ges of warming and moistening the air thus admitted, are very
important ones; for when either a cold or dry state, of the atmosphere
prevails, its influence is very injurious to plants in these confined
situations: cold raw air, when it comes in contact with the tender foliage
of the plants, has the effect of chilling the sap in its progress through
their tissue, and thus lessening their excitability, when it should be
increased; whilst dry air acts as an incessant drain upon the vegetable
juices, which it abstracts through the stomates and pores of the leaves
and stems. When cold air is admitted to any position where it can unite
with caloric, and not in an equal ratio with moisture, it necessarily
becomes arid, and in that state it eagerly combines with moisture in any
form with which it can come in contact therewith; and consequently if cold
air is admitted to a plant structure, where it can have the means of
combining with heat, faster than with moisture, it would be brought into
this arid state, and would supply its voracious appetite, by abstracting
the juices of the plant. It is a very important question how far this
state of things is connected with many of the diseases as they are called,
to which plants are subject; for my own part, I believe it to have a very
considerable influence in the production of many of them. A shallow bed of
soil (_k_), is all that would be required; for in the winter season, there
is nothing gained by encouraging a very luxuriant and gross state of
growth: the composition of this soil will be noticed hereafter: beneath
it, and resting on the top of the tank, should be placed a layer of coarse
open rubble, not less than six inches in thickness; and among this rubble
by means of tubes (_n_), placed at intervals along the bed, I would
occasionally pour considerable quantities of water, in order to maintain a
due regulation of moisture in, and throughout the soil, among which the
vapour arising from the water would ultimately rise. Beneath the tank a
space (_o_), might be provided, which would serve admirably either for the
cultivation of Mushrooms, or the forcing of Rhubarb, or Sea Kale.
Transverse partitions should be introduced into the bed of soil, so as to
divide the roots of each plant from those of its neighbours: this
arrangement will admit of a complete succession of plants being
maintained, by the removal of those which have become old and debilitated,
and the substitution of youn