819.] and on the Laws of the Communication of Heat. 323. 
First Case.—Thermometer in its natural state. 
Excess of the tem-|Total velocities of the 
perature of the| cooling of the ther- Velocities of cool-|Velocity of cooling 
thideiaiiheter: aGakeeer: ing in vacuo. due to the gas. 
80° 99-969 | 08° 17-93° 
60 16°14 3-54 12-60 
40 9-87 9-18 7-69 
20 4-28 0:95 3:33 
cas | 
ae rota vat of the then ¥Clotities of cool-|Velocities of cool- 
- Si Ley ly ba a ing in vacuo. ing due to the gas. 
30° 19-59° 1-77° 17-822 
60 13:97 1:29 12°68 
40 8:62 0:87 7:75 
20 3°74 0:37 3°37 
This comparison giyes for hydrogen a result similar to that for 
air. The equality being thus verified for surfaces differing so 
much from each other as glass and silver, and for gases of such 
different qualities as air and hydrogen, it is natural to generalize 
the result, and to deduce from it the following law. 
The loss of heat owing to the contact of a gas, other things bein 
a is independent of the state of the surface of the body sack 
cools. , 
This remarkable law of the communication, of heat has been 
already admitted by Mr. Leslie. But this skilful philosopher has. 
only given it as a probable consequence of two indirect experi- 
ments, which consist in proving that the state of the surface has 
only a very feeble effect on the time of cooling in those circum- 
stances in which radiation can contribute but a very small por- 
tion of the loss of heat. This is the case, for example, when a 
hot body is exposed to a violent wind, or when it is plunged 
into a liquid. But these experiments, however ingenious, cam 
never completely supply the place of direct observations. “And 
in the present case would it not have been possible, for example, 
to suppose that a property observed in air while violently in 
motion, could only be applied in a limited sense to air in a state 
of rest? This doubt would appear still better founded, or would 
be changed into certainty, if we admitted with Mr. Leslie that 
air in a state of rest deprives bodies of heat. by two different 
pays ; namely, by a conducting property such as exists in solids, 
and by the renewal of the fluid from ascending currents. Our 
process, by enabling us in the first place to show the existence 
of the same law in different gases, dissipates all the doubts 
which the experiments of Mr. Leslie still allowed to remain. 
This is one of the cases in which the advantages of the uniform 
method which we have adopted can be best seen. 
x 2 
