CHEMICAL CONCENTEATIOK 21 



ing ferric carbonate is exposed to the air, the iron at normal surface tem- 

 peratures and pressures will exchange its carbon dioxide for oxygen and 

 be precipitated as ferric oxide. In other words, there is the process in 

 nature whereby chemical precipitation leads to the concentration of 

 ferric oxide. 



The example cited above, of the precipitation of ferric oxide, is a 

 ■chemical process, normal, as one may say, to the established order of 

 physical conditions of the outer earth. Hence, together with other simi- 

 lar or analogous instances in which the bringing of bodies or volumes of 

 material into intermediate association leads to the precipitation or segre- 

 gation of one form of matter apart from other forms, it may be termed 

 concentration owing to chemical reactions. 



In an extension of our classification indicating methods of concentra- 

 tion the subdivisions of the results of chemical precipitation may be based 

 on the nature of the products or on the processes included in this concen- 

 tration. Suggestive categories may be formed by each of these methods, 

 but for our present purpose and for the sake of economizing space subdi- 

 visions based on the major features of the processes involved are here 

 considered. 



Chemical reactions resulting in the concentration and preservation of 

 Tarious forms of mineral matter take place in nature in the main by the 

 (1) union of gases with gases, (3) the union of liquids with liquids; 

 and here we include the union of substances or ions in solution (3) by 

 "the union of gases with liquids or solids and by the union of liquids with 

 gases or solids. The significant result of these various unions, so far as 

 the present discussion is concerned, is that solids of specific composition 

 are produced which may be reckoned as geological concentrates. 



1. The interaction of gases with gases in nature causes the precipita- 

 tion of such substances as ammonium chloride, borates of the alkaline 

 earths, sodium carbonate, ferric chloride, and a variety of other products, 

 some of which are of commercial importance. These substances when 

 deposited in and about volcanic conduits, as already stated, are commonly 

 associated with the products of sublimation, and the two classes have 

 been tentatively classed as fumarolic concentrates. 



2. The reactions between substances in solution, or, as is perhaps a 

 more accurate statement, the afiinity between different ions when sub- 

 stances are in solution, is the fundamental factor in chemical concentra- 

 tion. Principally, however, at or near the earth's surface, chemical 

 unions which lead to the precipitation of solids from solutions are con- 

 trolled in a conspicuous manner by temperature changes, evaporation, 

 relief of pressure, escape of gases from solutions, vital processes, etcetera, 



