GARDENING AS A SCIENCE. 179 
life. But besides these chief constituents of air, it is proved that a certain portion 
of aqueous vapour, (i. e., water in the form of air,) perhaps about one part, a 
quarter, or rather more, in every hundred parts is always present, and likewise, 
a still smaller part of another of the products of combustion. We find the 
components of air laid down in the form of a table, thus :■— 
Azote by measure ..... ... 77*5 
Oxygen „ 21- 
Vapour „ 1-42 
Carbonic Acid „ , -8 
100 • 
As animals cannot live in air so depraved, neither can plants. What therefore, 
we may inquire, can restore air, deteriorated as it is by the processes of respiration 
and of burning, always going on throughout Nature ? 
Dr. Priestley's experiments tend to prove that plants by their vital agency — 
that is groiving plants, pour from their leaves a volume of oxygen sufficient to 
restore that lost by the air, and also to attract so much of the other deadly gas, 
called carbonic acid, which is produced by combustion, and also by the respiration 
of man and animals ; for, in fact, breathing implies a slow but perfect phenomenon 
of combustion, as one of its processes. 
It unfortunately happens, that all our experiments upon living plants are 
conducted with machinery, and under circumstances, which do not tally with the 
order of Nature. Therefore, as bell-glasses, certain gases, powerful sun-beams 
admitted through glass, and other concomitants, place a plant in an unnatural 
position, we must be somewhat jealous of our inferences. However, results have 
been observed whicli lead to some accurate conclusions ; as for instance that of the 
deterioration of air by combustion and respiration, and also, to a shrewd suspicion 
that plants may be the renovators of the atmosphere. 
Whether this latter doctrine be correct or not, certain it is, that azote is one 
of the components of ammonia (volatile smelling salts). And if it meet with the 
other — i. e. hydrogen^ which must be liberated whenever water is decomposed, a 
chemical union may be formed by the agency of natural electricity, by which the 
azote will be brought into a condition wherein it is rendered not only soluble in 
water, but capable of being absorbed by the earth. 
It will thus be apparent that the machinery of Nature is adequate to maintain 
all its elements within one harmonizing circle. Under certain circumstances 
indeed, not appreciable by man, disorder does for a time occur, epidemic poisons 
prevail, and sickness or debility is the inevitable consequence ; thus the gardener 
perceives a healthy plant to flag, and dwindle. 
But, as a whole, the harmony goes on ; and that which threatens to become an 
instrument of destruction, is found to constitute a link only of that great chain 
of reconstruction which is never interrupted. 
