AMERICAN JOURNAL OF SCIENCE 



[FIFTH SERIES.] 



Art. VIII. — Colloids in Geologic Problems; by George 



D. Hubbard. 1 



Introduction. — It lias long been recognized that solid 

 matter occurs in more than two states. The terms 

 "crystalline" and "amorphous" exclude quite a large 

 body of materials, some of which are organic and some 

 inorganic. The other condition has been called the col- 

 loidal state. Many substances which occur crystalline 

 can be prepared in the colloidal state without any com- 

 positional difference. Amorphous substances were in 

 many instances once in colloidal condition. Perhaps all 

 substances can ultimately be prepared in this state 

 whether they are normally crystalline or amorphous. 



In the case of some substances, there seems to be no 

 line fixed between the crystalline and the colloidal state. 

 In the same way, there seems to be no line which can be 

 definitely drawn between a true solution and a colloidal 

 solution. In practice we say a substance is in the col- 

 loidal state when it will not dialyze or diffuse through 

 certain membranes, as egg skin, bladder, goldbeater's 

 skin, or parchment paper. Molecular dispersions are in 

 true solution; dispersions of much larger aggregates (a 

 hundred, more or less, of molecules in one particle) are 

 in colloidal solution. Such particles are too large to pass 

 through the membrane. 



Bancroft 2 says, "Colloid chemistry is the chemistry of 

 grains, drops, bubbles, filaments, and films." Grains of 

 solid particles, drops of fluids, bubbles of gas, filaments 

 very small in two directions, films small only in one direc- 



1 The author is deeply indebted at many points in the paper to Dr. Harry 

 X. Holmes of Oberlin College for suggestions and criticism. His General 

 Chemistry and Colloid Manual have been of much help also. This paper was 

 presented to the Geologic Section of the Ohio Academy of Science, April 15, 

 1922. 



2 Bancroft, W. D., Applied Colloid Chemistry, p. 2 (McGraw-Hill Co.). 



Am. Jour. Sci.— Fifth Series, Vol. IV, No. 20.— August, 1922. 



