THE RADIATION AND CONDUCTION OF HEAT. 
133 
external air, considerable pain will be experienced from the loss of caloric con- 
sequent on such an act. 
We have chosen these commonplace examples the more completely to demon- 
strate our position ; although the experience of the cultivator of extensive practice 
might at once decide the point. There is the most apposite analogy, in respect to 
heat between an individual in a confined room, and a plant in a sort of tent-like or 
any other protective frame. Both may be kept from perishing by frost if situated 
in the middle of the area, but both must undergo the loss of those parts which 
are in contact with a thin covering closely communicating with the outer air, if 
frost be sufficiently severe. Proofs of this have often been furnished in the 
case of plants w T hose shoots touched the glass or mats by which they were 
surrounded. 
Herein, therefore, lies the art of protecting plants. They must, first, be 
enveloped in a material which is known to be an imperfect radiator of heat, so that 
their own temperature may, for the most part, be retained within or around them. 
And secondly, that material should be so disposed, that no part of it be nearer 
than about two or three inches to the exterior shoots. The importance of confining 
and tying in the outer branches of shrubs that are wished to be covered, will thus 
be plainly discernible. 
Plant-houses and frames have yet to be treated of. It is generally imagined 
that no resemblance is traceable between the operation of sheltering these, and that 
of protecting isolated plants, because specimens in the former are already guarded 
by a sheltering surface. The principle, however, remains unaltered and unalter- 
able, whatever may be the conditions in which the plants exist ; and is as 
applicable in the one instance as in the other. Glass, it is well known, radiates 
heat with astonishing rapidity, and the temperature which a glazed surface derives 
from the house or frame beneath it is so great, that were the additional covering 
made use of allowed to lie flatly upon it, heat would be conducted from the entire 
apartment with very little less celerity than if the glass were exposed, or this last 
removed, and mats substituted for it ; the only difference of result from the cases 
before mentioned being, that the whole plant would be rather more slowly robbed 
of its caloric, instead of at once merely having a single member frozen. 
Common garden mats are exceedingly well adapted for placing over the roofs of 
frames or houses, but they should never be so thrown on as to touch the glass. To 
avoid this, one or two small strips of wood can be fastened across the middle, as 
well as to the top and bottom of the frame ; and if the mats are drawn tightly over 
these, and secured by strings, observing to have the former long enough to prevent 
any apertures being left, they will answer every desirable purpose infinitely better 
than they could if not sustained at a trifling elevation above the roof. "Where 
Harrisons mode of glazing is adopted, the peculiarity of which is to dispense with 
wooden bars raised higher than the level of the glass, and thus present a perfectly 
flat surface ; such a precaution is especially indispensable. 
In a recent paper on Pelargoniums (p. 88), we have noticed a kind of protection 
