518 
On tlic Food of Plants. 
taking the very lowest estimate, and then calculate how much of 
this poisonous gas is annually poured into the atmosphere by one 
single large city. 
Processes going on beneath the surface of the earth are often 
fruitful sources of carbonic acid. A German writer, Bischofi of 
Bonn, quoted by Liebig, considers that from the ancient volcanic 
district of the Eifel, near Coblentz, on the Rhine, no less than 
ninety thousand pounds of carbonic acid are daily sent into the 
air from this small spot alone. Consider the number of active 
and extinct volcanoes in various parts of the world — along the 
range of the Andes, for example — and think how largely these 
contribute to the contamination of the air. And yet, after these 
actions, or at least some of the most potent among them, have 
been going on in all probability for periods of time so vast as to 
surpass conception, the quantity of carbonic acid in the atmosphere 
amounts but to YuVoth part of its volume. It is impossible to 
doubt that some efficient method for its removal has been pro- 
vided, and that method is the agency of plants. The carbonic 
acid thrown into the air from so many sources is by them re- 
moved : its carbon furnishes them food, while the oxygen (useless 
in such abundance to vegetable, but absolutely required for the 
maintenance of animal, life) is sent back again to perform its 
proper function. Animals and plants are thus mutually sub- 
servient to each other's existence. 
The source of the hydrogen of plants will present little diffi- 
culty. It is derived from the decomposition of Mater — at least in 
great part. As a large portion of every plant consists of sub- 
stances in which the oxygen and hydrogen are in the relation to 
form water, no excess of oxygen will attend the production of 
these bodies ; but in resinous and oily products generally, oxygen 
will be liberated, and will add itself to that set free from the car- 
bonic acid. 
Nitrogen, in the shape of albuminous matter, appears to be in- 
dispensable to the development of living plants in every stage of 
their existence. The immense provision of that substance in the 
seeds of most vegetables is a proof of its importance ; and the 
care which nature has thus taken to guard against any deficiency 
in this respect, to the young plant, is very remarkable. We are 
ignorant, however, of the precise duty it fulfds, and mere con- 
jecture will advantage us but little. 
The origin of this nitrogen, present in all vegetables, is an 
inquiry of very great importance. There is no difficulty in 
showing, by a process of reasoning similar to that used in relation 
to the carbon, that, if not the whole, by far the greater part of 
the nitrogen contained in wild plants — forest-trees, for example — 
