•PRODUCTION OP COLD. 
117 
scarcely gives more heat in its combustion than would be fur- 
nished by lib, of the same sort of wood burned, and in its na- 
tural state. 
V. 
A Mode of producing intense Cold. By M. H. B. 
To Mr Nicholson. 
SIR, 
I HOPE you will give early insertion to the following pro- 
posal for producing intense cold. The proposal itself is all 
that I can communicate at present ; perhaps I may be able, in 
a short time, to send you an account of some experiments on 
this subject. 
The diminution of temperature observed during the exhaus- 
tion of the receiver of an atr-pump, appears to suggest a prin- 
ciple on which the degree of artificial cold may be very much 
increased. The degree of cold produced in this way appears to 
be proportionate to the degree and rapidity of the rarefaction 
of the air contained in the receiver ; to increase the quantity 
of rarifaction, therefore, would be to increase the intensity of 
the cold. The quantity of rarefaction will be measured by the 
difference between the density of the air employed, before and 
after the experiment j the denser the air is before, and the 
rarer after the experiment, and the greater the rapidity with 
which this change is effected, the more intense will be the cold 
induced. 
To generate artificial cold, it is proposed, that a cylinder be 
filled with air, which is, by means of an accurate piston, to be 
subjected to very strong pressure. The cylinder and contained 
air are to be cooled as much as possible by the best pijocific 
mixture, and in this state the air is to be allowed to escape 
through an orifice, into a large exhausted receiver. Any sub- 
stance contained in the cylinder, or exposed to the stream of 
expanding air in the receiver, will have its temperature very 
much reduced. And as the air may be compressed to an inde- 
finite 
Diminution 
of tempera- 
ture by ex- 
haustion pro- 
portioned to 
difference of 
density pro- 
duced. 
It is proposed 
to condense 
the air of a 
receiver, and 
then suddenly 
rarify it. 
