322 Royal Society : — 



screwed on at each end of the main tube ; these caps are closed 

 by a glass plate at one end, and have each a small side tube 

 for the purpose of passing hydrogen or other gases through the 

 hot tube. The furnace is supplied with coke or charcoal; an 

 electric lamp, connected with thirty Grove's cells, is placed at one 

 end of the tube and a one-prism spectroscope at the other. The 

 temperatures reached by this furnace may be conveniently divided 

 into four stages : — 



I. When the continuous spectrum of the tube extends to the 

 sodium-line D, this line not being visible. 



II. When the continuous spectrum extends a little beyond D, 

 this line being visible as a bright line. 



III. When the spectrum extends into the green, D being very 

 bright. 



IV. When the spectrum extends beyond the green and D be- 

 comes invisible as a line, and the sides of the furnace are at a red 

 heat. 



I may add (1) that I have only within the last few days been 

 able to employ the third and fourth stages of heat, as the furnace 

 was previously without a chimney, and the necessary draught 

 could not be obtained ; and (2) that I was informed, a little time 

 ago, by Prof. Eoscoe that, with a white-hot tube, he had observed 

 new spectra in the case of Na and K. These spectra, which I now 

 constantly see when these temperatures are reached, I shall call 

 the " new spectra." 



9. The results of the experiments, so far as the visible spectrum 

 is concerned, between the stages indicated may be stated as 

 follows :— 



H. No absorption. 

 N. No absorption. 



K. I have observed, either separately or together : — 

 (a) The line absorption-line near D. 

 (/3) Continuous absorption throughout the whole spec- 

 trum. 

 (y) Continuous absorption in red and blue at the same 

 time, the light being transmitted in the centre of 

 the spectrum (as by gold-leaf). 

 (3) Continuous absorption clinging on one side or 

 other of the line. (This phenomenon, which, so 

 far as I know, is quite new, will be described in 

 another note.) 

 (e) The new spectrum. 

 Na. I have observed, either separately or together : — 

 (a) D absorbed. 



(/3) Continuous absorption throughout the whole spec- 

 trum, 

 (y) Continuous absorption clinging on one side or the 



other of D. 

 (2) The new spectrum. 



