VI OF CHEMISTRY. 



Chemistry analyzes and explains the true nature and con- 

 stitution of soils. It reduces mineral and organic matter 

 to their primitive elements, and shows the relation subsist- 

 ing between them. 



The elementary bodies into which plants are capable of 

 being resolved, are sixteen in number ; of these the soil 

 constitutes twelve, and water and air the remainder. 



The materials constituted by^ the soil are of four kinds 

 earths, alkalies, acids, and metals. 



The earths are lime, alumina, silica, and magnesia. 

 The three first form, in various proportions, the principal 

 ingredients of all our cultivated soils. Magnesia is of rare 

 occurrence. When lime prevails in a soil, it is denominated 

 calcareous ; when alumina abounds, it is called a clay soil ; 

 and when silica is the principal constituent, it is sandy or 

 gravelly, according as the material is fine or coarse. 



The alkalies are potash, soda, and ammonia. These are 

 detected by their effects on colours, acids, and oils, on 

 which they exercise a peculiar and striking influence. 



The acids are sulphuric and phosphoric. The latter, in 

 combination with lime, forms the material of the bone of 

 animals. 



The metals are iron and manganese, in the form of oxides. 



To the above we may add chlorine, the gas which is so 

 disagreeably perceived by its smell in bleaching salts. 



From water and air are derived the gases, oxygen, 

 hydrogen, nitrogen, and carbonic acid. These are the 

 great constituents of organic life. They constitute from 

 eighty-five to ninety-nine per cent, of the structure of plants. 



Besides the elementary substances above mentioned, 

 there exists in every soil, in a greater or less degree, a 

 quantity of decomposed vegetable matter called humus, 

 which, when in any considerable quantity, imparts a dark 

 colour to the earth, and is always a principal cause of its 

 fertility. 



To ascertain, accurately, the proportions in which these 

 materials exist in a soil could scarcely be effected, except 



