EMPLOYED IN AGRICULTURE. 150 



is, to liave their constituents broug'ht into that state of inti- 

 mate union wliicli render them almost unattackable by ordi- 

 nary acids, and of course, therefore, scarcely decomposable by 

 atmospheric ag'ents — the greater portion at least of sub- 

 marine lavas, or of volcanic products which have cooled 

 slowly under pressure, consists of an intimate mixture of 

 g'lassy felspar with some member of the zeolitic family, the 

 characteristic of which is its easy solubility in acids. 



Nor is this property confined to submarine products, for 

 Dufresnoy has shown that the modern lavas of Vesuvius are 

 distinguished from those of the Monte Somma by being- 

 soluble in acids in the proportion of 4 to 1. 



Nature, therefore, appears to have provided means for the 

 slow diseng'ag-ement of alkali, not only in the case of plutonic 

 rocks, but also in those of more modern volcanic origin, by 

 causing- the materials of which they consist not in general to 

 assume a vitreous condition, but to separate in the act of 

 cooling- into a certain number of definite combinations, some 

 of which, at least, acids are capable of attacking-, and which, 

 consequently, would be liable to yield to the continued in- 

 fluence of atmospheric agents. 



Svlphuric Acid. — Another chemical agent which seems to 

 operate in promoting vegetation, somewhat upon the same 

 principle as lime has been represented to do, is sulphuric 

 acid. An example of its operation is appended in the now 

 familiar process of treating bones with this acid, as the 

 earthy phosphates are by this means brought into a soluble 

 form, and are therefore rendered more fit to be assimilated 

 by plants. 



This, at least, I apprehend to be the true theory of the 

 advantage derived from its use when mixed with bones ; for, 

 although the phosphoric acid is introduced to the soil as a 

 superjdiosphate, still it is not as such that it enters into the 

 structure of the plant ; and it therefore follows, that it must 

 have combined with its usual quantity of lime before it was 

 taken up by the roots. 



A case, however, of a more precisely parallel description 

 is afforded by the ])ractice, adopted in some parts of the 

 continent, of sprinkling the soil with sulplnu'ic acid. 



Here, in the first instance, a certain amount of gypsum is 

 g'enerated, which acts beneficially upon certain crops, by 

 supplying them with a material needed for their inorganic 

 constitution. 



This end, however, might be attained at a cheaper rate 



