96 G. D. Hubbard — Colloids in Geologic Problems. 



tion, — these constitute the field of the student of colloids. 

 The smallest particle visible with the aid of the best com- 

 pound microscope is about 100 millimicrons in diameter, 

 while the largest molecules approach a diameter of 

 1 millimicron. Materials in the colloidal state have parti- 

 cles ranging between these limits. 3 



The term "colloid" or "colloidal state" has been 

 expanded to include not only solid gels, but suspensions 

 and emulsions. The former may be solids in liquids, as 

 the very fine sediments in roily water, liquids in liquids, 

 or even solids in solids, as the blue rock salt which is a 

 suspension of finely divided metallic sodium in crystalline 

 halite. In suspensoids the solid particles do not power- 

 fully absorb or hold large quantities of water. In emul- 

 soids, on the other hand, the particles very powerfully 

 hold or absorb large quantities of water, forming gels. 

 Such gels have been made solid, containing as high as 

 99.87% water. 



Gels are substances in the colloidal state which have 

 set or become apparently solid, even though they contain 

 a large percentage of water. The fruit juice (acid), 

 water, and sugar in the jelly cup have set because of a 

 small percentage of pectin. All are essential to the 

 phenomenon. Certain minerals, for example most ortho- 

 silicates and some zeolites, on some evaporation after 

 solution in hydrochloric or nitric acid, will set in beautiful 

 gels because of the silicic acid liberated. Sodium silicate 

 or water glass sets in the same way when treated with 

 acetic or some other acid, which makes a salt with the 

 sodium and leaves the silica to gel. 



As early as 1861 Thomas Graham 4 knew that dissolved 

 crystalline substances diffused in and out of silica gels, 

 and that reactions occurred between salts in solution in 

 the gels and solutions from outside which diffused in. 

 The famous Liesegang rings were discovered in 1896. 

 Many of these were rings of metallic crystals or crystal- 

 line salts, formed in silica gels by reactions between two 

 solutions, one of which was in the gel and the other was 

 gradually diffusing through the gel. The well-developed 

 crystal had time to form because the solutions came in 



3 Holmes, H. N"., Colloid Chemistry, 1922 (Wiley and Sons). 

 4 Hatschek, E., and Simon, A. L. Trans. Inst. Mining and Metallurgy, Lon- 

 don, 21, 451-480, 1911-12. 



