106 
BOTTOM HEAT. 
no doubt on the subject ; but we have read and observed enough to authorise a 
few inquiries and suggestions. A good and efficient tank ought to comprise an 
expansive pair of water-channels, one of which is connected with the " flow," the 
other with the "return-pipe" of the furnace. These channels should be enclosed 
in a deep, oblong chamber of brick-work, so constructed as to contain an air- 
chamber, heated by radiation from the surfaces of slates laid sloping from the edges 
of the water-channels, and expanding to the top course of the sides of the brick 
walls, where they are joined by a broad, covering-slab of thick slate, upon the four 
flat edges of which a brim or border of bricks is placed. By this arrangement a 
space, the entire breadth of the slab, is obtained, capable of containing a body of 
materials wherein to plunge pots of various dimensions, either to their rims, or to 
any extent which may suit the object of the gardener. Beside the heated dry air 
obtained from the outer surface of the slates, and which can pass through pigeon- 
holes in the sides of the brick chamber into the air of the house, a body of vapour 
can be conveyed from the hot- water channels through orifices made at the two 
ends of the brick case, which can be closed wholly or partially, according to 
circumstances. 
We have thus all the appliances of a well-regulated source of bottom-heat ; 
but what are the materials which should constitute the plunging medium ; and is 
heat or temperature the sole object worthy of consideration ? 
It has been stated that some gardeners who have tried the tank, are recurring 
to tan, or other fermenting sources of heat ! Is this true in the abstract — and if it 
be, why is a medium ever subject to sudden mutations, substituted for another that 
is comparatively permanent and certain in its effects ? Again, let us consider the 
inconveniences of tan, or of a bed of leaves covered with a layer of tan, or a 
compound of stable-dung and leaves. All of these are, in the first instance, liable 
to heat violently ; insomuch that whole collections of most valuable plants have 
been irremediably destroyed in a few hours. This degree of temperature declines, 
but not always regularly ; it is influenced by atmospheric conditions ; it fluctuates, 
becomes irregular— is lost ; and even under the most propitious course, the bed 
subsides, requires renewal, is changed in its texture, and favours the production of 
worms, grubs, and myriads of devouring millepedes. On the contrary, if we 
superpose upon the slab of the water-tank a stratum of sand and powdered 
charcoal, to the depth of from five or six to seven or nine inches, and bring these 
materials to the moist condition of tan, or of vegetative mould, we acquire a 
plunging medium perfectly at command with respect to temperature, ever clean, 
scarcely, if at all, liable to change ; and by no means favourable to the introduction 
of predatory vermin. 
The question now presents itself at once ; is this clean and convenient medium 
equal in every respect to that of the old fermenting bed ; or does the latter surpass 
in some qualities which give it essential pre-eminence ? 
Experience must decide, for the subject is quite new. In the mean time it is 
