434 Prof. Tyndall on the Absorption and 



Many eminent writers, it is true, have supposed the upper atmo- 

 spheric regions to be colder than space, the depression of tempe- 

 rature being due to the radiation of the aerial particles, just as a 

 grass-blade is lowered, by its radiation, below the air which sur- 

 rounds it. This notion must, I think, be abandoned; for, as 

 far as experiment goes, it leads us to conclude that air, and 

 particularly air in the higher atmospheric regions, behaves as a 

 vacuum both as regards radiation and absorption. 



§ 15. In his paper on the conduction of heat by gases, Pro- 

 fessor Magnus examines the question of convection, and has 

 adduced some striking experiments to show that the cooling of 

 an incandescent wire in hydrogen is not due to the convection of 

 the gas. He finds that when the wire is enclosed in a narrow 

 tube, with only a thin film of the gas surrounding it, and where 

 therefore currents, in the ordinary sense, are hard to be con- 

 ceived of, the gas still exercises its cooling power. It had often 

 occurred to me to make this experiment ; and when I first heard 

 of its successful performance by Professor Magnus I adopted his 

 conclusion, that the cooling was due to conduction. 



Reflection, however, caused me to change my opinion. Sup- 

 pose the wire to be stretched along the axis of a wide cylinder 

 containing hydrogen, we should have convection, in the ordinary 

 sense, on heating the wire. Where does the heat thus dispersed 

 ultimately go ? It is manifestly given up to the sides of the 

 cylinder. The transfer by convection is a transfer ultimately to 

 the sides of the cylinder, and if we narrow our cylinder we 

 simply hasten the transfer. The process of narrowing may 

 continue till a tube like that used by Professor Magnus is the 

 result; the convection between centre and sides will still con- 

 tinue, and produce the same cooling effect as before. Whether 

 we assume conduction or convection, the tube surrounding the 

 wire must be supposed to possess sufficient conducting power to 

 carry the heat off, otherwise it would become incandescent itself 

 by the accumulation of the heat. 



The reasoning of Professor Magnus in connexion with this 

 subject is of extreme ingenuity. He contends that there is no 

 reason why stronger currents should establish themselves in 

 hydrogen than in other gases. Currents are due to differences 

 of density produced by the expansion of a portion of the gas by 

 heat. Now hydrogen actually expands less than other gases, 

 and hence the differential action on which the currents depend 

 is less in this gas than in the others. Professor Magnus alludes 

 to the friction of the particles against each other, but considers 

 this ineffective. 



This reasoning leads us to the threshold of a question which 

 might form the subject of a long and profitable investigation. 



