Radiation of Heat by Gaseous Matter. 425 



While, however, in the case of the elementary gases the dis- 

 crepancy between Professor Magnus and myself consists in a 

 defect on my part, or an excess on his, with the powerful gases 

 I obtained a considerably stronger action than he does. Thus 

 with olefiant gas his absorption amounts to less than 54 per cent., 

 whereas in mine it amounts to more than 72. This last result 

 is only what might be expected, inasmuch as the length of gas 

 traversed by the radiant heat was in the one case a little under 

 15 inches, and in the other 33. 



Professor Magnus has further published an account of experi- 

 ments in which a powerful gas-flame surrounded by a glass 

 cylinder furnished the source of heat, the latter being aug- 

 mented by a parabolic mirror of polished metal placed behind 

 the lamp. In this case the gases were enclosed in a glass tube 

 1 metre long and 35 millims. in diameter, the two ends of which 

 were stopped with plates of glass 4 millimetres thick. 



Two series of experiments were executed with this tube, in 

 one of which the interior surface was covered with black paper, 

 while in the other the glass was uncovered within. The former 

 method is that pursued by Dr. Franz ; and the result obtained 

 by Professor Magnus in the case of atmospheric air and oxygen 

 closely agrees with that obtained for the same gases by Dr. 

 Franz. Professor Magnus makes the absorption in the case of 

 the blackened tube about 2J, and Dr. Franz about 3 per cent., 

 for air and oxygen. 



In the case of the unblackened tube, however, the absorption 

 was found to be much more considerable. Here the absorp- 

 tion by air and oxygen amounted to 14*75 per cent., and with 

 hydrogen it reached 16 '23. This great difference between the 

 unblackened and the blackened tube is ascribed by Professor 

 Magnus to a change of quality which the heat undergoes by its 

 reflexion from the interior glass surface. 



One of my motives in introducing a glass tube into the pre- 

 sent inquiry was, that I might be enabled to investigate the in- 

 teresting question raised by this surmise of Professor Magnus. 

 I have failed, however, to obtain his result. My naked glass 

 tube, which is nearly of the same length as his, gives me a result 

 which is more than 140 times less than his in the case of air and 

 oxygen, and more than 160 times less than what he has obtained 

 with hydrogen. Our sources of heat are, it is true, different, 

 but the disadvantage is on my side ; for assuredly the rays from 

 a gas-jet are, if anything, less affected by the transparent ele- 

 mentary gases than those from my source. Had I time, I would 

 repeat the experiments with a flame ; but this, I regret to say, 

 is out of my power at present. 



Another difference between Professor Magnus and myself has 

 Phil Mag. S. 4. Vol. 24. No. 163. Dec. 1862. 2 F 



