36 FAMILIAR LETTERS ON CHEMISTRY 



their assimilation. It is obvious that the rapidity of the decomposition of a solid 

 body must increase with the extension of its surface ; the more points of contact 

 we offer in a given time to the external chemical agent, the more rapid will be its 

 action. 



The chemist, in order to prepare a mineral for analyis, to decompose it, or to 

 increase the solubility of its elements, proceeds in the same way as the farmer deals 

 with his fields he spares no labor in order to reduce it to the finest powder; he 

 separates the impalpable from the coarser parts by washing, and repeats his me- 

 chanical bruising and trituration, being assured his whole process will fail if he is 

 inattentive to this essential and preliminary part of it. 



The influence which the increase of surface exercises upon the disintegration of 

 rocks, and upon the chemical action of air and moisture, is strikingly illustrated 

 upon a large scale in the operations pursued in the gold mines of Yaquil, in Chili. 

 These are described in a very interesting manner by Darwin. The rock containing 

 the gold ore is pounded by mills into the finest powder; this is subjected to wash- 

 ing, which separates the lighter particles from the metallic ; the gold sinks to the 

 bottom, while a stream of water carries away the lighter earthy parts into ponds, 

 where it subsides to the bottom as mud. When this deposit has gradually filled up 

 the pond, this mud is taken out and piled in heaps, and left exposed to the action 

 of the atmosphere and moisture. The washing completely removes all the soluble 

 part of the disintegrated rock ; the insoluble part, moreover, cannot undergo any 

 further change while it is covered with water, and so excluded from the influence 

 of the atmoshere at the bottom of the pond. But, being exposed at once to the air 

 and moisture, a powerful chemical action takes place in the whole mass, which 

 becomes indicated by an efflorescence of salts covering the whole surface of the 

 heaps in considerable quantity. After being exposed for two or three years, the 

 mud is again subjected to the same process of washing, and a considerable quantity 

 of gold is obtained, this having been separated by the chemical process of decom- 

 position in the mass. The exposure and washing of the same mud is repeated six 

 or seven times, and at every washing it furnishes a new quantity of gold, although 

 its amount diminishes every time. 



Precisely similar is the chemical action which takes place in the soil of our fields ; 

 and we accelerate and increase it by the mechanical operations of agriculture. By 

 these we sever and extend the surface, and endeavor to make every atom of the soil 

 accessible to the action of the carbonic acid and oxygen of the atmosphere. We 

 thus produce a stock of soluble mineral substances, which serve as nourishment to 

 a new generation of plants, and which are indispensable to their growth and 

 prosperity. 



LETTER XIII. 



MY DEAR SIR : 



Having in my last letter spoken of the general principles upon which the 

 science and art of agriculture must be based, let me now direct your attention to 

 some of those particulars, which will more forcibly exhibit the connexion between 

 chemistry and agriculture, and demonstrate the impossibility of perfecting the 

 important art of rearing food for man and animals without a profound knowledge 

 of our science. 



All plants, cultivated as food, require for their healthy sustenance the alkalies 

 and alkaline earths, each in a certain proportion ; and in addition to these, the 

 ceralia do not succeed in a soil destitute of silica in a soluble condition. The com- 

 binations of this substance, found as natural productions, namely, the silicates, 

 differ greatly in the degree of facility with which they undergo decomposition, in 

 consequence of the unequal resistance opposed by their integral parts to the dissolv- 

 ing power of the atmospheric agencies. Thus, the granite of Corsica degenerates 

 into a powder in a time which scarcely suffices to deprive the polished granite of 

 Heidelberg of its lustre. 



Some soils abound in silicates so readily decomposable, that in every one or two 

 years, as much silicate of potash becomes soluble and fitted for assimilation as is 

 required by the leaves and straw of a crop of wheat. In Hungary, extensive dis- 

 tricts are not uncommon where wheat and tobacco have been grown alternately 



