ATMOSPHERIC AIR AS THE FOOD OF PLANTS. 39 
burned or decays in contact with the air. It is like oxy- 
gen, colorless, but it has a peculiar pungent odor and 
pleasant acid taste. 
The composition of carbonic acid is evident from what 
has been said as to its production from carbon and oxygen. 
Tt consists of two atoms, or 32 parts by weight, of oxygen, 
united to one atom, or 12 parts, of carbon. Its symbol is 
CO, In the subjoined scheme are given its symbolic, 
atomic, and percentage composition. 
At, wt. Per cent. 
c = B 27.27 
oO = 8 72.73 
CO, = 44 100.00 
Tn a state of combination carbonic acid exists in nature 
in immense quantities. Limestone, marble, and chalk, 
contain, when pure, 44 per cent of this acid united to lime. 
These minerals are in chemical language carbonate of lime. 
Common saleratus is a carbonate of potash, and soda- 
saleratus is a carbonate of soda. 
From either of these carbonates it is easy to separate 
this gas by the addition of another and stronger acid. 
For this purpose we may employ the Rochelle or Seidlitz powders so 
commonly used in medicine. If we mingle together in the dry state the 
contents of a blue paper, which contains carbonate of soda, with those of 
a white paper, which consist of tartaric acid, nothing is observed. If, 
however, the mixture be placed at the bottom of a tall bottle, and a little 
water be poured upon it, at once a vigorous bubbling sets in, which is 
caused by the liberated carbonic acid.* 
Some important properties of the gas thus set free may be readily 
made manifest by the following experiments. 
a, If a burning taper or match be immersed in the gas, the flame is 
immediately extinguished. This happens because of the absence of free 
oxygen. 
b. If the mouth of the bottle from which carbonic acid is escaping be 
held to that of another bottle, the gas can be poured into the second ves- 
sel, on account of its density being one-half greater than that of the air. 
Brook that the invisible gas has thus been transferred is had by placing 
* Chalk, marble, or saleratus, anal chlorhydric Quuriatic) acid, or strong vine: 
gar (acetic acid) can be equally well employed. 
