30 CHYMISTRT APPLIED TO AGRICULTURE. 



which the two earths combine to form marl. Numerous 

 analyses have been made by me of the marls of the cen*- 

 tre and south of France, and I have found them to con- 

 tain from 10 to 60 per cent, of sub-carbonate of lime, 

 from 15 to 50 of alumina, and from 15 to 66 of siliceous 

 sand. Marl is often produced by the decomposition of 

 silex or flint. 



ARTICLE V. 



Of the Properties of the different Earths. 



As the several earths contained in the soils of which 1 

 have just spoken, do not all possess the same qualities, and 

 are very differently affected by air, water, and heat, the 

 most powerful agents of vegetation, the excellence of a 

 soil depends upon its containing the right proportion of 

 each species of earth ; and that is supposed to be the best 

 soil, in which the virtues of one portion of its constituent 

 principles correct the faults or defects of the rest. In 

 order to produce these mixtures, to supply the deficiencies 

 of poor soils, and to be able to render them, by art, suita- 

 ble to the production of some particular article of cultiva- 

 tion, it is necessary to know the particular properties of 

 each kind of earth ; and it is upon this subject that I shall 

 now speak. 



Siliceous earth, or silica, exists in all hard primitive 

 rocks, and forms nearly the whole of quartzeous moun- 

 tains. In order to obtain it in its greatest degree of purity, 

 it is fused with six parts of potash ; it is then dissolved in 

 water, and separated from the alkali by muriatic acid ; the 

 solution is evaporated to dryness, and the residuum wash- 

 ed in water affords pure silica. In this state silica has 

 the appearance of a white impalpable earth, rough to the 

 touch ; when thrown into water it sinks with extreme 

 rapidity, but its particles have no tendency to unite into 

 one mass. The weight of silica compared to that of water 

 is 2.5 to 1.* 



[* Silica exists pure in rock crystal, and nearly pure in flint. It 

 may be obtained pure by heating rock crystal to redness, quenching 

 it in water, and reducing it to a fine powder. Fuse 1 part of this 



