ARTIFICIAL NITRE BEDS. 161 



it is obvious that in whatever proportion we may suppose the ammonia 

 of the air to reach the leaves and roots of plants, in no less proportion 

 must the nitric acid, with which it is associated, he enabled to enter into 

 the circulating system of the various tribes of living vegetables, that 

 flourish on every quarter of the globe. 



3°. Again, we have seen that, during the decay of vegetable substan- 

 ces in moist air, ammonia is formed at the expense of the h^^drogen of 

 the water and of the nitrogen of the air. In consequence of, or in con- 

 nection with, such decay, nitric acid is also largely produced in nature. 



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 by mixing earth of different kinds 

 with stable manure or other animal and vegetable matters, and exposing 

 the mixture to the air in long ridges or conical heaps, which are occa- 

 sionally 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 comes off is found to contain a variable 

 quantity of the nitrates of potash, soda, lime, and magnesia, which are 

 employed for the manufacture of saltpetre. In these nitre beds it has 

 been observed that the production of nitric acid either does not take plaec 

 at all, or only with extreme slowness, unless animal and vegetable mat- 

 ter be ])resent in considerable proportion. And yet the quantity of nitric 

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

 oxidation of the whole of the nitrogen contained in the organic matters 

 present in the mixture.* It is also observed that the nitre beds are more 

 productive when a portion from one outer face of the heap is lixiviated 

 from time to time, and the washed earth added to the other side, than 

 when the whole is lixiviated at once, and again formed into a heap and 

 exposed to the air. 



It appears, therefore, that organic matters are in our climate necessa- 

 ry to cause the formation 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 are in general only artificial nitre beds, often unskil- 

 fully prepared and badly managed, producing, however, a certain quan- 

 tity of nitrates, to the presence of which their effect on vegetation may 

 not urfrequently be ascribed. To this fact we shall hereafter recur. 



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

 ical regions, may be regarded as natural nitre beds, in which, the decay 

 of organic matter being vastly more rapid than in our temperate regions, 

 the production of nitric acid is rapid in proportion. f 



4°. But in many localities in which the presence of organic matter is 



* Dnmas, Traite de Chemie, II., p. 725. He ailds, that 100 lbs. of nitre contain the nitrogen 

 of 75 lbs. of ordinary animal matter, supposed in a dry state, or of 300 or 400 lbs. in its ordi- 

 nary state of moisture, — a much greater relative proportion of animal matter than is ever 

 added to the heap. 



1 We are as yet too little acquainted with the natural history of the district of Arica in 

 South America, in which, as already stated (p. 56), the nitrate of soda has been accumulateu 

 in such large quantity, to be able to say to wtiat special cause the accumulation is due. But 

 as, from the description of Mr. Darwin, the locality appears to have been the site of an an- 

 cient lake, it is not unlilcely that the nitrate may have been derived from the successive 

 washings of a soil similar t(^iat of India, by rains or periodical floods, which for a long pe- 

 riod emptied themselves imWor fed the lake. 



