46 LESSONS IN CHEMISTRY. 



unite with more oxygen, and the change which the farmer 

 frequently observes in the colour of the newly turned up 

 mould from dark brown to an ochrey red, is produced by the 

 protoxide of iron existing in the soil being converted, 

 by exposure to the air, into the reddish brown peroxide. 

 Protoxide of iron and its compounds are considered to be 

 injurious to plants. You will, therefore, perceive how the 

 various mechanical processes which tend to expose the particles 

 of the soil to the influence of the oxygen of the atmosphere, 

 may not only improve their texture, but produce important 

 chemical changes in the ingredients which they contain. 



49. Oxide of Manganese. Manganese is a metal which, in 

 many respects, resembles kon. It unites with oxygen in 

 several proportions, and a small amount of some of its com- 

 pounds is discovered in the ashes of plants. 



50. Silica is the earthy substance which constitutes the 

 bulk of flint. Hence, it is frequently termed " earth of 

 flints." It also forms a large part of sandstone, sand, and 

 of the greater number of rocks with which we are acquainted. 

 Itock crystal — beautiful specimens of which are found in the 

 granite of Mourne — is almost pure Silica, and the white sand 

 produced by the v^ethering of Muckish, and other mountains 

 in Donegal, also contains it nearly free from foreign ingre- 

 dients. Pure Silica is a snow-white tasteless poAvder. It 

 is insoluble in water, and in all acids, except one named 

 Fluoric Acid. When heated with potash or soda, it forms, 

 according to the quantity employed, an insoluble transparent 

 glass, or a compound that dissolves in water. In combining 

 with these substances silica performs the part of an acid, and 

 the compounds are termed silicates; when the alkali largely 

 predominates the silicates dissolve readily in water, but when 

 only a small amount is present, the compound is not dissolved 

 by water, and is only slowly acted upon by strong acids; 

 thus common window glass is an insoluble silicate, and the 

 greater number of rocks consist chiefly of silica united with 

 variable proportions of iron, lime, and other elements. The 

 carbonic acid, and probably other acids, produced during the 

 decay of vegetable matters slowly decompose the compounds 

 of silica existing in the soil and in the straw of the manure 

 heap, and when thus separated from the elements with which 

 it was combined, silica becomes soluble in water, and capable 

 of being taken up by the roots of plants. 



