86 Prof. Magnus on the Propagation of Heat in Gases. 



The above experiments with cotton and eider-down speak against 

 this ; for it can scarcely be assumed that the heating takes place 

 through radiation. Moreover, the experiments which Dr. Franz 

 has published * on the radiation through hydrogen, show that 

 more heat-rays do not pass through hydrogen thau through at- 

 mospheric air. It appeared, however, necessary, before main- 

 taining that gases can conduct heat, to determine by new- 

 experiments how far that doubt was founded. Hence the deter- 

 minations on the passage of heat-rays through various gases 

 were concluded when I communicated to the Academy, in July 

 of last year (1860), the investigation on the conduction of 

 heat. 



As far as I know, Dr. Franz's are the only experiments which 

 have hitherto been published on the diathermancy of the gases. 

 These, which moreover only refer to atmospheric air, hydrogen, 

 and carbonic acid, could not be sufficient for the present pur- 

 pose, because an argand lamp was used as a source of heat. 

 But it was not merely possible, but even probable that the 

 transmission of thermal rays would differ with the source whence 

 they came. If therefore the experiments were to be conclusive, 

 the transmission must be investigated for rays proceeding from 

 the same source of heat, that of boiling water. 



Boiling water as a source of Heat. 



Dr. Franz in his experiments enclosed the gases in tubes 

 closed at both ends by pieces of plate glass. Now from Mel- 

 loni's experiments-]-, rays from so low a source of heat penetrate 

 plate glass in scarcely perceptible quantities. Even when I used 

 plates of rock-salt to close a tube a metre in length, the action 

 which the rays of boiling water produced upon the thermo-pile 

 were so small, that a comparison of the different gases furnished 

 no satisfactory results. It fui'ther appeared desirable to avoid any 

 kind of plates, even of rock-salt; for although, from Melloni's 

 experiments, the rays which have passed through this substance 

 comport themselves exactly like those which proceed directly 

 from the source of heat and have only passed through air, yet 

 the rock-salt might possibly alter the rays, and exert an influence 

 on their subsequent passage through different gases. I have 

 accordingly undertaken new experiments on the diathermancy 

 of gases for obscure heat, in which it was my object to allow the 

 rays of heat to pass through the gases without necessitating 

 their passage through any plate. 



When these experiments were finished, I saw from the ' Pro- 



* Poggendorff's Annalen, vol. xciv. p. 337. 

 t Ibid. vol. xxxv. p. 3D3. 



