FIXATION OF ATMOSPHERIC NITROGEN 397 



history of the planet there can be no doubt that the animal 

 organism would have adapted itself to it. It is pretty certain 

 that the composition of the earth's atmosphere has changed 

 considerably since life appeared on the globe, and the physio- 

 logical processes going on in the present animal and vegetable 

 inhabitants of the earth are the result of adaptation. It is true 

 that atmospheric nitrogen appears to take no direct part in the 

 animal economy. But it is now known that certain plants by a 

 mechanism of their own, namely the nodules swarming with 

 bacteria which are formed on the rootlets of plants of the natural 

 order Leguminosce, the bean and pea tribe, have the power of 

 taking in the nitrogen of the air and using it in building up some 

 of the albuminous or protein compounds contained in their 

 tissues. Plants, however, usually derive their nitrogen from 

 chemical compounds which are formed in minute quantity in 

 the atmosphere. 



Of these probably the ammonia results from products of decay 

 of animal and vegetable remains escaping into the air. But the 

 oxides of nitrogen which are formed, and which come down to 

 the soil in the form of nitrous or nitric acid with the rain, are 

 undoubtedly produced by electric discharges taking place through 

 the atmosphere, perhaps more or less at all times, but especially 

 during thunderstorms. This production of nitrate by electricity 

 can not only be demonstrated in a few minutes on the lecture 

 table, but has become the basis of a most important manufacture. 

 It has also been long known that certain metals when heated in 

 nitrogen gas combine with it forming a class of compounds 

 called nitrides. 



The companion element in the atmosphere, oxygen, enters 

 directly into a considerable number of combinations giving rise, 

 for instance, to the rusting of moist iron, and as everyone knows 

 it is taken up in the lungs of animals and changes the venous into 

 arterial blood. It is, however, a comparatively inactive gas 

 unless its temperature is raised. 



It would be useless, for example, to expect the ordinary 

 materials of fuel to burn in the air unless at some point it is 

 heated by the application of a flame. But by a silent electrical 

 discharge at the common temperature oxygen can be converted 

 into the very active substance known as ozone. This attacks all 

 sorts of substances which, under the same circumstances, would 

 be indifferent to common oxygen. Ozone consists of the 



