214 SOIL ITS ANALYSIS. 



chemists, and a compound, according to them, of oxygen and siliconi 

 in the proportion, Berzelius says, of 100 of the radical to 108 of 

 oxygen. 



Silica in a state of purity occurs in the form of a white powder, 

 and having a density of 2.7. It is infusible in the most violent fur- 

 nace, but it not only melts in the intense heat which results from the 

 combustion of a mixture of hydrogen and oxygen gas, but it is even 

 dissipated in vapor. As generally obtained, silica is held insoluble 

 in water; still, when in a state of extreme subdivision, it is s«)luble ; 

 and then its insolubility is probably not so absolute as is generally 

 supposed, for M, Payen has found notable quantities in the water of 

 the Artesian well of Grenelle, and in that of the Seine. Silica ex- 

 ists especially in very appreciable quantity in certain hot springs 

 where the presence of an alkaline substance favors its solution ; 

 the water of the hot springs of Reikum in Iceland contain about 

 Y75^oth parts of its weight of silica ; and the thermal spring of Las 

 Trincheras, near Puerto Cabello, deposites abundant silicious concre- 

 tions. The water of this latter spring, which is at the temperature 

 of 210° F., besides silica contains a quantity of sulphurated hydro- 

 gen gas, and traces of nitrogen gas. Rock crystal when colorless 

 and transparent may be regarded as pure silica ; in the varieties of 

 quartz which mineralogists designate as chalcedony, agate, opal, &c., 

 the silica is combined with different mineral substances, particularly 

 oxide of iron and of manganese, alumina, lime, and water. 



Carbonate of lime, considered as rock, belongs to every epoch in 

 the geological series, and frequently constitutes extensive masses. 

 When pure it is composed of lime 56.3, carbonic acid 43.7 ; and its 

 density is then from 2.7 to 2.9. It dissolves with etfervescence 

 without leaving any residue in hydrochloric or nitric acid. Exposed 

 to a red heat its acid is disengaged, and quick-lime remains. Car- 

 bonate of lime is insoluble in water, but it dissolves in very consid- 

 erable quantity under the influence of carbonic acid gas. When 

 such a solution is exposed to the air the acid escapes by degrees, 

 and the carbonate is deposited, by which means those numerous 

 deposites of carbonate of lime are produced, which we see constitu- 

 ting tufas and stalactites. The solubility of carbonate of lime in 

 water acidulated with carbonic acid, enables us to understand how 

 plants should meet with this salt in the soil, inasmuch as rain-water 

 always contains a little carbonic acid. 



The mineral substances which we have now studied, taken iso- 

 latedly, would form an almost barren soil ; but by mixing them witii 

 discretion a soil would be obtained, presenting all the essential con- 

 ditions of fertility, which depend as it would seem much less on the 

 chemical constitution of the elements of the soil than on their physi- 

 cal properties, such as their faculty of imbibition, their density, their 

 power of conducting heat, &c. It is unquestionably by studying 

 these various properties that we come to form a precise idea of the 

 causes which secure or exclude the qualities we require in arable 

 poiis. This has been done very ably by M. Schiibler, and his admj- 



