838 Dulong and Petit on the Measure of Temperatures, &c.[May, 
eooling for all possible circumstances. We must recollect that 
by velocity of cooling we mean always the number of degrees 
which the temperature of the body would sink during an infinitely 
“ginal and ‘constant interval of time. 
First Law.—If we'could observe the cooling of a bedy placed 
jn a vacuum surrounded by a wall, totally destitute of heat or 
deprived of the faculty of radiating, the velocities of coolin 
would decrease in a geometrical progression, while the temper- 
atures diminished in an arithmetical progression. 
Second Law.—For the same temperature of the walls of the 
vacuum in which the body is placed, the velocities of cooling for 
excesses of temperature in arithmetical progression decrease as 
the terms of a geometrical progression, diminished by a constant 
number. The ratio of this-geometrical progression is the same 
for all bodies, and is equal to 1:0077. 
Third Law.—The velocity of cooling in vacuo for the same 
excess of temperature, increases in a geometrical progression, 
while the temperature of the walls of the vacuum increases in an 
arithmetical progression. The ratio of this progression is like- 
wise 1-0077 for all lodies. 
Fourth Law.—The velocity of cooling due to the sole contact 
of a gas is entirely independent of the nature of the surface of 
the body. ¢i 
Fifth Law.—The velocity of cooling due to the sole contact 
of a fluid varies in a geometrical progression, while the excess of 
temperature itself varies in a geometrical progression. If the 
ratio of this second progression be 2, that of the first is 2°35, 
whatever be the nature of the gas and its elastic force. This 
law may be likewise announced by saying, that the quantity of 
heat carried off by a gas is in all cases proportional to the excess 
of the temperature of the body raised to the power 1-233. 
Sixth Tats Phe cooling power of a fluid diminishes in a 
geometrical progression when its tension itself diminishes in a 
geometrical progression. Ifthe ratio of this second progression 
is 2, the ratio of the first is 1°366 for air; 1°30] for hydrogen ; 
1431 for carbonic acid ; and 1-415 for olefiant gas. 
This law may likewise be presented in the following manner: 
The cooling power of a gas is, all other things being equal, 
proportional to a certain power of the pressure. The exponent 
of this power, which depends on the nature of the gas, is 0°45 
for air; 0°315 for hydrogen; 0°517 for carbonic acid ; and 0°501 
for olefiant,gas. ; 
Seventh Law.—The cooling power of a gas varies with its 
temperature in such a manner that if the gas can dilate, and if 
it preserves always the same elastic force,’ the cooling power 
will be as much diminished by the rarefaction of the gas as itis 
increased by its augmentation of temperature ; so that ultimately 
it depends only on its tension. 
e see from these propositions that the total law of cooling, 
