.14 THE CHEMISTRY OF THE FARM. 



The atmosphere also furnishes a small amount of nitric 

 acid. The nitrogen and oxygen of the atmosphere com- 

 bine under the influence of electric discharges, nitrous 

 acid being formed ; tjiis is converted into nitric acid by 

 the action of ozone, or peroxide of hydrogen. This forma- 

 tion of nitric acid in the atmosphere is the only original 

 source of combined nitrogen on our globe the existence of 

 which has been placed beyond dispute. Nitric acid may 

 also be formed in the atmosphere by the oxidation of 

 ammonia by ozone and peroxide of hydrogen. 



The total amount of nitrogen, in the form of ammonia 

 and nitric acid, annually carried to the soil by rain, varies 

 in different years and places. The average of many experi- 

 ments on the continent gives 10.23 lb. of nitrogen per acre. 

 The average of two years' experiments at Rothamsted gave 

 7.29 lb. The continental average is probably rather above 

 the truth for the open country, many of the determinations 

 having been made near towns. 



Rain also furnishes small quantities of alkaline chlorides, 

 especiall}'- in the neighbourhood of the sea ; sulphates are 

 also present. At Cirencester the chlorides in the rain are 

 on an average equal to about 53 lb. of common salt per 

 acre per annum ; at Rothamsted in Hertfordshire the 

 quantity is about 22 lb. 



The Soil. — All soils have been produced by the dis- 

 integration of rocks, generally through the prolonged 

 action of water, air, and frost. The character of a soil 

 largely depends on the character of the rock from which 

 it has been derived. Primitive and igneous rocks yield 

 soils rich in potash ; fossiliferous rocks produce soils rich 

 in phosphoric acid. The principal ingredients of soils are 



