690 
“Nature of to which it may be referred. Now we believe that few 
sons will assert that this is the case; on the other 
d, we conceive every one will admit, that any at- 
tempt to explain the intimate nature of the motion suppo- 
sed to produce heat, and the manner in which vibrations 
can excite a sensation such as that which we refer to this 
inciple, must be a most arduous task. Still more so 
will it be to explain, how it can be the immediate cause 
of liquidity and of elastic fluidity ; how it can be trans- 
ferred in definite quantities from one body to another; 
how this can be done even while it is lying dormant, 
or while we have no proof that the motion actually ex- 
ists. In short, we shall find, that the hypothesis of vi- 
brations is far from being unencumbered with difficul- 
ties; and that we have only removed one, to become 
involved in a much greater number. Besides, although 
we have admitted, that there is no direct experiment to 
prove the independent existence of heat, or at least 
none against which some exception has not been taken ; 
yet there are facts brought forwards, perhaps as deci- 
sive on this side of the question as those respecting 
friction are on the other. We refer to the transmission 
of heat through a vacuum. Pictet proved that this takes 
place in the vacuum of the air-pump; and Rumford, 
the great advocate for the immateriality of heat, has 
shown that it is capable of passing even through the 
Torricellian vacuum. There seems no method of re- 
conciling this fact with the hypothesis, except by ta~ 
‘king for granted the existence of some kind of vapour 
or elastic fluid, along which it is propagated; a suppo- 
sition equally gratuitous, and equally unsuppo by 
direct and independent facts as that for which it is 
substituted. Indeed, the same remark may be applied 
to all the phenomena in which the radiation of heat is 
concerned ; it seems extremely improbable, if not im- 
possible, that these rays are ‘carried along by the air, 
even when near the surface of the earth, and they 
- must necessarily traverse.an immense track totally de- 
void of air. If we, in this case, suppose the existence 
of a subtile medium, invented for the purpose of carry 
ing them through space, we are here, as in the former 
instance, creating an agent at least as hypothetical as 
the matter of heat. Upon the whole, we are strongly 
inclined to the opinion which regards heat as an elas« 
tic fluid, of a proper immaterial nature, although of 
extreme subtilty, the particles of which are repulsive 
with respect to each other, but are attracted by other 
bodies, with different degrees of force, according to 
their respective nature. 
_ Before we conclude this discussion concerning the 
immateriality of heat, it will be proper to notice the 
experiments which have been made, in order to ascer- 
tain whether it be actually possessed of gravity, or ra- 
ther, whether its weight can be measured by a balance. 
The best contrived experiments of this description were 
those of Fordyce. He very carefull weighed a quan- 
tity of water, froze the water, and then again weighed 
it. Now he argued, that in this process, the water must 
have parted with the latent heat which maintains it in 
the liquid form!; so that if heat be a ponderable sub- 
Stance, it might be expected that the ice would exhibit 
a diminution in its weight, equivalent to that of the ca- 
lorie which had escaped. The result, however, did not 
correspond with this idea; and indeed, in some of the 
most accurate trials, it seemed as if the body that had 
parted with its heat had even acquired a slight addition 
of weight. It is, however, generally admitted, that no 
decisive conclusion can be drawn from such experi- 
‘ments ; and that, from the conception that we have of 
HEAT. 
the extreme tenuity of the gee of heat, it is net pro- 
bable, that any portion which we can have it in our 
power to impart to a body, could be detected by the in- 
struments that we employ in ascertaining the’ t of 
favourof the hypothesis of the materiality of heat, be 
‘avour e sis e materi of heat, 
cause wethink it explains the ena in general with 
greater facility, is encum with less difficulties 
than the immaterial hypothesis. Yet we must remember, 
that it is not decisively proved by any direct and un- 
exceptionable experiments; and it must also be acknow= 
it |, that it has not received the sanction of some 
eminent philosophers, both at the revival of letters, and 
in our own times. We have already mentioned the opi- 
nion of Rumford; and Professor Leslie, to whom science Le 
is so peculiarly indebted for his numerous experiments byp 
and discoveries on the subject of heat, also adopts the 
hypothesis, which ascribes its effects to a certain mo- 
‘tion among the particles of bodies. He conceives that 
the BE Bp 9 and transmission of heat, is very simi-« 
lar to of sound, and that it in fact consists in cer« 
tain aerial undulations. A hot body communicates a 
portion of its heat to the contiguous stratum of air; 
this immediately expands, and by this expansion, a vi- 
bration is excited in the adjoining stratum ; this is pro- 
pagated to the next, and so on until the equilibrium is 
produced. The of heat is therefore of the same 
velocity with the undulation of the air, or rather is 
identical with it; and according as the surfaces’ of bo- 
dies act upon the air in contact with them, so will they 
radiate heat with greater or less readiness. The facility 
of radiation is supposed to be principally owing to the 
approximation of the air to the surface ; those bodies ra- 
diating the best, to which the air comes into the closest 
contact. This view of the subject explains why the best 
radiators should likewise be the best absorbers of heat ; 
and it is also supposed to afford a reason for the effect of 
roughening the surface, or covering it with substances 
which destroy its polish. Professor Leslie seems to have 
advanced his hypothesis, merely as a convenient manner 
of accounting for some of his own experiments; he has 
not stated it in such a way as to apply to all the pheno-~ 
mena of heat, nor has he attempted to reconcile it with 
the experiments of Herschel, and others which appear 
decidedly adverse to it. Indeed, we much doubt whe- 
ther it can be adapted even to some of his own experi« 
ments on the radiation and reflection of heat; for it 
must be supposed, that the undulations of the air are 
propelled from the surface of bodies, impinge u 
others, and are again reflected, exactly as they radiate 
heat, with the same velocity, and in the same direction ; 
a circumstance which we apprehend it would be ex- 
tremely difficult to prove. It may be farther remarked, 
that although Professor Leslie's hypothesis supposes that 
the undulations of the air are the cause of heat, or that 
which produces the phenomena in question ; yet there 
are many expressions which would seem to imply, that 
this heat is something distinct from the undulations 
themselves, and that the heat is transferred or transmit 
ted from one wave to the other, net that the waves are 
themselves the actual cause of the effect. The farther 
consideration of this question would involve us ina dis« 
cussion that would exceed the limits to which we are 
necessarily restricted. 
In the former 
chemical properties of heat, but we proposed 
more fully acquainted with its nature and its effects. 
On this subject there are two points to be determined : 
rt of this article we alluded to the Chemical 
to defer properties 
the consideration of them until we had made ourselves °f bests 
