XII.] ORIGIN OF METALLIFEROUS DEPOSITS. 223 



exterior portions of our planet, shows that there was, at least, 

 but a very partial concentration and separation of these ele- 

 ments during the period of cooling and condensation. The 

 superficial crust of the earth, from which all the rocks and 

 minerals which we know have been generated, must have 

 contained, diffused through it, from the earliest time, all the 

 elements which we now meet with in our study of the earth, 

 whether still diffused, or accumulated, as we often find the 

 rarer elements, in particular veins or beds. 



The question now before us is, How have these elements 

 thus been brought together, and why is it that they are not all 

 still widely and universally diffused 1 Why are the compounds 

 of iron in beds by themselves, copper, silver, and gold gathered 

 together in veins, and iodine concentrated in a few ores and 

 certain mineral waters 1 That we may the better discern the 

 direction in which we are to look for the solution of this 

 problem, let us premise that all of these elements, in some of 

 their combinations, are more or less soluble in water. There 

 are, in fact, no such things in nature as absolutely insoluble 

 bodies, but all, under certain conditions, are capable of being 

 taken up by water, and again deposited from it.* The al- 

 chemists sought in vain for a universal solvent ; but we now 

 know that water, aided in some cases by heat, pressure, and 

 the presence of certain widely distributed substances, such as 

 carbonic acid and alkaline carbonates and sulphides, will dis- 

 solve the most insoluble bodies ; so that it may, after all, be 

 looked upon as the long-sought-for alkahest or universal men- 

 struum. 



[* It is well known that many chemical compounds when first generated by 

 double decomposition in watery solutions remain dissolved for a greater or 

 less length of time before separating in an insoluble condition. The solubility 

 of recently precipitated carbonate of lime in water holding certain neutral 

 salts, as already described (ante, page 140), is a fact in the same order. In this 

 connection may also be recalled the great solubility in water of silicic, titanic, 

 stannic, ferric, aluminic, and chromic oxides when in what Graham has 

 called their colloidal state. There is reason to believe that silicates of in- 

 soluble bases may assume a similar state, and it will probably one day be 

 shown that for the greater number of those oxygenized compounds which we 

 call insoluble there exists a modification soluble in water.] 



