ANNALS 
OF 
PHILOSOPHY. 
APRIL, 1819. 
ARTICLE I. 
Researches on the Measure of Temperatures, and on the Laws of 
the Communication of Heat. By MM. Dulong and Petit. 
(Continued from p. 182.) 
Of Cooling in a Vacuum. 
THE observations on the cooling in vacuo, calculated as before 
explained, are all affected by an error very small indeed, but 
which it is requisite to correct. This error comes from the small 
quantity of air remaining in the balloon, and which, in the 
greater number of the experiments, amounted only to two milli- 
metres. 
This correction cannot be applied immediately to the series of 
temperatures furnished by observation; but it can be easil 
applied to the velocities of cooling obtained by calculation. It 
is merely necessary to diminish them by a quantity corresponding 
to the heat eaet off by the air remaining in the balloon. 
To determine the amount of this correction in each case, we 
observed the cooling of our thermometer in air of different 
degrees of density ; and we calculated for the different excesses 
of temperature the velocities of cooling Sg Beane to each 
density, By subtracting from these velocities those which take 
place in vacuo, we obtain exactly the quantities of heat carried 
off by the air in its different states of rarefaction. We shall have 
nearly accurate values of these same quantities by subtracting 
the velocities already very near each other, which are given by 
the observations of cooling in the balloon, when it contains only 
a very small quantity of gas. 
Vou. XIII. N° TV. Q 
