Radiation of Heat by Gaseous Mattet\ 435 



For a given difference of density, is not the mobility of hydrogen 

 greater than that of the other gases ? The experiments above 

 recorded, where different gases were brought into direct contact 

 with the source of heat, seem to answer this question in the 

 affirmative. I have had no time to pursue the question regard- 

 ing hydrogen ; but I have made a few experiments which show 

 the influence of density on the mobility of a gas in a very stri- 

 king manner. 



Having first so purified atmospheric air as to render it sen- 

 sibly 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 withdrawn from the radiating surface ; this quantity, expressed 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 nnits 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 atmosphere was used, they would have withdrawn sensiblv 

 double the amount of heat ; but the fact is that half an atmo- 

 sphere carried off 62, 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 In fact, the amount of heat withdrawn will be proportional on 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 1 

 be these velocities, we have 



68 ~ W ° r v' " 34 



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 subsequent convection. 



Here, then, we see what a powerful effect density, or the 

 internal resistance which accompanies density, has on the mobi- 

 lity of a gas ; and there is every reason to suppose that the 

 mobility of hydrogen is due to the comparative absence, in its 



