204 Professor Dewar [Juno G, 



WEEKLY EVENING MEETING, 



Friday, June G, 1879. 



George Busk, Esq. F.R.S. Treasurer and Vice-President, 

 in tlie Chair. 



James Dewar, M.A. F.R.S. 



PULLEKIAN rXtOFESSOU OF CHEMISTUT, ROYAL INSTITUTION, KTC. ' 



Spectroscopic Investigation, 



In Kirchhoff's celebrated paper " On the Relation between tho 

 Radiating and Absorbing Powers of different Bodies for Light and 

 Heat," the remarkable experiments of reversing the briglit lines of 

 lithium and sodium by causing sunlight to pass through the vapours 

 of those metals, volatilized in the flame of a Bunsen's burner, are 

 described. Bunsen and Kirchhoff reversed tho stronger lines of 

 potassium, calcium, strontium, and barium by deflagrating their 

 chlorates with milk-sugar, before the slit of the solar spectroscope. 

 Recent researches on tho artificial formation of Fraunhofer lines 

 have been made by Cornu, Lockycr, and Roberts. 



Cornu improved upon a method previously used by Foucault. 

 It depends upon so arranging the electric arc that the continuous 

 spectrum of the intensely heated poles is examined through an atmo- 

 Bphero of the metallic vapours volatilized around them. By this 

 means Cornu succeeded in reversing several lines in the spectra of the 

 following metals, in addition to those above mentioned, viz. thallium, 

 lead, silver, aluminium, 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 drew tho conclusion that a very thin layer 

 of vapour was sufficient for the reversal. In almost every case the 

 lines reversed are the more highly refrangible of the lines character- 

 istic of each metal. 



Lockyer's plan was to view the electric arc through tho vapours of 

 the metals volatilized in a horizontal iron tube. The iron tube had 

 its ends covered with glass plates, and was heated in a furnace, a 

 current of hydrogen passing during tho experiment. He did not 

 succeed in observing any new reversal of bright lines, with tho 

 oxce2)tion of an unknov/n absorption line which sometimes appeared 

 when zinc was experimented on. He confirmed, however, the 

 chaunelled-space absorption spectra observed by Roscoe and Schuster 

 in tho cases of potassium and sodium, and recorded channelled-space 



