MISCELLANEOUS ESSAYS. 
firongly attracted by thefe fub- 
ftances, is confined, and forms a 
barrier which not only prevents the 
cold winds from approaching the 
body of the animal, but which op- 
pofes an almoft infurmountable ob- 
ftacle to the efcape of the heat of 
the animal into the atmofphere. 
In the fame manner does the air 
in fnow ferve to preferve the heat 
of the earthin winter. The warmth 
of all kinds of artificial clothing may 
be fhown to depend on the fame 
caufe; and were this circumftance 
more generally known, and more 
attended to, very important im- 
provements in the management of 
heat could not fail to refult from it. 
A great part of our lives is {pent in 
guarding onrfelves againft the ex- 
iremes of heat and of cold, and in 
operations in which the ufe of fire 
is indifpenfable ; and yet how little 
progreis has been made in that moft 
ufeful and moft important of the 
arts, the management of heat ! 
Double windows have been in’ 
ufe many years in moft of the north- 
ern parts of Europe; and their great 
utility, in rendering the houfes fur- 
nifhed with them warm and comfor- 
table in winter, is univerfally ac- 
knowledged ; but I have never 
heard that any body has thought of 
employing them in hot countries to 
keep their apartments cool in fum- 
mer : yet how eafy and natural 
is this application of fo fimpie and 
fo ufeful an invention! If a dou- 
ble window can prevent the heat 
which is in aroom from pafiing out 
of it, one would imagine it could 
require no great effort of genius to 
difcover that it would be equally 
efficacious for preventing the heat 
without from coming in, But na- 
425 
tural as this conclufion may appear, 
I believe it has never yet occurred 
to any body; at leaft, 1 am quite 
certain that I haye never feen a dou- 
ble window either in Italy, or in 
any other hot country I have had 
occafion to vifit. 
But the utility of double windows 
and double walls, in hot as wel! as 
in cold countries, is a matter of fo 
much importance, that I fhall take 
occafion to treat it more fully in 
another place. Inthe mean time I 
fhall only obferve here, that it is 
the confined air fhut up between the 
two windows (not the double glafs 
plates) that renders the paflage of 
heat through them fo difficult. Were 
it owing to the increafed thicknefs of 
the glafs, then a fingle pane twice 
as thick would anfwer the fame pur- 
pofe ; but the increafed thicknefs of 
the glafs of which a window is form- 
ed, is not found to have the leaft 
fenfible effect in rendering a room 
warmer. 
But air is not only a non-conduc- 
tor of heat, but its non-conducting 
power may be greatly increafed. 
To be able to form a juft idea of the 
manner in which air may be ren- 
dered a worfe conductor of heat, 
or, which is the fame thing, a bet- 
ter non-conduétor of it than it is in 
its natural unconfined ftate, it will 
be neceflary to confider the manner 
in which heat paffes through air. 
Now it appears, from the refult of 
a number of experiments which I 
made witha view to the invettiga- 
tion of this fubjeét, and which are 
publifhed in a paper read before the 
Royal Society*, that though the 
particles of air, each particle for it- 
felf, can receive heat from other bo- 
dies, or communicate it to them, yet 
* See the Philofophical Tranfactions and our Regifter for 1792. 
there 
