105 
OF GARDENING AS A SCIENCE. 
NO. y. 
We resume our inquiry at tlie point where we quitted it at page 84, with the 
allusion to that most wonderful provision of nature, by which a gas of very 
poisonous and deadly quality is removed from the atmosphere, and converted by 
the agency of vegetation to the food of plants, and the comfort of man. 
Carbonic Acid, or fixed aerial acid, is the inevitable product of combustion 
and respiration ; and the atmosphere is the recipient of the gas so produced. Any 
one may convince himself that it is always present, by exposing a little pure fresh- 
burnt lime, or a glass of strong, and brilliantly clear, lime-water to the air, for a 
day or two. Neither of these would hiss, (eifervesce,) were a drop or two of any 
acid applied to it ; but, after being so exposed to the air for some time, the lime 
would be converted to carbonate of lime, and the lime-water would lose its pellu- 
cidity, and be covered with a scale of a brittle, insoluble substance. Both would 
effervesce, with the extrication of frothy air-bubbles ; and these changes are pro- 
duced by the attraction exerted by the pure lime upon the carbonic acid of the 
atmosphere, which causes a union of the two, and the formation of that neutral 
salt of lime commonly known by the term chalk. We approach to an estimate 
of the actual quantity of this acid contained in the air by perusing the note at 
page 17 of Liebig's work. 
" The air contains, in maxim 0, -rirf thrTT of carbonic acid, and tVwVo" of oxygen 
gas. A man consumes in one year 166075 cubic feet of oxygen gas (or 45000 
cubic inches in one day, according to Lavoisier, Seguin, and Davy) ; a thousand 
million men must accordingly consume 166 billion cubic feet in one year ; this is 
equal to two of the quantity which is contained in the air, in the form of 
carbonic acid. The carbonic acid in the air would thus be doubled in a 
thousand years, and man alone would exhaust all the oxygen, and convert it 
into carbonic acid, in three hundred and three times as many years. The con- 
sumption by animals and the process of combustion is not introduced into 
the calculation." 
There is not, perhaps, in the whole economy of wonder- displaying nature a 
fact more conclusive, more simple, and yet more astounding than the one which 
the foregoing extract reveals. For if some agency were not unceasingly at work 
to withdraw that volume of poisonous gas, which every act of combustion and 
respiration pours into the air, the whole would inevitably be converted into a pes- 
tilential vapour. Nitrogen is destructive of breathing life — and of it we have seen 
that no less than fths of the entire atmosphere consists ; the remaining fifth of 
oxygen is the sole meliorator — and of it every breath we inspire consumes a portion. 
Then, must we not inquire with Liebig, " How does it happen that the proportion 
VOL. VIII.—NO. LXXXIX. P 
