20 



Dr. W. Huggins. 



[Dec. 18, 



the pile from the source renders it difficult for heat to pass by con- 

 vection from the one to the other. 



Following the method pursued by Professor Buff, the radia- 

 tion through hydrogen was compared with the radiation through 

 air. To make the experiments more strictly comparable, the tem- 

 perature of the source was in all cases rendered the same. Instead 

 of having to wait 8 minutes for the needle to reach its maximum 

 deflection, it was steady there in less than a quarter of this time. 

 The result was very different from that obtained by Professor Buff. 

 Instead of the one gas absorbing from 50 to 60 per cent, of the 

 radiation, and the other none, no difference whatever was to be 

 detected between the deportment of hydrogen and that of air. Both, 

 as before, proved practical vacua to the rays of heat. 



Professor Buff describes other experiments, to one only of which I 

 need refer. He makes " the diathermancy of olefiant gas somewhat 

 higher than that of air." It mast have been a strangely defective 

 apparatus which could yield such a result. Air differs as widely 

 from olefiant gas as rock-salt differs from ice. Pursuing Professor 

 Buff's own method of experiment,* but insulating the glass cylinder 

 from the source of heat, the diathermancy of air was found to be 

 perfect, whereas 11 inches of olefiant gas absorbed 33 per cent, of the 

 total radiation. 



Such is my examination of the paper to which my attention was 

 directed two years ago by Dr. Hofmann. 



III. " On the Photographic Spectra of Stars." By William 

 Huggins, D.C.L., LL.D., F.R.S. Received December 11, 

 1879. 



(Abstract.) 



The author presented, in December, 1876, a preliminary note on 

 the subject of this paper, together with a diagram of the spectrum of 

 Vega compared with that of the sun. 



The author refers to a paper by Dr. William Allen Miller and himself 

 in 1864, in which they describe an early attempt to photograph the 

 spectra of stars. 



Other investigations prevented the author from resuming this line 

 of research until 1875, when a more perfect driving clock, by Grubb, 

 enabled him to take up this work with greater prospect of success. 



The author describes the special apparatus and the methods of 

 working which have been employed. 



The spectrum apparatus consists of one prism of Iceland spar and 



* A defective method even when every care is bestowed upon it. 



