324 Dulong and Petit on the Measure of Temperatures, (May, 
The principle which we have just established being well veri- 
fied, we may confine ourselves in the remainder of our experi- 
ments to observe the cooling of the thermometer with the bulb 
naked in air and the different gases. Henceforth we shall give 
only in our tables the effect produced solely by the contact of the 
gas. They have been always calculated, as we have said before, 
by subtracting from the total velocities of cooling, those which 
would take place in the same circumstances if the thermometer 
were cooling in vacuo. ; 
We shall now enter into the examination of the different cir- 
cumstances which may modify the action of the elastic fluids in 
the production of the phenomenon which occupies our attention. 
We shall study the influence of each of these causes, first in 
air, then in hydrogen, carbonic acid, and olefiant gases. We 
made choice of the first two, in consequence of the great differ- 
ence of their physical properties. Air and olefiant gas, on the 
contrary, offer the curious ne oi of two gases of almost 
the same specific gravity, but of very different composition. The 
example of the influence of the temperature of the surrounding 
medium has upon the rate of cooling in vacuo, naturally led us 
to examine in the first place, if the temperature of the gases does 
not produce an analogous effect upon the quantity of heat which 
they carry off. It is needless to say that such experiments had 
not yet been attempted, the philosophers who have tumed their 
attention to this subject having always supposed that the veloci- 
ties of cooling depend only on the excess of temperature. 
Without stopping to detail our first attempts, we shall state 
immediately those tables in which the law shows itself mani- 
festly. In the experiments in question, the temperature of the 
gases was varied by heating sufficiently the water of the balloon; 
but the gas was at the same time allowed to dilate itself, so that 
it always preserved the same elasticity. The following table 
contains the results of such a set of experiments made 
upon air. 
lVelocities of 
Excess of thejcooling due to 
thermometer the contact of 
above the sur-jair under the 
rounding air. |pressure 0°72m, 
Ditto pressure! 
0-72 m. temp, Ditto temp. 60°. 
0° | 
Ditto temp, 80°. 
| 
temp, 20°. 
200° 548° | 546° | —_ _ 
180 4:75 470 | 4-79 —_ 
160 4:17 416. | 4:20 4:13 
140 351 3°55.) | 3°55 3°49 
120 2:90 2935505 2°94 2°88 
100 2:27 2:28 2°24 2°25 
80 Ld 1:73 1-7) 1:78 
60 1:23 1-19 1:18 1-20 
Nee nn nn UE EEE EEE EERE 
