Jo8 PHYSICAL CHEMISTRY IN MEDICINE. 



allows not only a survey of what has been accomplished, 

 but also allows us to state in advance what may be ex- 

 pected to happen in colloidal solutions, has, moreover, 

 the advantage that its suppositions are capable of being 

 tested experimentally in fact, have already been tested 

 in many cases. 



We have optical means at our disposal, for example, 

 by which we can determine the number and the size of 

 the colloidal particles. If a colloidal solution is placed 

 in a bundle of intense light rays, the fine particles of the 

 solution reflect the light in part, as can be determined 

 through its polarization. The absorption of different 

 portions of the spectrum may also give a clue regarding 

 the size of the particles. Finally, SIEDENTOPF and 

 ZSIGMONDY have made it possible with their ingenious 

 ultramicroscope to determine the number and size of the 

 colloidal particles. The electrical condition of the sus- 

 pended particles we can recognize from their behavior 

 in the electric current, in that they migrate, according to 

 their electrical charge, either toward the positive pole 

 when they are negatively charged, or toward the negative 

 pole when they are positively charged. 



These facts allow us to understand a process which 

 has long served as the prototype of most of the colloid 

 reactions and which explains many important questions 

 in physiology and pathology, namely, the precipitation of 

 colloids. 



It has already been pointed out that the salts, acids, 

 and bases dissociate in part in aqueous solution into 

 their oppositely charged constituents, the ions. Let us 

 suppose now that a sufficient number of such ions are 

 introduced into a colloidal solution of a metal which 



