44 ANNUAL'REGISTER, 1797. 
To confine heat is nothing more 
than to prevent its efcape out of 
the hot body in which it exifts, and 
in which it is required to be retain- 
ed; and this can only be done by 
furrounding the hot body by fome 
covering compofed of a fubftance 
through which heat cannot pafs, or, 
through which it paffes with great 
difficulty. If a covering could be 
found petfeétly impervious to heat, 
there is reafon to believe that a hot 
body, completely furrounded by it, 
would remain hot for ever; but we 
are acquainted with no fuch fub- 
ftance ; nor is it probable that any 
fuch exifts. 
Thofe bodies in which heat paffes 
freely or rapidly, are called con- 
duétors of heat; thofe in which it 
makes its way with great difficulty, 
or very flowly, non-conductors, or 
bad conductors of heat. The epi- 
thets, good, bad, indifferent, excel- 
lent, &c, are applied indifferently 
to conductors and to non-conduc- 
tors. A good conduttor, for in- 
ftance, is one in which heat paffes 
very freely ; agood non-conductor 
is one in which it paffes.with great 
difficulty ; and an inditferent con- 
ductor may likewife be called, with- 
out any impropriety, an indifferent 
non-conduétor. 
Thofe bodies which are the worft 
conduétors, or rather the. beft non- 
condudtors of heat, are beft adapted. 
for forming coverings toconfineit. 
All the metals are remarkably 
good condudtors of heat ; wood, 
and in general all light, dry, and 
fpungy bodies, are non-conductors. 
Glafs, though a very hard and com- 
pact body, is a non-conductor :- mer- 
cury, water, and liquids of all kinds, 
are conduétors; but air, and in ge- 
neral all elaftie fluids, fteam even 
not excepted, are non-conductors, 
Some experiments which I have 
lately made, and which have not 
yet been publifhed, have induced 
me to fufpect that water, mercury, 
and all other non-elaftic fluids, do 
not permit’ heat to pafs through 
them from particle to particle, ‘as it 
undoubtedly paffes through folid bo- 
dies, but that their apparent ’‘con- 
ducting powers depend effentially 
upon the extreme mobility of their 
parts. In fhort, that they rather 
tranfport heat than allow it a paf- 
fage. But I will not anticipate a 
fubjec&t which I propofe to treat 
more fully at fome future period. 
The conduéting power of any fo- 
lid body in one folid mafs, is much 
greater than that of the fame body 
reduced to a powder, or divided in- 
to many {maller pieces. An iron 
bar, or an iron plate, for inftance, 
is amuch better conductor of heat 
than iron filings; and faw-duft is a 
better non-conduétor than wood. 
Dry wood-afhes is a better non.con- 
ductor than either; and very dry 
charcoal, reduced to a fine powder, 
is one of the beft non-conductors 
known: and as charcoal is perfeét- 
ly incombuftible when confined in 
afpace where frefh air cay have no 
accefs, itis admirably well caliculat- 
ed for forming a barrier for confin- 
ing heat where the heat to be con- 
fined is intenfe. 
But among all the various fub- 
{tances “of which coverings may be 
formed for confining heat, none can 
be employed with greater advan- 
tage than common atmofpheric air. 
It is what Nature employs for that 
purpofe; and we cannot do better 
than to imitate her. 
- The warmth of the wool and fur 
of beafts, and of the feathers of birds, 
is undoubtedly owing to the air in 
their interftices; which air, being 
ftrongly 
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