EVOLUTION OF COLLOIDS 131 



is preserved by the crystals growing larger 

 and increasing in number. In a solution of 

 a colloid as a rule no saturation point is 

 reached, the solution becomes thicker and 

 thicker, and finally forms a viscid gum 

 especially at lower temperatures, and the 

 gum holds tenaciously to the last portions 

 of the solvent. As typical examples there 

 may be mentioned aqueous solutions of com- 

 mon salt (a crystalloid) and of ordinary gum 

 arabic (a colloid). Having defined this pro- 

 perty of crystallization as something properly 

 typical of crystalloids, which caused Graham 

 to give them their name which distinguishes 

 them from colloids, let us now briefly take 

 up the subject of molecular combinations, 

 molecular affinities and molecular valencies, 

 before passing on to the consideration of the 

 other main properties of the colloids. 



As a typical example of molecular com- 

 pounds amongst crystalloids, there may be 

 taken three crystalline substances, closely 

 related chemically, viz., sodium chloride 

 (common salt), sodium bromide, and sodium 

 iodide. When their respective solutions in 

 water concentrate, each substance, at a 

 definite concentration in each case, separates 

 out in crystalline form from the water. 

 Now each of these salts is completely saturated 



