PHYSICAL CHEMISTRY 23 



of larger molecules such as those of sugar, proteins, etc., the 

 process is called dialysis. Dialysis is of value in freeing from 

 salts, etc., a solution of protein or other nondialyzable sub- 

 stance. 



There has been much discussion as to whether osmosis and 

 diffusion or these plus simple nitration could account for the 

 absorption from the intestine, the excretion of urine, the forma- 

 tion of lymph, the passage of gases through the alveolar walls 

 in the lungs and many other processes. Whereas these phe- 

 nomena doubtless are influenced by the laws governing osmosis 

 and diffusion, they evidently are controlled also by other fac- 

 tors. For example an isolated loop of intestine will absorb 

 substances from within itself which are present in concentration 

 no greater than that in which they exist in the blood, whereas 

 the concentration of many substances in the urine is much above 

 their concentration in the blood. On the other hand, the trans- 

 ference of gases in the lungs probably follows simple laws of 

 diffusion except in time of stress, when cellular activity appears 

 to come into play. 



Electrical Properties of Solutions. It is a well established 

 fact that substances in solution often carry electric charges. 

 This can be demonstrated by passing an electrical current 

 through a solution. The substances in solution will migrate 

 toward the positive or negative pole according as they carry 

 negative or positive charges. This migration in the current is 

 called cataphoresis. Only substances which are dissociated into 

 their ions will migrate thus. Those which are not dissociated 

 do not migrate, and thus do not conduct an electric current. 

 Those which conduct are called electrolytes, those which do not, 

 nonelectrolytes. Knowledge of the electrical properties of dis- 

 solved substances has been of the greatest value in the study of 

 colloidal solutions, hydrogen-ion concentration and other im- 

 portant fields. 



H-ion Concentration. It is a well known fact that acids 

 dissolved in water and dissociating into their ions give off 

 hydrogen-ions. The extent to which an acid is dissociated de- 



