292 Mr. G. A. Hemsalech and the Comte de Gramont on 



raised and lowered by means of some mechanical device, 

 is placed vertically above the lump of metal and commu- 

 nicates with the negative end of the circuit. The arc is 

 started by lowering the carbon rod and bringing it into 

 contact with the piece of metal. As soon as the arc is 

 struck the metal melts and forms a globular mass from 

 which the arc continues to burn quite steadily. The role 

 of the copper plate is, naturally, to dissipate the heat generated 

 by the arcing, and thus to prevent the rapid boiling away of 

 the metal. With this device good continuous arcs have been 

 obtained with the following metals : Sb, Bi, Mn,Na, Sn, Cd, 

 Zn, Pb, TI, Al, Mg, and Li. 



§ 3. Spectroscopic methods employed. 



Nearly all our observations were made photographically. 

 In order to cover the whole region of the spectrum between 

 X 2150 and X 7000 the following three instruments were 

 made use of : 



1. Quartz spectrograph provided with one 60° Oornu- 



prism for the region from X 2150 to X 3200. 



2. Uviol glass spectrograph by Hilger with two 60° 



prisms for the region X 3170 to X 5000. 



3. Glass spectrograph covering the region X 3600 to 



X 7000 and consisting of three 45° prisms by 



Steinheil. 



Lockyer's method of projecting an image of the source 



upon the slit was used throughout, and the focussing and 



adjustment of the electrodes upon the slit were done in the 



manner previously described by one of us *. 



§ 4. Preliminary observations. 



An arc was established between metallic lead and a carbon 

 cathode with the arrangement described in §2. Both visual 

 and photographic observations failed to reveal any trace of 

 spark lines even in the immediate vicing of the molten 

 metal. During these experiments our attention was attracted 

 by the strong heating of the carbon cathode, the tip of which 

 became white hot almost from the moment of starting the 

 arc. We concluded from this observation that large numbers 

 of the electrons expelled by the incandescent carbon were 

 shot through the arc gap and, by reason of their intense 

 ionizing action, were lowering the resistance of the medium 

 through which the arc was passing. Accordingly, in a 



* Hemsalech, Philosophical Magazine, vol. xl. p. 316 (1920). 



