PRACTICAL INFERENCES. 277 



of nitric acid is owing to the decomposition of ammonia, 

 and it is conceived by him, that the last products of the de- 

 composition of animal bodies present themselves, in the 

 form of ammonia in cold, and in that of nitric acid in warm 

 climates. * Hence, in proportion to the amount of nitric 

 acid formed, and of nitre absorbed by the plant, that of the 

 nitrogen, and consequently that of the saccharine matter, 

 present in it, may be diminished. 



*'We may also be guided in the management and selec- 

 tion of manures, by the principles above laid down. The 

 solid excrement of animals varies of course in composition 

 according to the nature of their food : thus that of herbivo- 

 rous animals, which are fed principally on grasses, contains 

 much silicate of potass, as well as phosphoric salts, but 

 comparatively little nitrogen ; whilst human. faeces contain 

 little of the former ingredient, but much phosphate, and a 

 larger proportion of nitrogen. There will be seen even a 

 difference in these respects between the manure afforded by 

 the inhabitants of towns, fed principally upon animal food, 

 and that of peasants, who subsist in a greater degree upon 

 vegetables. 



**In like manner, the excrement of cattle is more effica- 

 cious as manure, when the animal is well fed, and under- 

 going the fatting process, than when it is more scantily 

 nourished. 



'* According to Sprengel, there is a difference between 

 different kinds of herbivorous animals in this respect, cows 



* " I have seen no attempt to account for the formation of nitrate of 

 soda in such large quantities in Peru, and may therefore offer the fol- 

 lowing, as at least a plausible solution. 



*' Wherever salt lakes occur, which become partially or wholly dried 

 up during a part of the year, carbonate of soda will be formed from the 

 decomposition of common salt. This I have observed myself on the 

 sandy plains of Hungary, in the neighborhood of Pesth. Now if any 

 circumstances should concur in such spots, calculated to generate nitric 

 acid, the latter, by its stronger affinity for the alkali, would take the 

 place of the carbonic acid, and nitrate of soda would result. 



" This, however, being a deliquescent salt, would not accumulate on 

 the surface, except in countries like Peru, remarkable for their extreme 

 dryness. 



" But how are we to account for the generation of so large a quanti- 

 ty of nitric acid in this locality .'* 



" If we suppose with Mr. Darwin, that the district in which the salt 

 is found was once a lake or inland sea, its change to dry land must have 

 caused the destruction of all its marine inhabitants. Now the decom- 

 position of their exuviae would, in a warm climate, present themselves, 

 as stated in the text, in the form, rather of nitric acid, than of ammonia. 



" Hence the production of so much nitrate of soda in Peru, is attrib- 

 utable to the heat ; its preservation to the dryness of the climate." 



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