COLLOIDS AN D LIFE 69 



The disperse phase of a sol then has enormous sur- 

 face, which gives rise to the various sorption phenom- 

 ena. Some of the properties of colloids vary 

 according to the degree of dispersion. Thus, col- 

 loidal gold changes color with the size of the par- 

 ticle — it may show red or blue or purple. 



Colloidal solutions are distinguished from true 

 solutions in that the disperse phase of colloids is 

 heterogeneous, giving the Tyndall (optical) effect, 

 instead of homogeneous as in true solutions. A 

 true solution is a molecular solution, whereas the 

 particles in a sol are many times larger than mole- 

 cules. The dispersed particles of a sol show lively 

 movement, the Brownian movement, which is the 

 more violent the smaller the particles. This move- 

 ment is independent of external conditions, and per- 

 sists for months or years — as long as the dispersion 

 medium permits. The movements are due to col- 

 lisions of the molecules of the medium with the 

 particles, and thus the particles, being knocked 

 about, do not settle down, but with reference to 

 gravity rather behave similarly to the molecules of 

 the gases of the air. It is the thermal agitation of 

 the molecules of the medium that causes the molec- 

 ular bombardment of the particles of the disperse 

 phase, the Brownian movement. 



The laws governing the displacements of these 



