44 ILLINOIS ACADEMY OF SCIENCE 



The first investigators in the subject thought that but few- 

 substances can be obtained in the colloidal condition. Later, 

 and as the result of an enormous number of researches, it has 

 become evident that practically any solid or liquid substance 

 may, by proper manoeuvering, be obtained in colloidal solu- 

 tion in some solvent. The method of preparing colloidal solu- 

 tions are quite diverse as a consideration of the following typ- 

 ical examples will show. 



(a) A colloidal solution of silver chloride may be prepared 

 by mixing a very dilute solution of silver nitrate with a very 

 dilute solution of common salt. The use of very dilute solu- 

 tions is a method of producing colloidal solutions of many 

 slightly soluble substances. 



(b) An interesting series of colloidal solutions of gold may 

 be prepared by treating gold chloride solutions with reducing 

 agents. These vary from the translucent blue to the trans- 

 parent orange red solutions. In the latter the gold is in such 

 a fine state of division that the separate particles are scarcely 

 visible in the ultramicroscope. 



(c) A colloidal solution of ferric hydroxide may be pre- 

 pared by simply pouring a few drops of ferric chloride into 

 boiling water. 



(d) Bredig's method, which is applicable to a number of 

 metals, consists in causing an electric arc to strike, under 

 water, between electrodes made of the metal of which a col- 

 loidal solution is desired. With the use of high frequency alter- 

 nating currents this method has recently been used in the pre- 

 paration of colloidal solutions of a number of metals in a great 

 range of degrees of dispersion. 



(e) Colloidal tungsten and chromium may be made by treat- 

 ing the finely ground metals alternately with acid and alkali. 

 This is known as the etching method. Colloidal tungsten pre- 

 pared in this way was used in the manufacture of the early 

 fragile tungsten filaments. 



(f) It has also been found possible to prepare colloidal solu- 

 tions of a number of metals simply by keeping them for a long 

 time in boiling water in the absence of oxygen. 



The question now arises; what properties in common have 

 solutions prepared by such different methods and involving 

 such different substances? The size of the particles has al- 

 ready been mentioned, some other properties will be considered 

 in the following paragraphs. 



