THE PROBLEMS OF SOLUTION 117 



redlssolvable substances is termed setting, that 

 of substances in the second class, which form in- 

 soluble precipitates, is termed coagulation. 



The mechanism of gelation in the first, or 

 reversible class of colloidal systems, has been 

 studied experimentally by Van Bemmelen and by 

 W. B. Hardy. The process of solidification seems 

 to consist in the growth of a solid framework 

 containing more liquid portions. The tempera- 

 ture at which this separation into two phases 

 occurs depends on the amount of water present. 



The coagulation of irreversible colloidal 

 solutions, as already stated, can be effected by 

 the addition of small quantities of the solution 

 of an electrolyte, such as an ordinary salt or acid. 

 Graham, who originally investigated the subject, 

 found that a minute trace of salt was often 

 sufficient. Thus, hydrated alumina, prepared from 

 a solution of the chloride in distilled water, was so 

 unstable that a few drops of well-water produced 

 coagulation, and the same change was brought 

 about by pouring the colloidal solution into a new 

 glass vessel, unless the vessel had previously 

 been washed repeatedly with distilled water. 



Several experimenters, including Schulze, 

 Linder and Picton, and Hardy, have investigated 

 this coagulative power of electrolytes, with very 

 curious and interesting results. The coagulative 

 power of a salt is found to vary in a remarkable 

 manner with the chemical valency of one of its 

 ions.^ The average of the coagulative powers of 



^ The valency of a chemical atom may be defined as the number 

 of hydrogen atoms it will combine with or replace. Thus the 

 normal valency of oxygen is two, since two hydrogen atoms unite 

 with one oxygen atom to form water. Faraday's work showed that 

 the electric charge carried by an ion is proportional to its valency. 



