MORSE. — SPECTRA OF WEHNELT. 



525 



POTASSIUM. 



Spark. Potassium chloride on carbon electrodes. 

 Arc. Metallic potassium on carbon electrodes. 



Wehnelt. Carbon point in solution of potassium carbonate. (Impurities in 

 carbons same as in litliium.) (Plate 1, Fig. 3.) 



The spectrum resembles the arc more closely than the spark. The 

 difficulty of obtaining the spark spectrum unobstructed by air lines and 

 the lack of a resolution of the doublets make comparisons in the case of 

 little value. 



Magnesium. 



In the table the following points are of especial interest : 



1. The triplet A 3720, X 3721, \ 3730. These lines do not appear in 

 the spark or arc spectrum under ordinary circumstances, but are strong 

 in the spectrum of magnesium in the oxyhydrogen flame, and have 

 always been considered lines belonging to a low temperature. Liveing 

 and Dewar * observed that when the arc passed between electrodes of 

 metallic magnesium these lines were visible, provided the atmosphere 

 about the arc was one which could provide oxygen. They come out 

 clearly in air, oxygen, and carbon dioxide, but do not appear in hydrogen, 

 nitrogen, cyanogen, chlorine, or ammonia. 



2. The presence of the arc line at A 4352 and the strong spark line 

 at A 4481. One or the other of these is usually very faint under ordi- 

 nary conditions. In the arc between magnesium electrodes both lines 

 are strong, A 4481 sometimes quite as strong as in the spark.* Hart- 



* Proc. Roy. Soc, 32, 189 (1881) ; and ibid , 44, 241 fl888). 



