rate of diffusion is very much slower than that of 

 crystalloids. 



Moreover, although a substance may, under given condi- 

 tions, produce a colloidal solution, it does not follow that it 

 will do so under all conditions ; thus, a solution of soap in 

 water is colloidal whilst a solution in alcohol is not ; again, 

 many substances, such as haemoglobin, egg albumen, etc., which 

 were formerly regarded as typical colloids, have been obtained 

 in a crystalline form, while, on the other hand, most crystal- 

 loids can be made to give colloidal solutions ; for instance, 

 sodium chloride, if generated by some reaction taking place in 

 benzene, gives a colloidal solution in that 'solvent. 



Appended are some of the more important general charac- 

 teristics of colloids. 



GENERAL PROPERTIES OF COLLOIDS. 

 A. Diffusibility. 



Graham considered that colloids were either incapable cf 

 diffusing through parchment or else possessed the power to a 

 very limited degree ; it would, however, appear that the dif- 

 fusibility of colloids depends to some extent on the membrane, 

 for a colloidal solution of congo red will diffuse through some 

 varieties of parchment but not through others. In any case, 

 however, the rate of diffusion is much slower than that of a 

 crystalloid would be. 



Osmotic Pressure. Experimental determinations have 

 shown that substances in colloidal solution produce a very 

 small lowering of the freezing point or elevation of the boiling 



