The Atmosphere as a source of Nitrogen to Plants. 
251 
The discoveries of Bergman, Priestley, Lavoisier, and others, 
have shown that the air consists of two gases — oxygen and nitro- 
gen, in the relation of one part of the former to four parts of the 
latter; these gases not being in chemical combination, but merely 
in mechanical mixture. Further, it is believed that the chief 
function of the nitrogen of the air is of a negative character, that 
is to say, it dilutes the oxygen and prevents the violent action 
which it would exert in nature if it were not so diluted. 
Black investigated the nature of carbonic acid, or " fixed air " 
as it was first called, demonstrating its production by combustion 
and the respiration of animals ; and subsequent observers proved 
the existence of this gas in very definite and uniform quantity in 
the air at all elevations.* 
Priestley and Saussure, by the most ingenious and interesting 
experiments, determined the action of plants in decomposing car- 
bonic acid and appropriating its carbon. 
Cavendish made us acquainted with the nature of water, re- 
solving it into its elements, hydrogen and oxygen. He was also 
the author of a very singular experiment to which we shall have 
to allude presently. 
In air and water, then, we have apparently all the organic 
elements necessary for the growth of plants ; in air, oxygen 
and nitrogen in abundant quantity ; and in lesser but still ade- 
quate proportion, carbon in the form of carbonic acid. In 
water — hydrogen, and a further supply of oxygen. Of the de- 
composition of carbonic acid and water by vegetation, and the 
convertibility of their elements into the proximate principles of 
plants, we have not here to speak ; abundant evidence exists of 
such decomposition and conversion, a<id it is taken for granted, 
that in this way provision is made for three out of the four orga- 
nic elements necessary to vegetable growth. If the power of a 
plant to appropriate the atmospheric nitrogen were equally clear 
and incontestable, we should have no occasion to go further — 
the great abundance of this gas as an ingredient, and the chief 
ingredient of the air, render it superfluous to look for any other 
source of its supply. The evidence on this point, however, 
is far from proving the capacity of nitrogen, as one of the con- 
stituents of the bulk of the air, to furnish the nitrogen, which, 
in the form of gluten, vegetable albumen, vegetable casein, &c,, 
* It may be remarked in passing, that the proportion of carbonic acid in air 
has been found to vary from .37 to 62 parts in every 100,000 parts of air. This 
quantity, small as it may at first sight appear, is, in reference to the whole bulk 
of the atmosphere, very great. It has been computed liy Professor Liebig that it 
would be adequate to supply the carbon contained in all the deposits of coal on the 
crust of the globe, and that it is abundantly sufiBcient for all the purposes of a 
natural vegetation. Without entering upon the question of the supply of carbon 
to plants in the sense of their artificial cultivation— a question of the highest 
