COLLOIDS. 243 



serums would receive some elucidation. Not much 

 advancement can be made, however, until the prop- 

 erties of colloids are more thoroughly understood. 

 Substances which go into solution were classi- 

 fied by the English physicist, Graham, as crystal- 

 loids and colloids. Crystalloids include many in- 

 organic salts. Usually they form clear solutions 

 in water and exert osmotic pressure, supposedly 

 because of the small size of their molecules. They 

 diffuse with some rapidity and many are conduc- 

 tors of electricity. Organic colloids comprise such 

 substances as albumin, starch, dextrin, tannin, 

 gelatin and many gums. By proper treatment of 

 certain metals and their salts, inorganic colloids properties of 

 may be prepared ; for example, ferric hydroxid and 

 the sulphids of antimony and arsenic. When col- 

 loids are dissolved in water the solutions are often 

 more or less opaque, and are sometimes opalescent 

 because the particles or molecules are of such size 

 that they polarize light. They exist in water either 

 as a solution of molecules of great size or as a sus- 

 pension of considerable particles or aggregates of 

 molecules. In some instances the particles are so 

 large that they may be seen by a magnification of 

 1.000 diameters, while in others no degree of mag- 

 nification renders them visible with the ordinary 

 microscope. By the use of the recently devised 

 ultramicroscope, however, the finest particles in 

 some colloidal solutions may be discerned. Col- 

 loidal substances, such as albumin, diffuse very 

 slowly and exert little or no osmotic pressure, sup- 

 posedly because of the large size of the particles. 

 They do not conduct electricity, but the particles 

 themselves react to the electric current by altera- 

 tions in the direction of their motion (i. e., toward 



