ANNALS 
OF 
PHILOSOPHY. 
Se 
MAY, 1819. 
ey 
——— 
ArrTIcLeE I. 
Researches on the Measure of Temperatures, and on the Laws of 
the Communication of Heat. By MM. Dulong and Petit. 
(Concluded from p. 251.) 
Of Cooling in the Air and in Gases. 
THE laws of cooling in vacuo being known, nothing is more 
simple than to separate from the total cooling of a body sur- 
rounded with air, or with any other gas, the portion of the effect 
due to the contact of this fluid. For this, it is obviously suff- 
cient to subtract from the real velocities of cooling those 
velocities which would take place, if the body, caterts paribus, 
were placed in vacuo. This subtraction may be easily accom- 
plished now that we have a formula which represents this velo~ 
city with great precision, and for all possible cases. We can 
then determine the energy of cooling due to the sole contact of 
fluids, and such as it would be observed directly if the body 
could be deprived of the faculty of radiating. This part of our 
labour required a very considerable number of experiments, 
because the laws which we wished to discover were to be studied 
with respect to the different gases, and for each of them at 
different temperatures, and under different pressures. Each 
experiment was made and calculated as we have explained 
above. We shall, therefore, satisfy ourselves with stating the 
mean results of these different observations. 
The first question with which we behoved to occupy ourselves, 
was to ascertain whether the modifications of the surface of 
Vor. XII. N° V. xX 
