132 
THE RADIATION AND CONDUCTION OF HEAT. 
it is chiefly from this cause that the geniality of our atmosphere arises. But it 
also imbibes other rays ; and the radiation of these serves to prevent a coldness 
during the night that would be wholly destructive to both animal and vegetable 
life. 
Having thus laid the basis of the application of artificial coverings to plants on 
the principles of science, the following hints on protection will not be misappre- 
hended. Plants being formed of earthy and atmospheric elements, combined and 
concentrated according to their peculiar powers, have, in proportion to their 
porosity, and with a slight reduction on account of their vital energy, the same 
tendency to radiation as soil. Unless, then, this be duly interrupted, the more 
susceptible kinds must be subjected to irremediable injury; whereas, if radiation 
can be effectually suspended, they will be perfectly safe in the most severe weather. 
We would most willingly print this last declaration in capitals, if we thought it 
could escape the culturist's notice ; so mightily influential do we deem its purport. 
It follows from these premises, as we have often before endeavoured to show, 
that the old method of supplying fire heat to plant-houses, where the sole design 
is to exclude frost, is radically wrong ; indeed, it is a positive and total waste, 
expended, too, on an object that cannot sometimes be realized by such a practice, 
and the failure of which is more or less likely to be fatal to the plants. If it be 
demanded what we would substitute for it, we would suggest — prevent the escape 
or radiation of heat, and there will be no need to essay the supply of a deficiency 
that does not exist. 
If these hints on radiation have received half the attention which their subject 
demands, the reader will be ready "patiently to pursue the investigation as it 
concerns conduction. We have distinguished the two processes by showing that 
the first phrase characterizes the transmission of heat to the superincumbent air, 
while the topic now to be discussed embraces the deprivation of temperature which 
a body suffers on being touched by a colder one. On the extent to which the latter 
circumstance is obviated, the merits of any system of covering plants, whether in 
houses, frames, or the open ground, must, to a very great degree, be wholly 
dependent. 
It is notorious to every inquiring person, that heat is much more speedily dissi- 
pated when the substance containing it is in direct connexion with some less highly 
heated material. But the more humble members of our profession find it difficult 
to comprehend or believe this fact. Let us, then, adduce an easy illustration. If 
a bar of cold iron, which, perhaps, is one of the most rapid conductors of heat, 
be placed within an inch of any individual's hand, the diminution of caloric which 
its vicinity occasions is scarcely perceptible ; but if the same substance be grasped 
or laid on the hand, the abstraction of heat is so sensibly felt, that, without a 
strong effort of volition, the iron would immediately be dropped. Again, the 
atmosphere of an apartment may be exceedingly comfortable as regards tempera- 
ture, and altogether consonant with the appetencies of a sensitive person, while, by 
pressing the hand against glass of which the outer side is exposed to a severe 
