206 CHEMISTRY OF PLANT LIFE 



(probably) of casein, etc., are suspensoids; while sols of egg- 

 albumin, of oils, etc., are emulsoids. The classification of these 

 substances into suspensoids and emulsoids is, however, more a 

 matter of convenience than of real difference in composition, since 

 it is practically impossible to say whether many of the organic 

 substances which normally exist in colloidal form are themselves 

 liquids or solids, when in the non-dispersed form. 



CONDITIONS NECESSARY TO THE FORMATION OF SOLS 



Suspensoids differ from mechanical suspension of solids in a 

 liquid in that in the latter the solid particles settle toward the 

 bottom of the mixture, because of the effect of the attraction of 

 gravity upon them. The rate at which such particles settle 

 depends upon the size and density of the particle and the vis- 

 cosity of the liquid, and can be roughly calculated from the formula 

 for Stokes' law for the rate of falling of a spherical body in a liquid. 

 This formula is 



V= velocity of the falling body, in millimeters per second; 



r = radius of the particle, in millimeters; 



s = specific gravity of the solid; 



s' = specific gravity of the liquid; 



g =the attraction of gravity, in dynes; 



n =the viscosity of the liquid. 



For example, if this formula be applied to determine the rate at 

 which the particles of gold of the size of those in a red gold sol 

 would settle, if they were in mechanical suspension in water 

 (r=10MM, or one-ten-thousandth of a millimeter; s=19.3; s' = l; 

 = 980, and n = 0.01), it will be found that such particles will 

 settle at the rate of approximately 0.0146 millimeter per hour, or 

 a little over 10 mm. (0.4 inch) per month. Hence, the settling of 

 such particles, if in mechanical suspension, would be measurable, 

 although very slow. Shaking up the suspension would cause the 

 particles to rise through the liquid again. But in a gold sol, or 

 suspensoid, which contains particles of gold of the size used in this 

 calculation, the gold particles do not settle, even at the slow rate as 

 calculated above. They remain uniformly distributed through- 



