376 Mr. J. N. Lockyer's Spectroscopic Notes. No. III. [June 11, 



tube closed at each end with glass plates ; and I have to express my ob- 

 ligations to Dr.*Bussell for allowing them to be conducted in his labora- 

 tory, and for much assistance and counsel concerning them. 



A stream of dry H was allowed to pass. The tube was heated in a 

 Hofmann's gas-furjiace, pieces of the metal to be studied having previ- 

 ously been introduced. It was found that the glass tube melted ; it was 

 therefore replaced by an iron one. The inconvenience of this plan, how- 

 ever (owing to the necessity for introducing the metal into the end of the 

 hot tube when the first charge had volatilized), and, moreover, the 

 insufficiency of the heat obtainable from the gas-furnace, soon obliged me 

 to replace both tube and furnace by others, which have now been in use 

 for many weeks, and which still continue to work most satisfactorily. 



The iron tube is 4 feet in length, and is provided with a central en- 

 largement, suggested to me by Mr. Dewar, forming a T-piece by the 

 screwing in of a side tube, the end of which is left projecting from the 

 door in the roof of the furnace. Caps are 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 hydro- 

 gen 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 becomes in- 

 visible 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 spectrum, 

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



