Chemistry. XXi 
rate of cooling in each degree. Thus they were enabled to 
appreciate the portion of heat carried off by the small quantity 
of air remaining in the balloon, and hence to determine the rate 
of cooling in an absolute vacuum. I have no doubt that this 
method of proceeding will appear unsatisfactory to Mr. Leslie. 
He has given it as the result of his experiments, that what is 
called radiation of heat is merely heat carried off by the air. It 
follows as a consequence, I conceive, from this opinion, that in 
an absolute vacuum hot bodies would not cool at all. Dulong 
and Petit have not only admitted the possibility of their cooling ; 
but have even calculated the law according to which they do 
cool in vacuo. Now as cooling in an absolute vacuum can only 
take place in consequence of radiation in the strictest sense of 
the word, it follows as a consequence, if their mode of reasoning 
be accurate, that heat is actually radiated from the surface of 
bodies, and not carried off, as Mr. Leslie supposes, by aerial 
pulses. 
If the common notion of radiation be correct, it is obvious that 
' the cooling of a hot body in vacuo must be the consequence of 
the excess of its radiation above that of the surface which sur- 
rounds the vacuum. It occurred to our authors to examine the 
rate of cooling, when the temperature of this surface was made 
to vary. From five sets of experiments which the reader will 
find given in the Annals of Philosophy, xii. 243, it appears that 
the rapidity of cooling increases as the temperature of the sur- 
rounding surface increases. This seems to mea very extraordi- 
nary fact. I do not know well how it can be reconciled to the 
commonly received doctrine of radiation. I wish very much, 
therefore, to see these experiments repeated and verified. Our 
authors have expressed this very curious law in the following 
manner : 
“ The velocity of cooling of a thermometer in vacuo for a con- 
stant excess of temperature increases in a geometrical progression 
when the temperature of the surrounding medium increases in an 
arithmetical progression. The ratio of this geometrical progres- 
sion is the same, whatever be the excess of temperature 
considered.” 
The law of cooling in vacuo, which our authors discovered by 
means of the experiments just alluded to, they express by the 
following proposition : 
“ When a body cools in vacuo, surrounded by a medium 
whose temperature is constant, the velocity of cooling for excess 
of temperature in arithmetical progression increases as the terms 
of a geometrical progression diminished by a constant quantity.” 
And this law holds whether the surface of the cooling body be 
glass or silver. 
If we were to suppose a body cooling in vacuo simply by 
radiation, and not to receive any heat by radiation, then the 
rate of cooling would follow the terms of a geometrical series ; 
