y the ordinary process of opening the
sashes of forcing houses, has been said to be unnecessary, or at least by
no means important, in so far as the function of vegetable respiration is
concerned, because the buoyancy of the air within all such structures,
would enable it to escape in sufficient quantity through their openings
and crevices, to counterbalance any thing like deterioration, which might
by any means result from the vital action of the plant. The admission of
external air, is also directly injurious to forced plants, during the
winter and spring months, when a very material difference of temperature
exists between it, and the internal volume, by contracting the vessels,
impeding the circulation of the juices, and thereby checking the regular
course of the growth of the plant. If these reasons fail to stamp it as a
practice which ought not largely to be indulged in, it is further
objectionable, as being productive of a prodigal expenditure of fuel:
there can be little doubt but that generally speaking, a far greater
quantity of fuel than is requisite, is expended in maintaining the
temperature of forcing houses, solely from this cause; for the cold air
when admitted, continues to abstract a portion of heat from the warmed
air, until the temperature of both becomes equal, and consequently an
increased application of fuel is requisite, in order to raise the newly
admitted air to the same temperature as that which has been suffered to
escape; and as the buoyancy of heated air is so great, an immense volume
must necessarily rush out through a very small aperture, and thus there
must also of necessity be an immense waste both of heat, and of fuel. A
given portion of fuel, in its combustion, can give off but a certain
proportionate ratio of heat, and if this is allowed unnecessarily to
escape, the prodigality is self-evident. It is but a weak argument, which
would seek to give to the admission of cold air, the office of regulating
the temperature of plant houses; this ought to be effected by limiting the
degree of heat _applied_, and not by attending to the _abstraction_ of
that which had been previously administered with two lavish an hand.
Besides the extravagance of such a course, the constitutional vigour and
energy of the plants is at the same time sacrificed by undue excitement.
The admission of cold air in large quantities, therefore, brings
condemnation in its train, since it is unnecessary, and extravagant, a