254 Messrs. A. Fowler and H. Payn. [June 10, 



that the magnesium line 4481 becomes stronger in the spectrum of 

 the metallic arc in air as the current strength is lessened.* 



The occurrence of enhanced lines in the spectrum of the arc at 

 reduced pressure, however, does not appear to have been previously 

 recorded, and it may, therefore, be useful to state briefly the results 

 which have been obtained. 



Method of Experiment. 



The experiments were conveniently made by enclosing the arc in 

 a glass globe of about a litre capacity having two necks and an out- 

 let by which connection was made with a Toppler pump. Short 

 rods of the metal to be experimented upon were attached to brass 

 rods passed through the two necks and made air-tight with a packing 

 of rubber tube and Chatterton's compound. 



The pole pieces were put nearly in contact, so that a slight pressure 

 on one of them sufficed to strike the arc. With this arrangement 

 the arc can only be observed for a comparatively short time owing 

 to the burning away of the poles, and the experiment is also inter- 

 rupted by the formation of a deposit on the interior of the globe. 

 Still it is usually possible to take two or three photographs in each 

 setting up of the apparatus. Ordinary commercial metals cast into 

 rods a quarter of an inch in diameter were used. 



[Note added July 16. In each case the exhaustion was carried as far 

 as the nature of the apparatus would permit. The pressure on starting 

 the arc was from 1 to 2 mm. 



The current was obtained from a 100-volt circuit, and in a repeti- 

 tion of the experiment with magnesium poles, measurements have 

 shown that the current strength usually employed was 7 amperes for 

 the arc in air and 8 for the arc under reduced pressure.] 



Magnesium. 



The well-known triplets beginning at 5183*84 (^) and 3838*44 are 

 apparently little affected when the globe is exhausted, but the lines 

 5528-75, 4703*33, 4352-18, 4167-81, 4058-45, and 3987*08, which 

 form a regular series,! are usually sharper than in the ordinary arc, 

 and in some photographs four additional members of the series are 

 seen (approximate wave-lengths 3938, 3904, 3879, 3860). 



The band 5007*5, as already remarked, is generally reduced in 

 intensity, while the "hydride" bands, beginning at 5618, 5210, 4849, 



* 'Sitz. der Preuss. Akad. der Wiss.,' 1903, XII, p. 234; ' Astrophysical 

 Journ.,' vol. 17, p. 270. 



t Rydberg, ' Ofversigt af Kongl. Vet. Akad. Forhandl.,' 1893, Stockholm. 



