428 ANNUAL REGISTER, 
confined air; and the knowledge 
of this fa& is very important, as it 
‘enables us to take our, meafures 
with certainty and with facility for 
confining heat, and directing its 
operations to ufeful purpofes. 
But aimofpheric air is not the 
only non-conduétor of heat. All 
kinds of air, artificial as well as na- 
tural, and in general all elaftic flu- 
ids, {team not excepted, feem to 
potiets this property in as high a de- 
gree of perfection as atmofpheric 
air. 
That fteam is not a conductor of 
heat, I proved by. the following ex- 
periment: A large globular boitle 
being provided, of very thin and 
very tran{parent glafs, with a nar; 
row neck, and its bottom drawn in- 
ward {o as to form a hollow hemi- 
{phere about fix inches in diameter ; 
this bottle, which was about eight 
inches in diameter externally, be- 
ing filled with cold water, was plac- 
ed in a fhallow difh, or rather plate, 
about ten inches in diameter, witha 
flat bottom, formed of, very thin 
fheet brafs, and raifed-upon.a tri- 
pod, and which. conlained a fmall 
quantity (about two-tenths of an 
inch in depih) of water; a {pirit- 
Jamp being then placed under -the 
middle of-this plate, in a very few 
minutes the water in the plate be- 
gan to boil, and the hollow formed 
by the bottom of the bottle was fill- 
ed with cloucs of fteam, which, af- 
ter circulating in it with furprifing 
repidity four or five minuies, and 
after forcing out agocd, deal of air 
from under the boitle, began gra- 
dually to clear up. At the end of 
eight or ten, minutes (when, as I 
fuppofed, the air remaining .with 
the fteam in the hollow cavity form- 
ed by the bottom. of the bottle, had 
acquired nearly the fame cempera- 
£797- 
ture as that of the fteam) thefe 
clouds totally difappeared; and, 
though the water continued to boil 
with the utmoft violence, the con- 
tents of this hollow cavity became 
fo perfectly invifible, and fo little 
appearance was there of fteam, that, 
had it not been for the ftreams of 
water which were continually run- 
ning down its fides, I fhould almoft 
have been’ tempted to doubt whe- 
ther any fteam was actually gene- 
rated. 
Upon lifting up for an inftant 
one fide of the botile, and letting 
in afmaller quantity of cold air, the 
clouds inftantly returned, and _con- 
tinued circulating feveral minutes 
with great rapidity, and then gradu- 
ally difappeared as before. This 
experiment was repeated feveral 
times, and always with the fame re- 
fuit ; the fteam always becoming vi- 
fible when cold air was mixed with 
it, and afterwards recovering its 
tranfparency when, part of this air 
being expelled, that which remain- 
ed had acquired the temperature of 
the fteam. 
Finding that cold air introduced 
under the bottle caufed the fteam 
to be partially condenfed, and clouds 
to be formed, I was defirous of fee- 
ing what vilible effects would be 
produced by introducing a cold fo- 
lid body under the bottle. I ima- 
gined that if {team was a conductor 
of heat, fome part of the heat in the 
iteam paffing out of it into the cold 
body, clouds would of courfe be 
formed ; but I thought if fteam was 
a non-conduc¢tor of heat; that is te 
fay, if one particle of {team could not 
communicate any part of its heat 
to its neighbouring particles ; inthat 
cafe, as the cold body could only 
affect the particles of {team aétually 
in contact with it, no cloud would 
appear 
