744 [Assembly 



Silica, is of all the elementary earths, most extended through na- 

 ture, where it is never found in absolute purity, but always mixed 

 with other minerals. Among those bodies into whose composition it 

 enters for the largest part, and which exhibit Silica in its highest 

 degree, we mention the following : All precious stones except the 

 diamond, the flint, quartz and sand. It has no affinity with water, 

 and is considered to be insoluble in it. Yet its presence in plants 

 would make us suppose that its msolubility is not so absolute as is 

 commonly supposed. 



Alumine is the principal base of clay, Avhich it forms by combin- 

 ing with the silex. Like silex, we never find alumine in a state of 

 absolute purity, but always combined with other elements. 



It is after silica, the most extended of the earths, generally. It 

 has a great affinity for water and a great tendency to unite with the 

 silex. 



Clay is a chemical compound of silica and alumine and nearly in 

 the proportion of 60 of the first, to 40 of the latter. But this is not 

 merely a mixture, it is a chemical combination, which cannot be 

 decomposed, except by the chemical agents. Soil contains besides a 

 large proportion of silex mixed with it in the form of sand and gravel. 

 One of the characteristics of clay is to adhere to the tongue, owing 

 to its avidity for watei'. It absorbs water r.nd becomes a greasy 

 paste, and when it is saturated with water, it will not permit water 

 to pass through it. This impermeability of wet clay is the cause 

 that beds of it which alternate with beds of sand, cau5>e springs of 

 water on the surface of the earth. 



Lime. — We find the carbonic and the sulphuric acids njost com- 

 monly combined with lime. Its combination with carbonic acid 

 produces carbonate of lime, and with sulphuric acid, sulphate of lime 

 or Gypsum, or Plaster. Carbonate of lime, subjected to a high heat, 

 loses the water and the carbonic acid it contains, and then it is cal- 

 led Quick lime. 



Marl is a combination of clay and carbonate of lime, in which the 

 latter predominates. It is considered to be insoluble in water, but 

 nevertheless it is dissolved in some measure by means of carbonic 

 acid gas. And this is the w-ay the soil transmits this salt to plants. 



Sulphate of lime is produced sometimes in great masses, sometimes 

 crystalized and sometimes pulverized and mixed with soils. 



