12 
APPLICATION OF HEAT. 
make some allusion to a method of heating pointed out in recent numbers of the 
Gardeners' Chronicle. 
We confess, and many readers must feel, that it is no easy matter to depict, and 
render intelligible by plan and letter-press, the arrangement of a forcing-house. 
The Polmaise mode of procuring a warm and vaporous atmosphere without flues, 
water-channels, or tanks, is very interesting ; and it is hoped will be brought to 
rigid and impartial trial by those who have either funds at command to render outlay 
a secondary consideration, or sufficient energy to undertake in confidence what may 
appear to promise success. 
Still we must insist upon one chemical fact — and it is self-evident— namely, 
that fuel, in the act of combustion, can do no more than yield that volume of 
radiating matter, (call it caloric , latent heat, electrized vapour, or what you will), 
which is developed by some sulphuretted hydro-carbon, like coal, combined with a 
portion of bitumen , when brought into a state of combustion by oxygen gas. 
We do not now discuss the philosophy of combustion, but certainly assume, that 
as there is a furnace connected with the drains of the Polmaise process, that the 
apparatus can by no means develope and distribute more heat by the circulation of 
heated and cool air, than it could liberate if conveyed through the media of brick 
flues, water-gutters, or any other channels with radiating surfaces. So far as fire is 
concerned, the expense cannot be thus economised ; therefore, the great points at 
issue are, the first expense, and the permanent equalisation of a healthy renewable 
atmosphere. Such points, it is to be hoped, will be satisfactorily ascertained, and 
frankly given to the public. 
As to the medium for conveying and retaining a temperate bottom-heat — (and, 
by the bye, where do we find its applicability at all within the compass of the 
Polmaise machinery ?) — we must at present retain our predilection for tan, leaves, 
and such like decomposable substances, but solely because the chemical play of 
their constituent elements secures a constant stream of vivifying gases to the soil, 
and developing roots which experience shows to be most congenial to vegetable 
progress. Those decomposing materials are subject to serious objections, as they 
are not only productive of insects and grubs — among the most vexatious of which are 
the woodlice ( Oniscus ) — with which tan abounds ; also again, by the attraction which 
their decomposition excites, to induce the roots to pass the holes of the pots and 
to ramify extensively in the bed. 
For the purposes of propagation, therefore, we should be content with some 
negative substances which will neither ferment, decay, nor foster vermin ; and such 
are nicely- washed, unbinding sand, mixed up with one third part of charcoal dust. 
The tank-bed ought to be six or more inches deep, and the plunging materials made 
to occupy the whole space to within an inch, or an inch and a half, of its top. If 
properly moistened and kept in that condition, a very pleasant sweet warmth will be 
created, and may serve every purpose of propagation. 
We perceive that allusions are now made to coloured glass ; and, it may be, with 
