826 Dulong and Petit onthe Measure of Temperatures, [May, 
From all these comparisons, we may deduce the following 
law: 
The velocity of cooling of a body, owing to the sole contact of a 
gas, depends, for the same excess of temperature, on the density and 
temperature of the fluid; but this dependance is such that the 
velocity of cooling remains the same, tf the density and the temper- 
ature of the gas change in such a way that the elasticity remains 
constant. 
‘Hence in experiments on cooling by the gases, we need only 
attend to their elasticity. Itis, therefore, the influence of this 
last element that we must endeavour to appreciate. 
_. With this view we have determined for each gas, at different 
elasticities, the velocities of cooling for the same excesses of 
temperature. Of each of these series of experiments, we shall 
merely give what is necessary to demonstrate the law which we 
have obtained. ) 
Let us begin with air. 
The following table exhibits the corresponding velocities of 
cooling, owing to the sole contact of air under the following 
pressures; 0°72 m. 0°36 m. 0°18 m. 0°09 m. 0:045 m.; that is 
to say, under pressures ‘decreasing as the numbers 1, 3, 4, 4, +5. 
Baceeatten Velesty, Mio, Prev[Ditn, Pre Die} Dito, 
above the air.|sure 0°72 m. [UTE aT, i 0°09m. | 0°045m. 
5°48 401° 2°95° 2°209 
200° 1-59° 
180 4:75 3:52 2°61 1-90 1:37 
160 a7 3-08 2°91 1-62 1:20 
140 3°51 2°62 1-91 1-40 1:02 
120 2-90 2°12 1-57 1-15 0°84 
100 2:97 1:69 1:23 0.90 0°65 
80 1°77 1-29 0:96 0°70 0-52 
60 1-23 0-90 0°65 0-48 0:35 
0-75 
0°32 
If we take the ratios of the corresponding number in the 
second and third columns, we find their values commencing 
with the uppermost as follows : 
P82, V86i o87 4184 ©. 1-37 5. 14a ber oo Fae 
“We have likewise for the ratios of the numbers in the third 
and fourth columns: 
PIO"... Poa aot Salaries. MOO). ghee ol lume? 
The ratios between the fourth and fifth columns are : 
1°34 (4).-1-37. BG .. WBON137).. HES .. 1:37> 0085 
And lastly, the ratios between the fifth and sixth columns - 
are: . 
BB 08: P86 P87 0 4986 - 1°87 5 285-137 
