QUANTITY IN WHICH IT IS REPRODUCEI . 163 



the decomposition of ammonia, much, by far the greatest proportion in 

 all probability, derives its origin from tlie union of the elements of (Jie 

 atmosphere itself. This direct anion is elTecled in the air, as has been 

 already shown, by the agency of atmospheric electricity; but it also 

 takes place in the soil during the oxidation of the other elements con- 

 tained in the organic matters which are there undergoing decay. The 

 combination of the elements of ammonia in such circumstances proceeds 

 on the principle that bodies, themselves undergoing oxidation, dispose 

 other substances in contact with them (in this instance the nitrogen of 

 the air) to unite with oxygen also. The presence of lime, potash, &:c. 

 in the soil, further induces to this oxidation by the tendency of these sub- 

 stances to combine with the acid which is formed by this union of the 

 elements of which nitric acid consists. — It is impossible precisely to es- 

 timate the quantity of nitric acid produced in these various ways, through 

 these various agents, and in these varied circumstances, or to balance it 

 accurately against the amount of ammonia continually reproduced, as 

 we have seen, in nature, wherever the necessary conditions present 

 themselves. But, as I formerly concluded, that the amount of nitric 

 acid actually existing in the superficial deposits of our globe is greater 

 than that of ammonia, so I think that, in regard to the reproduction also 

 of these two compounds, the balance is in favour of tjie former. 



Since, then, nitric acid is fitted, by the solubihty of its compounds, to 

 enter into the circulation of plants in any quantity — since, when applied 

 to them, it does undoubtedly promote, in a remarkable degree, the growth 

 of plants — and since, in nature, it is continually reproduced in every 

 country, and under such varied circumstances — I cannot withhold my- 

 self from the conclusion, that, over the general vegetation of the globe, 

 it holds with ammonia at least an equal sway, and is appointed to exer- 

 cise at least an equal influence over the growth of plants, both in their 

 natural and in their cultivated state. 



Still the influence of each is not unvaried by locality or by climate. 

 The extent of dominion exercised by the nitrates probably diminishes as 

 we recede from the equator, while that of ammonia increases, — it may 

 be in an equal proportion. The reason of this probable variation will 

 appear in the following section. 



§ 6. Theory of the action of nitric acid and ammonia. 

 These two compounds act so far in common as to yield a supply of 

 nitrogen to the plants into which they enter. They do so, however, un- 

 der conditions which may be considerably different, and may be attend- 

 ed by unlike chemical changes. 



I. THEORY OF THE ACTION OF NITRIC ACID. 



1°. The nitric acid of the nitrates entering into the circulation of the 

 roots will ascend to the leaf, and will there be decomposed in the same 

 way as the carbonic and other similar acids are, by the action of the 

 sun's rays. It is only in the light of day that carbonic acid is decom- 

 posed in the green parts of plants — so must it be, generally, with the 

 nitric acid which ascends to the leaf. Its oxygen will be given off*, 

 while its nitrogen may be retained in the circulating system of the plant. 

 The extent to which this decomposition will take place at each passage 



