Properties 
of Heat. 
—\y— 
Explana- + 
tion of the 
equilibrium 
of heat, 
Prevost’s 
hypothesis, 
Explains 
the radia- 
tion of cold. 
672 
the two powers of repulsion and attraction, the heat 
escaping from a body, in consequence of the repulsive 
pencilled exists between its particles, while, at the 
same time, it:is attracted by the particles of the body 
into which it enters. And perhaps this kindof double 
operation will serve to explain most of the facts, or at 
least will enable us to announce them in language 
which implies no contradiction, and gives an idea of 
their relation to each other. The equal distribution of 
heat, as it has been called by some writers, or the equi- 
librium of caloric, as it has been styled by others, con« 
stituting one of the specific properties which we enu- 
merated above, has been the subject of much  observa- 
tion and experiment, and has.also given rise to. much 
hypothetical discussion. 
The law by which bodies of different temperatures 
become the one hotter and the other colder, or by 
which the equilibrium of heat is produced, was first 
laid down by Newton, and may be thus expressed in 
general terms; that the heat lost by the one body, and 
gained by the other, is in proportion to the excess. of 
the temperature of one above the other ; or, stating it 
in a more scientific manner, that the difference between 
the temperatures diminishes in a geometrical ratio, 
while the times increase in an arithmetical ratio. The 
external circumstances which. influence the rapidity of 
the equalizing process, are'the radiating power of the 
bodies themselves, their conducting power, and, pro- 
vided they are not in contact, the nature of the medium 
which is sre gor between them, and the mechanical 
changes which take place in the medium, relative to 
its position with respect to the heating and cooling bo~ 
dies; constituting currents. These principally operate 
in what are styled the elastic fluids; but they have also 
considerable effect in the action of liquids. The hypo- 
thesis which is commonly adopted, and which. appears 
satisfactorily to account for this peculiar property of 
bodies, was proposed by M. Prevost. It is founded 
upon the following data. Heat is conceived te be a 
fluid, composed of distinct particles, which pass through 
space in right lines, and:are projected in all directions, 
with very great velocity. The particles are so far re- 
moved from each other, that, analogous to what takes 
place with respect to light, a number of currents ma 
flow in different directions, without interfering with: 
each other. All bodies, except such as we suppose to 
be absolutely deprived of heat, send out rays to: each 
other, although generally in very different. degrees. 
Two bodies, exactly of the same temperature, will mu- 
tually give and receive heat, and even a cold body will 
radiate heat to a hot one; but, in the former: case; the 
uantities given and received by each will be exactly 
e same, so that the temperature will not be changed; 
and, in the latter case, the one will give much more than 
the other, until the temperature of the two is equalized. 
Phil. Trans. 1802, p. 443. 
This hypothesis appears to have been originally form- 
ed, in order to account for the experiment, of which we 
gave an account above, the radiation of cold.. When 
a heated. bodyparts with its caloric to the neighbour. 
ing bodies, and raises their temperature, the idea that 
presents itself, as the most natural. explanation of the 
fact is, thatthe hotter body has merely given off its su- 
perabundant heat to the colder, in consequence of the 
tendency which. heat has to distribute itself uniformly 
through all bodies subjected to its influence. It may 
be conceived in this case to pass off, in'a greater or less 
degree, according to the excess of the temperature of 
the rs body over the other, modified by the nature of 
HEAT, 
the surface, and the other’ careraelanees to which: , 
have already alluded. But this simple view of the ope- ,° 
ration, will not explain the radiation of cold, or > 
least will not explain the apparent reflection of it from 
a concave mirror. Ifthe cold body acted only by at« 
tracting heat from the neighbouring bodies, it would 
take it from the thermometer, the mirror, and _ all other 
contiguous substances; and there seems tobeno rea« 
son why the focus should be colder than any other part 
of the atmosphere, equally near the source whence the 
cold proceeds. According, however, to Prevost’s no» 
tion, when ice is placed in one of the foci, it: sends out 
radiant heat, which strikes, against the mirror, and is 
reflected into the opposite focus ; but these rays being 
comparatively colder than those which proceed from 
other bodies in the vicinity, have the effect of generating 
absolute coldin the second focus, and thus: to de 
the thermometer which is suspended there. A mutual ex- 
change of heat thus takes place between the ice and the 
thermometer, and the equilibrium is. established, by the 
ice acquiring, and the thermometer losing, whe ly 
of caloric. (Journ. Phys. t. xxviii. p. 3.). The faci 
with which this h: esis explains the radiation of cold, 
is itself an argument.in favour of its validity; and it 
must be admitted, that it applies equally well to all the 
other phenomena in which caloric is concerned. An ob« 
jection has indeed been urged —— it, that it does not 
take into account the effect of the conducting power of - 
bodies, which must have an important effect in the equa- 
lization of their temperature. This is not, however, 
properly an objection against the general doctrine of 
the rage interchange of radiant. but an omis« 
sion in ost’s manner of applying it ; and it seems 
that the two operations are not in any degree incompa-~ 
tible with each other. Still, however, M. vost’s opi« 
nion must be ded rather as a plausible conjecture, 
which has the merit of satisfactorily cipaden O phe- 
nomena, than asa founded upon any direct ex- 
imental proofs. It has indeed been conceived, that 
ofessor Leslie’s researches afford considerable support Supp 
to it, as they tend to establish the existence ofa radiating »Y Le 
energy in bodies, quite independent of their conducting (*P 
power ; an energy, by which even the worst conductors 
of heat, under certain circumstances, become the most’ 
active radiators of it. Yet this radiation can never be 
proved to exist, except there be a previous difference of 
temperature between the bodies; because the thermo- 
meter, which is our only measure of heat, and the onl} 
index which we of its presence, is never af- 
fected except by an unequal distribution of it. 
Having now described the manner in which heat Conductiz 
tends to pass from one body to another, we shall next powers 
proceed to the second of its specific properties, the °°" 
power by which it moves among the cles of the 
same body ; or is‘conducted, as it is styled, through their 
substance. As bodies appear under the three states of 
solids, fluids, and gases, we should consider the power 
which heat exercises in its transmission through each of 
these different forms of matter. Our remarks will, 
however, be chiefly confined to the action of' solids and 
liquids upon heat ; for in consequence of the tenuity of 
gases, or the distance at which their particles are situa- 
ted from each other, it does not eppes that any very 
notable effects can be attributed to them upon the pas- 
sage of free caloric, at least in comparison with what 
we observe in the two other classes of bodies. 
When heat, in its uncombined state, radiates through 
air, or through a vacuum, it moves with a velocity yon, 
which has not been accurately measured, but which, jas, ~ 
heat 
