132 Profs. Liveing and Dewar [Feb. 28, 



More generally we may obtain 



/ g9 \ f" x 2r+1 + ft 2 '* 1 < fo /go x f 71 " cos r6> , dO 



(84.) f'd&log.(l+iirooB^). (85.) [* delo & ^ + gr c _gL r g). 

 Jo Jo l + '2?Mr cos + m r 



and many others. 



II. " On tbe Reversal of tbe Lines of Metallic Vapours." By 

 G. D. Liveing, M.A., Professor of Chemistry, and J. Dewar, 

 M.A., F.R.S., Jacksonian Professor, University of Cam- 

 bridge. No. I. 



Since tbe celebrated paper by Kirchhoff, " On the relation between the 

 radiating and absorbing powers of different bodies for light and heat," 

 in which he detailed the remarkable experiments of reversing the 

 lines of lithium and sodium by sunlight and by the vapours of those 

 metals in the flame of a Bunsen's burner, and mentioned the reversal 

 of the brighter lines of potassium, calcium, strontium and barium 

 when the deflagration of the chlorates with milk-sugar was used 

 instead of the flame of a Bunsen's burner, further researches in 

 the same direction have been made by Cornu, Lockyer, and Roberts. 

 The method adopted by Cornu, which had been previously used 

 by Foucault, is one of great ingenuity, dependent upon so arrang- 

 ing the electric arc that the continuous spectrum of the intensely 

 heated poles is examined through an atmosphere of the metallic 

 vapours volatilized around them. By this means Cornu succeeded in 

 reversing several lines in the spectra of the following metals in addi- 

 tion to those above-mentioned, viz., thallium, lead, silver, aluminum, 

 magnesium, cadmium, zinc, and copper. He observed that in general 

 the reversal began with the least refrangible of a group of lines, and 

 gradually extended to the more refrangible lines of the group ; and he 

 drew the conclusion that a very thin layer of vapour was sufficient for 

 the reversal. It may be noted that in almost every case the lines seen 

 by him to be reversed were the more highly refrangible of the lines 

 characteristic of each metal, confirming generally the opinion expressed 

 by Stokes in a letter to Lockyer in the Proceedings of the Royal 

 Society for 1876, in which he introduces for the first time the idea of 

 the persistency of different rays with reference to temperature. 



The method adopted by Lockyer in the first instance was to view 

 the electric arc through the vapours of the metals volatilized in a 

 stream of hydrogen in a horizontal iron tube. 



