1816.] the Absorption of Gases ly Water. 228 
I must now leave it to be determined whether <‘ it would appear 
from these experiments of De Saussure, that Mr. Dalton’s theory 
fof the absorption of gases by liquids] is erroneous in every parti- 
eular.” I remain respectfully yours, 
Joun Darron, 
ArticLte VI. 
Defence of the Objections to Prevost’s Theory of Radiant Heat, 
By John Murray, M.D. F.R.S.E., Lecturer on Chemistry in 
Edinburgh. 
(To Dr. Thomson.) 
SIR, Edinburgh, Jan, 20, 1816. 
In the general view you have given in your last Number of the 
late improvements in science, you mention that Mr. Davenport had 
written a very complete refutation of some objections that had been 
started against Mr. Prevost’s theory of Radiant Heat. This refers, 
J believe, tothe answer given by that gentleman to some objections 
to the application of that theory to radiant cold. One of these 
objections J had advanced, and 1 now take the liberty of making a 
few observations on Mr. Davenport’s reply to it, which I was pre- 
vented by circumstances from doing at the time it was published. 
When a tin cannister containing a freezing mixture is placed 
opposite to a reflecting metallic mirror, in the focus of which a 
thermometer is placed, if one of the surfaces of the cannister be 
covered with a coating of lamp-black, it is known that the depres- 
sion of temperature which is indicated by the thermometer, is 
much greater than when the clear metallic surface is opposed. 
This appears to me inconsistent with Prevost’s explanation of 
radiant cold. ‘That explanation assumes that the effect depends 
merely on the interchange of rays of heat between the thermometer 
and the cold surface, regulated by the mirror, the thermometer 
being at a higher temperature, and therefore giving off more 
radiant heat than it receives in return from the cold body; so that 
its temperature falls. Now it seems obvious that of different sur- 
faces giving off different portions of caloric by radiation at the 
same temperature, the one which gives heat will allow of the 
greatest depression of temperature in the thermometer, for it is 
the one which will make the least return. A metallic surface is 
that which radiates least, it therefore should cause the greatest 
degree of cold when opposed to the thermometer; but it causes 
the least, and the blackened surface which discharges the largest 
quantity of caloric by radiation is the one which, in this experi- 
ment, causes the greatest depression of temperature in the ther- 
mometer. 
Mr. Davenport’s reply (which has been considered as satisfactory 
