410 
periment No. 4, it was 1284 seconds, 
as has just been observed) ; and this 
shews that the silk nov only did not 
eonduct the ‘heat itself, but that ‘it 
prevented the air by which its inter- 
stices were filled from conducting it: 
or, at least, it greatly weakened its 
power of conducting it. 
_ The next question which arises 
is, How air can be prevented from 
conducting heat? and this necessar- 
ily involves another, which ‘is, ‘How 
does airiconduct heat ? 
Tf air conducted heat, as it is pro- 
bable, that the metals and water, 
and all other solid bodies and un- 
elastic fluids conduct it; that is to 
say, if its particles remaining in their 
places, the heat passed ‘from one 
particle to another, ‘through the 
whole mass, as there is no reason to 
suppose that the propagation of heat 
isinecessarily in right lines, I cannot 
conceive how the interposition of 
sorsmall a quantity of any solid body 
as =!; part of the volume of the air, 
could have effected so remarkable a 
diminution of the conducting power 
of the air, as ‘appeared in the ex- 
periment (No. 4,) with raw silk, 
above mentioned. 
If air and water conducted heat 
inthe same manner, it is more than 
probable that their conducting pow- 
ers might be impaired by the same 
means; but when I made the expe- 
riment with water, by filling the glass 
globe, in the centre of which the 
bulb of the thermometer was sus- 
pended, with that fluid, and after- 
wards varied the experiment, by add- 
ing 16 grains of raw silk to the wa- 
ter, I-did not find that the conduct- 
ing power of the water was sensibly 
impaired by the presence of the'silk. 
But we ‘have just seen that the 
game'silk, mixed with an equal vo- 
ANNUAL REGISTER, 1792. 
lume of air, diminished its conduét- 
ing power in avery remarkable de- 
gree; consequently there is’ great 
reason to conclude that water and air 
conduct heat in a different manner. 
But the following experiment, f 
think, puts the matter beyond all 
doubt. 
It is well known that the power 
which air possesses of holding water 
in solution, is augmented by heat, 
and diminished by cold; and that, 
if hot air is saturated with water,'and 
if this air is afterwards cooled, a part 
of its water is necessarily deposed. 
I took a cylindrical bottle of very 
clear transparent glass, about,$ inches 
in diameter, and 12 inches high, 
with a short and narrow neck, and 
suspending ‘a small piece of linen 
rag, moderately wet, in the middle 
of it, I plunged it into a large vessel 
of water, warmed to about 100° of 
Fahrenheit’s thermometer, where I 
suffered it to remain till the con- 
tained air was not only warm, ‘but 
thoroughly saturated with the mois- 
ture which it attracted from the lin- 
en rag, the mouth of the bulb being 
well stopped up during this time 
witha good cork; this being done, 
I removed the cork for a moment, 
to take away the linen rag, and 
stopping up the bottle again imme- 
diately, I took it out of the ‘warm 
water, and plunged it into a large 
cylindrical jar, about 12 inches in 
diameter, and 16 inches high, con- 
taining just ‘so much ice-cold water, 
that, when the bottle was plunged 
into it, and quite covered by it, the 
jar was quite full. 
As the jar was of very fine trans- 
parent glass, as well as the bottle, 
and as the cold water contained in 
the jar was perfectly clear, I could 
see what passed in the bottle most 
distinctly 5 
eee 
