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