25 ELEMENTS OF 



bined with oxygen, forms '-alumina," or the pure earth 

 of clay) are the most widely diflfused on the surface of 

 the earth ; and all fertile soils, or soils capable of culture, 

 invariably contain alumina. 



There must, therefore, be something in aluminous 

 earth which causes it to exercise an influence on the life 

 of plants, and to assist in their growth. 'I'he property 

 on which this depends is, that clay invariably contains 

 potash and soda. Besides which, alumina attracts and 

 retains water and ammonia from the atinosphere. Al- 

 umina is itself very rarely found in the ashes of plants ; 

 but silica (or the earth of flints) is always present, hav- 

 ing, in most places, entered the plants by means of alka- 

 lies. Among aluminous minerals, feldspar, which is one 

 of them, contains 17 per cent, of potash ; mica from 3 to 

 5 per cent, of soda : clay slate contains from 2 to 3 per 

 cent, of potash ; and loam from 1 1-2 to 4 per cent, of 

 the same alkali. 



So that, in a layer of soil formed by the breaking 

 down of 40,000 square feet of one of these rocks, to the 

 depth of 20 inches, we should find that so much feldspar 

 would contain more than a million pounds of potash ; if 

 the soil were formed by the disintegration of clay slate, 

 about 200,000 ; if loam were the material, from 87,000 

 to 300,C00 ; and similarly of other rocks of partially 

 aluminous character. 



Potash is present in all clays, and in marl ; it has been 

 found in all aluminous earths in which it has been sought. 

 Alum (which is a sulphate of alumina, combined with 

 sulphate of potash) may be procured by digesting clay 

 in sulphuric acid, which takes up both the alumina and 

 the potash. 



A thousandth part of loam mixed with the quartz in 

 red sandstone, or with the lime in the diflferent limestone 

 formations, alfords as much potash to a soil twenty 

 inches in depth as is sullicient to supply a forest of pines 

 growing upon it with potash for a hundred years. 



Water. iin[)regiiated with the carbonic acid of the at- 

 mosphere, decomposes rocks which contain alkalies, and 

 then dissolves a part of the alkaline carbonates formed 



