CALCAREOUS COMPOSTS ARE NITRE-BEDS. 265 



diatc products of nitric acid on the soil), are well known to Tbo 

 highly fertilizing. The foregoing passages show (besides other 

 known sources) that the air supplies both, and that the surface 

 of the earth, everywhere, is sure to be more or less supplied from 

 the air, with ammonia and nitric acid. Nitrogen, which is one 

 of the two constituent parts of both these fertilizing compounds, is 

 also the richest and the most important element (for the small pro- 

 portion required), in the nutriment of plants, and the most power- 

 ful promoter of their luxuriance and perfection of growth* It may 

 be inferred, that it is by furnishing their element nitrogen to 

 plants, that both ammonia and the nitrates are such important aids 

 to vegetable growth, and to the fertility of soils. Ammonia is 

 produced and evolved in large quantity by the putrefaction of all 

 animal substances. Also, <{ during the decay of vegetable sub- 

 stances in moist air, ammonia is formed at the expense of the hy- 

 drogen of the water and of the nitrogen of the air. In conse- 

 quence of, or in connexion with, such decay, nitric acid is also 

 largely produced in nature." (Johnston, p. 161.) 



" The most familiar, as well as the most instructive examples of 

 this formation of nitric acid, is in the artificial nitre-beds of France 

 and the north of Europe. These are formed of earth [calcareous 

 in part], stable manure, or other animal and vegetable matters, the 

 mixture laid in ridges, occasionally watered with liquid manure, 

 and turned over, to expose fresh portions to the air. After a time, 

 perhaps once a year, the whole is washed, when the water which 

 conies off is found to contain a variable quantity of the nitrates of 

 potash, soda, lime, and magnesia, which are employed for the manu- 

 facture of saltpetre. In these nitre-beds, it has been observed that 

 the production of nitric acid either does not take place at all, or 

 only with extreme slowness, unless animal and vegetable matter be 

 present in considerable proportion. And yet the quantity of nitric 

 acid which is formed is much greater than could be produced by 

 the oxydation of the whole of the nitrogen contained in the organic 



matters present in the mixture It appears, therefore, 



that organic matters are, in our climate, necessary to cause the for- 

 mation of nitric acid to commence ; but that after it has begun, it 

 will proceed in the same heap for an indefinite period, and at the 

 expense apparently of the nitrogen of the air only. 



" Compost heaps [of manure, formed of rich soil, animal manure, 

 and lime or calcareous earth] are in general only artificial nitre- 

 leds, often unskilfully prepared, and badly managed, producing, 

 however, a certain quantity of nitrates, to the presence of which, 

 their effect^on vegetation may not unfrequently be ascribed. . . . 

 The soils in the plains of India, and in other similar spots in the 

 tropical regions, may be regarded as natural nitre-beds, in which 

 the decay of organic matter being vastly more rapid than in our 

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