﻿380 Prof. H. Crew and Mr. R. Tatnall on a 



which these same gentlemen have attempted to vaporize 

 strontium, and thus obtain the strontium triplet* at X3800, 

 free from the cyanogen band. They say, however, that the 

 arc worked so badly as to give only one line out of the 

 three. 



The well-known difficulty with the spark spectrum is that 

 it is almost as characteristic of the slight differences in 

 physical condition under which it is obtained as of the 

 chemical element from which it is obtained. Not only so, 

 but owing to its streaks, as it were, of high temperature 

 (" luminescence " ?) there is obtained, at the same time with 

 the spectrum of the metal, also the spectra of the gases in 

 which the discharge takes place. 



In the case of the carbon arc, nature has fortunately 

 grouped its many thousand lines into bands, leaving here and 

 there comparatively clear spaces in which the lines due to 

 substances deliberately introduced into the arc can be studied 

 and measured with a high degree of accuracy, as exemplified 

 in the work of Rowland and of Kayser and Runge. 



Fortunately also, in the case of some metals, especially the 

 easily volatile ones, the metallic vapour acts as if it shunted 

 off the current from the carbon vapour ; and the metal comes 

 out strong in comparison with the carbon. 



At the same time, the carbon and cyanogen bands stretch 

 practically through the whole spectrum from X 3500 into the 

 infra-red. Not only so, but many of these carbon lines have, 

 as a rule, intensities quite comparable to those of the metallic 

 lines. One ingenious effort has been made by Kayser and 

 Runge (I. c.) to rid themselves of the cyanogen bands by 

 working the carbon arc in a current of carbon dioxide. This 

 is partially successful ; but, at best, it only diminished the 

 intensity of the band. Messrs. Lewis and Ferry f , speaking 

 of the infra-red spectra of the metals, say : — " It seems as 

 though little more could be done in the discovery of new 

 metallic lines unless the carbon lines are first carefully 

 mapped, or some means is devised for raising the substances 

 investigated to sufficiently high temperature without placing 

 them directly in the [carbon] arc" 



We have, therefore, devised and used during the past year 

 the following method for obtaining the arc spectrum of the 

 metallic elements free from carbon, free from air-lines, and 

 free also from any continuous spectrum. 



The idea is simply that of an arc in which one pole rapidly 



* Kayser and Runge, Wied. Ann. lii. p. 115 (1894). 

 t Johns Hopkins University Circular, May 1894. 



