150 
ON CHEMISTRY. 
by Davy that, in places where there is a free circulation of air, it is 
probably never more than ^ nor less than ^ of the volume of the 
• n 
air. 
Upon the composition of this gas, formerly known by the terms 
aerial acid, and fixed air, he observes, that if thirteen grains of well 
burnt charcoal be inflamed by a burning-glass, in one hundred 
cubical inches of oxygen gas, the charcoal will entirely disappear; 
and provided the experiment be correctly made, all the oxygen ex¬ 
cept a few cubical inches, will be found converted into carbonic acid; 
and, what is very remarkable, the volume of gas is not changed. On 
this last circumstance, it is easy to found a correct estimation of the 
quantity of pure charcoal and oxygen in carbonic acid gas: the 
weight of one hundred cubical inches, is to that of one hundred cu¬ 
bical inches of oxygen gas as forty-seven to thirty-four; so that 
forty-seven parts in weight of carbonic acid gas must be composed of 
thirty-four parts of oxygen, and thirteen of charcoal.” (See 5tli 
Agricultural Lecture.) Now by Henry’s table, upon the authority 
of Kirwan, one hundred cubical inches of oxygen gas weigh thirty- 
four grains: hence, if the one hundred inches be attracted during 
the combustion of thirteen grains of charcoal, and united therewith, 
the result must be about forty-seven grains of carbonic acid gas, and 
this was the fact to be demonstrated. Carbonic acid derives its 
name from the latin word Carlo , (see Vol. 2, page 437,) and from 
the combination of that base, with the supposed acid, producing 
agent, oxygen (ib. page 436.) It is soluble in water to perhaps, the 
extent of rather more than the bulk of that liquid to which it then 
communicates acid qualities. In its free state, it exists in the form 
of gas; but it is attracted, and fixed in the solid form, by many che¬ 
mical bases, by lime particularly, which it renders mild. This mild 
lime is chalk, chemically termed carbonate of lime: the facts con¬ 
nected with carbonic acid as a vegetable aliment, will come under 
notice in a future article. 
Another substance is traceable in atmospheric air, as one of its 
apparent components; though chemists generally consider Azot and 
Oxygen to be the sole essential constituents of true respirable air. 
Water is stated to exist in the form of vapour at all times, but in 
varying proportions, according to the temperature of the atmospheric 
volume. 
I have spoken at some length upon the chemical nature of water 
in my second article, but much remains to be said when that fluid 
is viewed in connection with the constitution of atmospheric air; 
and I request the reader to bend all his attention to the following 
