164 THEORY OF THE ACTION OF NITRIC ACID AND OF AMMONIA. 



of the sap through the leaf will depend, in some degree, on the nature 

 of the base (whether potash, soda, or lime,) with which the acid is in 

 combination, but much more on the intensity of the light to which the 

 green parts of the plant are exposed, and on the temi)erature of the air in 

 which the plant hapj)ens to grow. ^ 



2°. It is still uncertain whether this acid is capable of being decom- 

 posed in the roots or stems of plants where it is excluded from the light, 

 though it is very probable that it may be so, especially in cases where 

 the juices naturally contain substances in which hydrogen is jiresent in 

 excess, or where such compounds make their way into the circulation 

 of plants from the manure that may be applied to their roots. 



Thus in the pines, in which turpentine (C40 Hg^) naturally abounds, 

 such a decomposition may the more readily occur, inasmuch as it would 

 not necessarily imply the production and evolution of any gaseous sub- 

 stance. Thus 



1 of Oiii OF Turpentine, = C40 H32 with the oxygen of 



1 of Nitric Acid (NO5) = O5 gives 



1 of Resin, = C40 H32 O5 



By uniting with the oxygen of the nitric acid, therefore, oil ot turpen- 

 tine, in such tree*, might be changed into resin during its passage 

 through the stem, while the nitrogen, being set free, might, at the mo- 

 ment of its liberation, unite with other elements to form those parts or 

 productions of the tree into which this element enters as a necessary 

 constituent. 



The above must be considered merely as an illustration of the kind of 

 changes which may possibly take place in the interior of certain plants, 

 and in the absence of light, when the nitrates happen to be present. 

 Were I to affirm that such changes actually do occur in the presence 

 of nitric acid, the theoretical chemist would have a right to expect that 

 several collateral questions should be discussed, the consideration of 

 which would here be out of place. 



3°. The nitrates may also act in another way, which does not involve 

 the necessity of the total decomposition of the acid they contain. We 

 know that in nature many substances are capable of inducing chemical 

 changes in other compound bodies, without themselves undergoing de- 

 composition. Some beautiful illustrations of this have already been 

 given in a previous lecture, when treating of the action of sulphuric acid 

 upon starch and woody fibre, [Lecture VI.. pp. 113, 114.] But the fact 

 which most immediately bears on the influence of the nitric acid in the 

 living plant, is that mentioned in p. 126, — that by solution in this acid 

 in the cold, starch is converted into a substance having the composition 

 of woody fibre. In the interior of the plant changes of this kind may 

 be produced by simple contact only, with the nitric acid, so that, with- 

 out being decomposed, it may be materially serviceable in promoting 

 those molecular changes which are necessary to the healthy and rapid 

 growth of the plant. 



II. — THEORY OF THE ACTION OF AMMONIA. 



1°. Ammonia is capable of contributing to ihe growth of the plant, 

 by means of the hydrogen, as well as of the nitrogen it contains. We 



