CARBONIC ACID. 79 
Carbonic-acid gas is the important ingredient 
as far as our present subject is concerned. This 
gas is contained in all effervescing liquids, is pro- 
duced by burning wood or coal, or gas in the 
air; and also by the breathing of man, animals, and 
birds. It is a very heavy gas, much heavier than 
air—so that, in fact, it can be poured from one 
jar into another. It is moreover a highly poison- 
ous, deadly gas, and if breathed for a little time 
by any human being, it would rapidly destroy 
his life. Yet, as we have said, every living be- 
ing, at every moment of his existence, throws 
from his lungs this same gas. 
The composition of wood when analysed by the 
chemists is very simple: it may be said, to speak 
roughly, to be composed of one half of carbon and 
one half of the constituents of water, or hydrogen 
and oxygen gases. ‘The composition of carbonic- 
acid gas is one part of carbon, and two of oxygen 
gas; so that, to produce wood, we only require to 
take away the two parts of oxygen gas, and to add 
to it the constituents of water. But the highest 
art of the chemist cannot effect this. He can 
unmake wood, and accurately tell us what are its 
components ; but to unite its dissevered parts so 
