202 
OF GARDENING AS A SCIENCE. 
acid in tliree liiindred and three times as many years. The consumption by 
animals, and by the process of combustion, is not introduced into the calculation." 
Every act of respiration, of combustion, and of fermentation, develops carbonic 
acid, which passes into, and blends with the atmospheric volume ; but we must 
postpone the consideration of this gas for the present ; it is too important to be 
slurred over. 
The theory above alluded to, that the atmospheric gases exist in the state 
of simple mixture — not in that of chemical union, is strengthened, 1st, by the 
following experiment by Dr. (then Mr.) Dalton : — Two jars of different kinds of 
gas being placed one above another, but properly connected together, the upper 
vessel containing the lighter of the two airs ; a portion of this lighter air descended 
through the heavier air, while a part of the latter ascended to, and through the 
lighter one, till an entire and uniform mixture was produced." Upon this great 
fact among others is founded the hypothesis that every gas is a vacuum to any 
other gas, lighter or more ponderous. 
2nd, By the comparative gravities of the gases, alone, and in a state of mixture. 
Nitrogen is lighter than atmospheric air, and still lighter than oxygen gas ; for one 
hundred cubic inches of nitrogen are estimated (under a mean pressure and tempe- 
rature) at 30'16 grains. Whereas the same volume of oxygen weighs 34*60 
grains. But if the two be blended together in the proportion of seventy-eight or 
seventy-nine parts of the former and twenty- one parts of the latter (or in round 
numbers about four and one), a mixture will be produced which, as respects weight 
and chemical properties, corresponds with those of the atmosphere, making allow- 
ance for the carbonic acid and aqueous vapour which are always existent in the 
latter ; thus the weight of one hundred cubic inches of the two is found to be very 
nearly 31*011 grains. 
3rd, By the chemical axiom that " whenever two gases or liquids unite chemically 
the compound has greater density than the mean density, thus the vapour of 
water, at the heat of boiling, occupies much less space than the hydrogen gas and 
oxygen gas which compose it would have occupied at the same temperature." 
With reason, therefore, it is concluded, that atmospheric air is not a chemical 
compound in the ordinary acceptation of the term ; and it now remains to inquire, 
what is the agency which it exerts upon the vegetable creation ? In order, however, 
to afford scope for mature reflection, it will not be irrelevant to refer to an article 
which appeared in the Gardeners Magazine of 1834, Vol. X. page 207, " O^i 
growing Ferns and other plants in glass cases in the midst of the smoke of London," 
by Mr. Ward, Surgeon. Before we cite the precise description of the cases by the 
author himself, it is proper to mention that, on the 2d of September of the present 
year, after a lapse of 7^ years, we inspected the collection, which now consists, not 
only of glass cases, in windows facing the south, the enclosed atmosphere of which 
is sometimes raised by solar heat to 100°, but of a close sort of fossil greenhouse at 
the back of the house, glazed with puttied laps, which resembles a kind of damp 
