68 THE ATMOSPHERE THE PRIMARY SOURCE OF NITROGEN. 



insipid. But I need not dwell on this point, since I shall hereafter lave 

 occasion to draw your attention to certain facts in reference to the con- 

 stitution of the atmosphere, which will satisfy you that, by the agency 

 of comparatively feeble causes, gigantic effects are continually produced 

 in nature, — and that we can scarcely fall into a graver error in reason- 

 ing of natural processes, than by overlooking the agency of forms of mat- 

 ter which present themselves to our senses in minute quantity only. In 

 reference to insect life this truth has been long established.^ In the coral 

 reefs you are familiar with the wonderful results of the persevering la- 

 bour of minute animals in one element. When I come to explain the 

 nature and origin of soils, I shall have occasion to show that even the 

 element on which you labour — the earth, on the cultivation of which 

 your thoughts and hands are daily employed — is occasionally indebted 

 for some of its most valuable properties to a similar agency, often un- 

 seen by you, and though working for your good, unheeded and un- 

 thought of. 



Whence, then, is this nitrogen derived by plants ? The primary 

 source it is not ditficult to see. We can arrive at it by a train of reason- 

 ing similar to that which led us. to the atmosphere as the original source 

 of the carbon of plants. Nitrogen does not constitute an ingredient of any 

 of the solid rocks,* nor do we know any other source than the atmosphere 

 from which it can be obtained in very large quantity. It exists, as we 

 have seen, in many vegetables, and it is more largely present in animal 

 substances, but these organized matters must themselves have drawn 

 this element from a foreign source, and the atmosphere is the only one 

 from which we can fairly assume it to have been originally derived. 



But though the nitrogen, like the carbon of plants, may thus be traced 

 to the atmosphere — as its orginal source — it does not follow that this 

 element is either absorbed directly from the air, or, in an uncombined 

 and gaseous state. Though the leaves of trees and herbs are continually 

 surrounded by nitrogen, the constitution of plants may be unfitted for 

 absorbing it by their leaves. The nitrogen may not only require to be 

 in a state of combination before it can enter into the circulation, but it 

 may also be capable of gaining admission only by the roots. These 

 points are considered in the following section. 



§ 6. Form in which the nitrogen enters into the circulation of plants. 



The question as to the form in which nitrogen enters into the circula- 

 tion of plants is one which at the present moment engages much attention. 

 It will be proper, therefore, to discuss it with considerable care. 



1°. It is considered an essential part of good tillage to break up and 

 loosen the soil, in order that the air may have access to the dead vege- 

 table matter, as well as to the living roots which descend to considerable 

 depths beneath the surface. When thus admitted to the roots, it is j*rf. 

 possible that some of the nitrogen of the atmosphere, as well as some of 

 its oxygen, may be directly absorbed and appropriated by the plant. 

 To what extent this absorption of nitrogen may proceed, however, we 



• Except coal, and coal itself is of vegetable origin. Throughout all rocks in which or- 

 pinic remains are found, more or less animal matter containing nitrogott is to be met with, 

 out these remains are only accidentally present, and they must have derived their nitrogen 

 during life, either directly or indirectly, from the atmosphere. 



