250 The Atmosphere as a source of Nitrogen to Plants. 
similar but improved conditions ; by degrees an accumulation of 
vegetable matter takes place, from the yearly increase in the 
number of plants, until, a true vegetable mould being formed, the 
bed becomes fitted for other classes of vegetation, and, in the 
lapse of time, the barren rock or the ruined tower becomes 
covered Avith luxuriant growth. 
So in the great forests of the western states of America, 
gigantic trees flourishing for centuries and shedding periodically 
their leaves and smaller branches, have formed a bed of vegetable 
mould, which fifty years of the most scourging crops hardly serve 
to exhaust. Again, covering hundreds of square miles, and of 
varying but great depth, we have enormous deposits of vegetable 
matter compressed into coal. No doubt exists of these beds having 
been formed by the growth and decay of successive vegetations, 
precisely in the same way that grass and turf give rise to vege- 
table mould in our present experience. 
]\ow it can hardly fail to have occurred to thinking men to 
inquire whence was derived the vegetable matter, which, on the 
barren rock or the ruined tower, in the great forest or the exten- 
sive coal bed, has year by year, and beginning from almost 
nothing, gradually accumulated : — tliat the soil does not furnish 
it is evident, and that the air must do so is equally plain. 
A further consideration of the natural composition of plants 
will show what it is which is thus supplied to them. Apart from 
the mineral matters of plants, which they may be supposed to 
derive from the earth, we have four different elements built up 
into every vegetable structure. It is as impossible for a plant to 
exist without these, or for a part of a plant to be formed and 
matured without the full proportion of any one of them, as it is 
for an infant to live and grow without food. 
These elementary substances are, as is Avell known to most 
readers, four in number, namely, carbon, oxygen, hydrogen, and 
nitrogen. It may be stated generally that there are no plants, 
and no organs of plants, in which the whole of these four elemen- 
tary substances are not found : they are, however, grouped into 
two very different classes of substances ; the one, as woody fibre 
or starch, containing the three first ; the other, of which the 
gluten of wheat may be taken as the type, containing in addition 
the fourth element — nitrogen. We have here little to do with 
these distinctions, the important point being to bear in mind that 
the existence of any plant is impossible without the necessary 
supply of every one of the constituents of its frame. 
As I said before, I do not propose to enter upon a history of 
the various discoveries in relation to the composition of the 
atmosphere, but with a view to a right understanding of our pre- 
sent inquiry, allusion must be made to some of the results. 
