674 
Properties filled with air, that the process of cooling was retarded 
of Heat. by the mixture of the body with the air; and hence he 
deduced the conclusion, that the presence of the solid 
matter impedes the motion of the air, and prevents 
those currents from being formed in it, which we 
shall afterwards find te act so important a part in the 
heating and cooling of fluids: (Phil. Trans. 1792, and 
Remarks on Essays, vol. ii. p, 428.) At the time when Rumford 
Rwnford’s _ published his Essay, the effect of the radiation of sur- 
expecl- faces was not thoroughly understood, and it is entirely 
be neglected in the view which he. takes of the subject. 
It must, however, have had a considerable share in the 
production of the phenomena; and we may imagine, 
that the more the particles or fibres of the solid matter 
are dispersed through the mass of air, the greater num- 
ber of rays will be interrupted by them. It is also pro- 
bable there is a real attraction between air and the dif- 
ferent substances that were examined, which must have 
had its share in affecting the nature of the results; but 
whether the attraction. differs in different substances, 
‘or whether the, attractive power of the air is precisely 
equal upon all of them, does not appear to be exactly 
Meyer’s ex- ascertained. Some experiments were made by Meyer 
periments of Erlangen, on the conducting power of the different 
von woods. kinds of wood... He procured balls of them, in which 
he formed a cavity to receive the bulb of a thermome- 
ter; the balls were then exposed to the same tempera- 
ture, and the effects upon the mercury were noticed. 
The experiments, although performed with apparent 
accuracy, are defective in not making any allowance of 
the action of radiation. Lime, fir, alder, and oak, were 
the worst, while ash, apple, and ebony, were found to be 
the best conductors of heat. Ann. Chim. t. xxx. p. 32. 
Effect of The different conducting power of bodies produces 
different —_ a great difference in their action upon the nerves in ex- 
conductors citing the sensations of heat and cold, although the 
on the sen- SON. ae 
batlone, bodies indicate the same temperature to the thermome- 
ter. Thus.it is well known, that in the same apart- 
ment, when it contains no source of local heat, the dif- 
ferent articles of furniture will convey to the hand very 
different degrees of warmth. Metals feel the coldest, 
stone or marble is the next in degree, while wood, and 
still more woollen stuffs, produce little or no feeling of 
any change of temperature. The effect in this case de- 
pends upon the different conducting power of these 
substances ; the human body being almost always warm- 
er than the objects that are contiguous to it, its heat is 
abstracted by them; but this operation goes on in pro- 
portion to their conducting power, i. ¢. to the respective 
velocity with which they are enabled to remove the 
heat from our body. It obviously follows, that when 
we wish to retain the heat in-any substance that is 
warmer than the surrounding medium, we must en- 
close it in bad conductors of heat ; and that exactly the 
same method must be pursued, if we wish to keep any 
substance at a lower temperature than the atmosphere, 
or other contiguous bodies. Thus we envelope our- 
selves in woollen cloth or furs when we wish to retain 
our natural warmth ; and we should employ the same 
method to prevent the melting of ice or snow. 
Cause of the | We-have already observed, that one cause which ob- 
different viously tends to affect the conducting power of bodies, 
ae is their possessing a spongy texture, by which portions 
of air become, as it were, entangled in its pores, and 
thus seem to prevent the communication of heat. But 
we observe a very marked difference in the conducting 
power.of bodies that are perfectly solid, and where no 
air, or at least no quantity that can be supposed capa- 
ble of producing any effect of this kind, is present. 
What is it, in this case, that causes the difference in 
HEA T. 
the conducting power of bodies? Is it an attraction 
which the heat ses for the cles of the solid, 
or do the experiments of Rumford and others lead us 
‘to regard the operation as of a more mechanical nature, 
as if there was something in the arrangement or shape 
of the icles, which retards the of the heat 
along them? These are questions which at present it 
appears to be beyond our power to answer. Those 
cases in which a greater or less degree of density, or 
of aggregation of the parts of bodies, produces a con- 
siderable effect upon their conducting power, would in- 
duce us to suppose, that the worst conductors should 
be regarded as merely the most effectual retarders of 
heat, and the best conductors as simply those that have 
the least power in retaining the heat that has been im- 
parted to them. But this view of the subject seems 
scarcely to apply to metals and other solids of a similar 
kind. - The radiation of the surface may be suppo 
to have some influence in these cases ; and M. Poisson Opinion o 
goes so far as to conjecture, that the a power Poisson 
of solids, generally, is to be regarded as a kind of rae 
diation from rticke to particle, operating at very small _ 
distances: (Journ. Phys. t. 1xxx. p. 434. et seq.) Upon. 
this point, however, it does not a that we have any 
data which can enable us to form a decided judgment. 
Without, however, entering upon any abstruse theo- gircun 
ry on the subject, for which the present state of our ces which 
information does not seem to afford a sufficient founda. affect 
tion, we may assume, as the most natural deduction 
from the facts, that the conducting power of bodies de. 
ends principally upon three circumstances. It is af= 
fected partly by the mechanical relation of their parti- 
cles to each other, partly by an attraction between the 
heat and the particles of which the body is composed, 
and partly by the radiating power of the heat. The 
heat which escapes from the surface will tend to draw 
from the interior a portion of its remaining heat, in or« 
der to supply what has been lost from the external 
4 There are also other causes, which, 
perhaps less efficacious, are not to be neglected. The 
consequence of adding heat to a body, is to expand it 
in all its dimensions; but, by this expansion, it ap-« 
pears to acquire a greater capacity for retaining heat, 
so that it become more disposed to carry it from 
other bodies, and to diffuse it over its own substance, 
and thus to have its conducting power increased. The 
effect which caloric has in altering the state of bodies, 
may likewise materially affect their conducting power, 
according to the nature of this Thus, if we 
throw acertain quantity of heat into a metal, it is re« 
duced to the fluid state, by which its relation to heat, 
“and the manner of conducting it, will be much affect~ 
ed. ‘There is some reason to suppose, from an experi« 
ment of Pictet’s, that heat passes more readily upwards 
powers 
than in the contrary direction. . He e a metallic 
bar vertically in a vacuum, and, eee it exactly in 
the centre, observed the effect produced upon thermo- 
meters attached to each end; when it appeared, that 
the one at the upper end was affected sooner than the 
one at the bottom: (Essay on Fire, § 33.) The expe- 
riment is ingenious: but there are some points connects 
ed with it, which render the result rather dubious. 
We now to consider the manner in which Cond: 
heat passes through fluids. Fluids differ essentially power of 
from solids in their. particles being moveable no | i 
each other; and this circumstance, as we shall find, 
acts a very im t in the transmission of heat. 
Fluids, like solids, are expanded by caloric, and of 
course become specifically lighter ; and therefore when 
heat is partially applied to them, in consequence of this 
