* there is reason to/su pose, is immensely great; so that, 
‘with respect to any lstances near the. sarth, it may be 
“e~ said to be infinite. Heat also passes through the most 
d we Ki solid posien, as, for example, seongh metals ; 
but in this case its p is igiously retarded, It 
‘ i also found to move h sire f theocbadie, with various 
_ degrees of velocity, which do not seem to bear an exact 
_Fatio to any other of their properties, and which can 
_only be ‘ascertained by direct experiment. This, in 
opposition to the rapid transmission of heat through the 
_alr, or through a vacuum, has been styled its slow com- 
‘munication ; and the power which solids possess of en- 
abling heat to pass along them, has been called their 
conducting power. When solids are in contact, and 
_are of different degrees of heat, they imme- 
diately tend to produce an equilibrium of temperature ; 
which is not brought about, as in the former case, by 
Tadiation, but by the one directly abstracting a ion 
of heat from the other, conducting this portion through 
its own substance, and diffusing it equally its 
es dif- particles. This faculty exists in bodies in very differ- 
theit ent degrees; but it is found that each individual body 
""8 always preserves the same degree of this power, unless 
some change takes place in its chemical or physical con- 
dition, when a change is at the same time produced in 
i ann o paras A wey but a correct exam- 
e ifferent degrees of this conducting power in 
ies, may be noticed in the different effects that are 
produced upon metal and upon glass. Ifa rod of each 
of these substances have one of its ends plunged into 
hot water, the metallic rod, will soon become so tho- 
roughly heated, through all its extent, that. it: will be 
impossible to apply the hand to the other extremity, 
while the glass rod will remain a long time in the wa- 
ter, before the upper end is sensibly affected. Hence we 
Say, that metals. are good conductorsof heat, and that glass 
snsest bo- 28 # Worse conductor of heat than metals. As a general” 
ss the best Principle, it may be stated, that the densest bodies are 
ductors, the best conductors; but to thisyrule there are many 
exceptions, Upon the whole, however, the principle 
seems to hold good, in so many instances, that we may 
infer the existence of a, necessary connexion) between 
the density and the conducting power of: bodies, and 
that when the ratio is not correctly maintained, it should 
be attributed, to. the interference of some other princi- 
le. Thus it is;remarked, that the same body, without 
haying experiencedany chemical, or any other physical 
change, except .a difference in its state of tion, 
has its conducting, power increased: or diminished; in 
proportion to its density, or to the contiguity of its par- 
_ ticles. Rumford found that a solid piece of irom is a 
ter conductor than.the filings of the same: metal ; and” 
* that. wood is, a, better conductor tham saw-dust. 
. Many, experiments have been: performed on the con- 
ducting power of solids, the object of which was to as- 
certain the amount.of this power, and to: learm whether 
it bore an, exact ratio to any other physical or chemical 
property. Metals, as was remarked above, are some of 
ae ben conductors of heat, but they differ considerably 
themselves in this respect, _Ingenhousz, insti 
t tuted a simple, but ingenious. process, for discovering 
usz’s ex- their relative power, which consisted in providing him- 
riments, self with reds, of different metals, all of the same diame- 
ter, and having a certain length covered) with: a coat- 
ing of wax of the same thickness.. The other ends. of 
the rods were them plunged: to :the same depth in. a 
heated fluid ; and ing to the quantity of wax that 
was melted, their conducting power was estimated. 
The best conductors. weve found to be silver, gold, tin, 
VOL, X, PART, Is 
HEAT. 
675 
and copper, while platina, iron, and lead seemed to be 
the worst: (Journ. Phys. tom. xxxiv. p. 68.) These 
experiments prove that the conducting er of the “~™ 
metals is not i in the same ratio with their den- 
sity. Richmann of Petersburg endeavoured to ascer- Richmann’s 
tain the same point by a different process. He pro- eperi- 
cured hollow balls of the several metals; into these he ™®"*- 
inserted the bulb of a thermometer, immersed the 
balls in. boiling water, and observed the effect upon the 
mercury: (Comment. Petrop, vol. iv. p. 241.) The 
results do not precisely agree with those of I - 
housz ; but although more elaborate, we think ‘ae 
less direct ; because in Richmann’s, other causes besides 
the mere effect of the conducting power might act upon 
the thermometer, an objection which does not seem to 
apply to Ingenhousz’s. 
. A set of interesting experiments was performed by Condueting 
Rumford on the conducting power of various animal power of 
and vegetable substances. The plan which he —o- 
was to provide himself with a glass cylinder, which |°f 
terminated in a globe of somewhat more than an inch 
in diameter. The bulb of a thermometer was suspend- 
ed in the centre of this globe, and was surrounded by 
the substance to be examined ; and the whole appara- 
tus was then plu into boiling water, and when 
xaised to a certain elevation of temperature, it was im- 
mersed in a mixture of ice and water, and the time 
noticed which was required to bring it down to this 
degree of heat. The heat from the water must ob- 
vious} s through the substance upon which the 
trial pe before it could reach the thermometer ; 
therefore its conducting power was estimated by the 
effect which it produced upon this instrument. He 
began by observing the length of time necessary for 
raising the thermometer enclosed in the cylinder, when 
it was surrounded only with air, and what length of 
time was also necessary to cool it, by afterwards im- 
mersing it in a mixture of ice and water. 
experi- 
taente. 
He then 
successively introduced into the globe, similar weights 
of wool, cotton, silk, linen, down, and fur, and com- 
pared their effects with air, taken as a standard; the 
two last bodies were found to be the worst conductors, 
and linen the best, a fact which agrees with our expe- 
rience of their effect when used as clothing; for we 
know that their only operation, as producing warmth 
to the body, must depend upon their retaining the heat 
which is generated in it, and preventing its escape. 
The Count concludes, that the relative i 
power of these substances — the See ratio of 
the quantity of air interposed between — or 
Gbres of which they are composed. He found that 
their non-condueti ér was not it proportion to 
the quantity of cali epaate whieh they contain, and 
therefore could not be from: any mutual attraction be- 
tween the solid matter and the air; for the power bore 
no ratio tothe actual quantity of the substance, but ob- 
range depended upon the: manner in which it was ar- 
rformed a direct experiment on silk, which 
establishes this point. ities of raw silk, 
the ‘ravellings of spun silk, and twisted silk thread, 
were respectively placed in the a us; and their 
conducting powers. were found to be in the proportion of 
9, 114, and 13.. He afterwards: examined’ how far the 
air itself in the globe and cylinder might be conceived 
to be the sole agent; when it Lcdaranpom | 
the space which the different substances oecupied, 
comparing the effect of the balk of air which would be 
left, with the. effect of the irstrument wher entirely 
4-@ 
