EADIATION OF HEAT BY GASEOUS MATTEE. 
97 
Having first so purified atmospheric air as to render it sensibly neutral to radiant 
heat, I allowed 15 inches of it to enter the front chamber F, and there to come into 
contact with the source of heat. Convection of course immediately set in, and its 
amount was accurately measured by the quantity of heat withdrawn from the radiating 
surface ; this quantity, expressed in the units adopted throughout this memoir, was 
62. 
The quantity of gas in the front chamber was now doubled, that is, it now had an 
atmosphere of tension ; the withdrawal of heat then was expressed by the number 
68 . 
In the last experiment we had double the number of atoms loading themselves with heat 
and carrying it away ; if their motion had been as quick as that of the atoms when half 
an atmosphere was used, they would have withdrawn sensibly double the amount of heat ; 
but the fact is that half an atmosphere carried off 64, while a whole atmosphere carried 
off 68 ; hence the absolute swiftness of the atoms in the case of the denser air must be 
very much less than in the case of the rarer. In fact, the amount of heat withdrawn 
will be proportional on the one hand to the number of carrying particles, and on the 
other to the velocity with which they move ; hence if v and v' be these velocities, we have 
62 V V 62 
v'~M 
Thus, while the atoms of the rarer gas travel 62 units in a second, those of the denser 
gas travel only 34. 
This retardation can, I think, arise from nothing else than the resistance offered by 
the particles of the air to the motion of their fellows. It must be borne in mind that 
the smallness of the increment observed on doubling the amount of gas was not due to 
the partial exhaustion of the source by the first quantity of gas. The heat of the 
source was such that the withdrawal of 64 of our units could not sensibly affect the sub- 
sequent convection. 
Here, then, we see what a powerful effect density, or the internal resistance which 
accompanies density, has on the mobility of a gas ; and there is every reason to suppose 
that the mobility of hydrogen is due to the comparative absence, in its case, of internal 
resistance. However this may be, the foregoing experiment enables us to draw some 
important inferences. 
Storms at great heights must be greatly facilitated by the mobility of the particles of 
the air. In fact storms are cases of convection on a large scale, and in our front cham- 
ber we had one in miniature. With the same difference of temperature on the summit 
of Mont Blanc, the motion of convection would be very nearly twice as great as at the 
sea-level. 
In the summer of 1859 I was fortunate enough to induce my friend Professor 
Franklaxd to accompany me to the summit of Mont Blanc, and to determine the com- 
parative rates of combustion there and in the valley of Chamouni. Six candles were 
MDCCCLXII. 0 
