36 
INFLUENCE OF CLIMATE ON PLANTS. 
produced, being a bad conductor of heat, is subsequently instrumental in preventing 
a like dispersion of heat from the lower strata. 
Different kinds of substances possess very variable capacities for absorbing, 
conducting, and radiating heat ; these depending chiefly on the nature of their 
surfaces and texture : radiation and absorption being the transmission of heat to 
and from the atmospliere, while conduction is the communication of it to and from 
any other materials. In general, those bodies which are of the loosest texture, 
are the best radiators and absorbents of heat, since they admit the air to all their 
parts most freely, while those are the more perfect conductors which possess a 
substance of a uniformly dense and compact nature. The extent to which any 
body acquires dew, when exposed to a clear and saturated nocturnal atmosphere, 
has been established as a criterion for determining its radiating powers. This 
ingenious theory is based upon the fact, that, when vapour is brought in contact 
with any substance, in proportion as the temperature of that substance diminishes, 
to the same degree will the vapour be condensed, and adhere to it in the form of 
dew. All metals, particularly those w^ith smooth and polished surfaces, are thus 
proved to be imperfect radiators of heat ; while glass, and those substances which 
are of a very porous nature, are spe.edily deprived of heat when placed in a cold 
atmosphere. Most vegetable bodies radiate heat profusely ; subject, however, to 
great variations, according to the nature of the fluids therein involved. Semi-fluids, 
such as oils and resins, dissipate heat much less freely than aqueous fluids ; hence 
the diff'erent capacities of plants containing either of them for enduring cold. 
The power of conducting or absorbing heat from contact with other substances, 
is seldom co-existent and at the same time co-extensive with the radiating power ; 
but generally resides in the same body in inverse proportion. Substances with a 
dense continuous tissue conduct heat most rapidly, there being more immediate 
and uninterrupted communication between their particles : whereas, those which 
are more porous admitting air, or being partially exposed to air throughout, 
radiate more liberally. The conduction of heat in porous substances is evidently 
obstructed by the irregularity of their tissue, and the interposition of minute 
vesicles of air, which, however, facilitate radiation ; while from a denser substance 
radiation is far less extensive, but, by the contiguity and uniformity of its con- 
stituent particles, conduction is readily effected. Metals possess the latter property 
in a very remarkable degree, as is evinced by the rapidity with which they reduce 
the temperature of any warmer substance which is applied to them. Glass, on 
the other hand, is an extremely slow conductor of caloric, as are also wood and 
other vegetable matters. The capacity of any body for the absorption of radiant 
heat, varies materially from its capacity of absorption by conduction ; the former 
being generally commensurate with its radiating power, since, in both instances, 
the heat is conveyed through the same medium. 
An increase of temperature may be produced by a variety of means. Chemical 
combinations, friction, percussion, and electrical elicitation, are the most efi*ective 
of them ; but the sun is the principal and most cogent natural source of heat. All 
