INFLUENCE OF AMMONIA OVER PLANTS. 55 



With (he ammonia, and form ammoniacal salts.* On the decomposi- 

 tion of these sahs by lime or otherwise — the organic acids which are se- 

 parated from them, are always more advanced towards that state in 

 vhich they again become fit to act as food for plants. 



8°. But the most interesting, and perhaps the most important proper- 

 ty of ammonia, is one which I have already had occasion to bring under 

 your notice, as possessed by water also, and as peculiarly fitting that 

 fluid for the varied functions it performs in reference to vegetable life. 

 This property is the ease with which it undergoes decomposition, either 

 in the air, in the soil, or in the interior of plants. 



In the air it is diflused through, and intimately mixed with, a largo 

 excess of oxygen gas. In the soil, especially near the surface, it is also 

 continually in contact with oxygen. By the influence of electricity in 

 the air, and of lime and other bases in the soil, it undergoes a constant 

 though gradual decomposition (oxidation), its hydrogen being chiefly 

 converted into water, and a portion of its nitrogen into nitric acid.f 



In the interior of plants this and other numerous and varied decom- 

 positions in all probability take place. 



The important influence which ammonia appears to exercise over the 

 growth of plants — the evidence for which I shall presently lay before 

 you — is only to be explained on the supposition that numerous transfor- 

 mations of organic substances are eflfected in the interior of living vege- 

 tables — which transformations all imply the separation from each other, 

 or the re-arrangement of the elements of which ammonia consists. In 

 the interior of the plant we have seen that water, ever present in great 

 abundance, is also ever ready to yield its hydrogen or its oxygen as oc- 

 casion may require, while these same elements are never unwilling to 

 unite again for the formation of water. So it is, to a certain degree, 

 with ammonia. The hj-drogen it contains in so large a quantity is ready 

 to separate itself from the nitrogen in the interior of the plant, and, in con- 

 cert with the other organic elements introduced by the roots or the leaves, 

 to aid in producing the dilTerent solid bodies of which the several parts 

 of plants are made up. The nitrogen also becomes fixed in the coloured 

 petals of the flowers, in the seeds, and in other parts, of which it appears 

 to constitute a necessary ingredient — passes oS'in the form of new com- 

 pounds, in the insensible perspiration or odoriferous exhalations of the 

 plant,— or returning with the downward circulation, is thrown off by the 

 root into the soil from which it was originally derived. Much obscurity 

 still rests on the actual transformations which take place in the interior 

 of plants, yet we shall be able in a future lecture, I hope, to arrive at a 

 tolerably clear understanding of the general nature of many of them. 



Such are the more important of those properties of ammonia, to which 

 we shall hereafter have occasion to advert. The sources, remote as 

 well as immediate, from which plants derive this, and other compounds 

 we have described as contributing to the nourishment and growth of 

 plants, will be detailed in a subsequent section. 



• Organic acids generally contain more oxygen in proportion to their carbon and hydro- 

 gen, than those which are alkaline or neulral. 



t It will be remembered that ammonia is represented by Nils, water by HO, and nitric 

 acid by NO5. It is easy to see, therefpre, how, by means of oxygen, ammonia should be 

 converted into water and nitric acid. 



