1819.] and on the Laws of the Communication of Heat. 331 
To enable the reader to judge of the accuracy of this law, 
we shall give in the following table, the velocities of cooling pro- 
duced by the contact of air under a pressure of 0°72 m.; the 
second column containing the values of these velocities observed ; 
‘and the third, their values deduced from the law which we have 
announced. 
Excesses of temperature. | Velocities observed. Ditto calculated. 
mee?) 5 CSUR. Aaee A SET ie ee, POG UD 5°45° 
180 ‘ ie i ae: a Sa 4°78 
mee C229 Sek ete <b ig Sa ae ee 4°14 
BD SON 20QR Ba Es ee $e te, pe. cee 3°51 
oO OS Pee eae a7 8d 9 RG 2°91 
TOMALES ELD GA oe php | A segrat en uae Zo 
og GB SB See oem 5 a iat a eb be 1:76 
LON ates aia SF Ae > Tides SOE ae ee 1:24 
LS orere as Srtuayoecurettt update: jaca tte ate 0°75 
phy doe aogsindbadd rigs". See, ae ee ic 0°32 
1t is needless to transcribe the similar comparisons which we 
have made on the other gases, and each of the pressures under 
which we have operated; for we have recognized above, that 
the series relative to each of them follow exactly the same law as 
for air, and that this law is observed under all pressures. But 
the comparisons of which we speak have afforded us as satis- 
factory results as the préceding ; and indeed this may be easily 
verified upon each of the series of observations which we have 
given above. 
To obtain a general expression of the velocity of cooling due 
to the contact of a fluid, it is necessary to collect all the parti- 
cular laws which we have made known. But the first law 
informs us that the state of the surface of the body has no 
influence on the quantity of heat which the fluid carries off from 
it, and the second law proves that. the density and the tempera- 
ture of this fluid do not affect the cooling but in as far as they 
contribute to vary the pressure ; so that the cooling power of the 
fluid depends ultimately upon its elasticity. This elasticity and 
the excess of temperature of the body are then the two only 
elements which can make the velocity of cooling vary. Denot- 
ing the first of these elements by p, andthe second by t, we 
shall have for V the velocity by the contact of a fluid. 
Vem. De ak 
b being for all gases and all bodies equal to 1-233; ¢ being like- 
wise the same for all bodies, but varying from one gas to 
another ; and m having a value which changes with the nature 
of the gas and with the size of the body. The values of c are, 
as we have found, 0°45 for air; 0°38 for hydrogen ; 0°517 for 
carbonic acid; and 0:501 for olefiant gas. The values of m 
depend, as we have said, on the dimensions of the body and the 
