166 MODIFYING EFFECT OF CLIMATE. 



The immediate and visible effect of an application of ammonia, or of 

 soot, or of any top-dressing containing ammonia, is to render the green 

 colour much more intense, and in the darkest weather. It is therefore 

 probable, I think, that the hydrogen of the ammonia contributes to this 

 immediate effect, and that the ammonia itself may be decomposed and 

 its elements appropriated to the nourishment of the living vegetable, 

 either by the unaided vital powers of the plant, or in the presence of a 

 feeble light only. Like water, ammonia is peculiarly liable to decom- 

 position, not always of that perfect kind which, /or the sake of simplicity ^ 

 I have endeavoured to explain in the present lecture, yet such as to ren- 

 der the elements of which it consists available to the general nourish- 

 ment of the plant. 



§ 7. Comparative influence jf nitric acid and of ammonia in different 

 climates. 



It follows, from what is above stated, that the beneficial influence of 

 ammonia upon vegetation will be readily perceived in all climates in 

 which plants are found to flourish. Its effects will be greater and more 

 rapid where the heat and light are more intense, — only because by these 

 agents the functions of all life are stimulated. 



Not so with the nitric acid in the nitrates. In the presence of organic 

 compounds, that is, in the sap of the plant, it is less easily decomposed 

 than ammonia. It requires the interference of more powerful agents — 

 of a higher temperature, or of more brilliant light, — and thus its efficacy 

 upon vegetation will be more dependent upon season and climate. 



Now, we have seen that in tropical countries the nitrates are produced 

 in the greatest abundance, and there the high temperature and the bril- 

 liant sun should render them most useful to vegetation. Such is well 

 known to be the case, and it may be regarded as one of those bountiful 

 adaptations with which all nature is full — that in these warmer regions, 

 the ammonia produced in the soil is first converted into nitric acid, that 

 it may remain fixed, and that this acid again is decomposed by the same 

 agents (light and heat), when it enters the living plant, and is required- 

 to minister to its growth. On the other hand, it may no less be regarded 

 as a wise provision, that in colder and more uncertain climates, where 

 warm and brilliant summers are less to be depended upon, that com- 

 pound of nitrogen (ammonia) should more abound, which is most easily 

 decomposed in the living plant, which is fitted in comparative darkness 

 to yield up its nitrogen, and by the hydrogen it contains, to corrtpensate 

 in some slight degree for the partial absence of the sun's rays. 



From these views, therefore, we should draw this further practical 

 conclusion — that in our climate, ammonia is sure to promote vegetation, 

 and in every season, while the nitrates will produce their maodmum e&^ecx^ 

 othei things being equal, in such only as have abundant warmth and 

 sunshine. Is this conclusion consistent with observation? Will it 

 serve to explain any of the apparent failures which have occasionally 

 been experienced in the employment of the nitrates ? 



§ 8. Stimulating influence of these compounds. 

 There remains one other point iix regard to the effect of these two 

 compounds upon vegetation, to which I would request your attention. 



