OF AGRICULTURE. 203 



seventy-nine per cent., or four-fifths of the atmos- 

 phere which surrounds us. Thus, the air is 

 twenty-one per cont. oxygen and seventy-nine per 

 cent, nitrogen. It being one of the constituents of 

 niter, or saltpeter, it is properly called nitrogen. 

 When combined with oxygen, in the proportion of 

 one combination weight of nitrogen to five combi- 

 nation weights of oxygen, thus : N, 14xO, 40. form 

 nitric acid. Nitrogen exists in all animal substances, 

 and in all plants, especially such as putrify with an 

 animal odor, as, cabbages and mushrooms. Nitric 

 acid, or aqua-fortis, combines with a great number of 

 bases, giving us an important class of salts, called 

 nitrates. 



When the ashes of plants are examined, we find 

 carbonates of bases that did not exist in the soil as 

 such. A large proportion of carbonate of lime and 

 potash is found in ashes ; the origin of these is to be 

 sought in the vegetable acids of plants, which, by 

 heat, produce carbonic acid. This is the effect of 

 heat upon all salts formed of vegetable acids, such as 

 tartaric, malic, citric, oxalic, and vinegar or acetic acids. 

 Each plant forms acids in definite quantity, propor- 

 tionate to its size, age, and parts of the plant ; thus is it 

 beautifully ordered by nature, the acids being 

 constant, the bases to saturate them will be equally 

 constant. 



It is an established fact, that plants growing in a 

 soil containing a due mixture of earthy ingredients, 

 always select a due proportion of each, according to 

 their functions, but if in such a soil an excess of 

 either of the alkalies, such as lime, potash, soda, or 

 magnesia, be present, then an excess of either of 



