162 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Apparatus. 



In searching for these lines I have employed the rotating metallic arc » 

 wh "h s'one to nse chemically pnre electrodes having httle or no 

 rhemical reaction with the gas employed. In this arc, then, one my 

 exne the gas to give off its characteristic radiations with greater m- 

 ensi v han "n one where the gas may enter into chemtcal compounds 

 W re a temperatnre is reached at which it becomes lum.nou. Tins ar 

 enables one also to select snch metals as do not have strong hues m the 

 neighborhood of the lines sought for, while in the spectrum of the carbon 

 arc there arc few spaces not already ocenpied by lines of carbon or of an 



Tie rotating arc, one electrode, either a disc or a rod of meUjl 

 rotates npon an axis, making abont 700 rotations per minute, while the 

 o her electrode has a slow movement of translation toward U-».. f 

 rotation The rotation not only prevents the excessive heating and 

 weTdtng together of the electrodes, bat it throws the hot gases to one 

 Tide o° thai the arc has the appearance of a small fan. The part of the 

 tne tl separated from the poles is very free from continuous 



SP TnTe apparatus used in these experiments the arc is enclosed in a 

 brass box or ■ « hood," having a volume of about 1* litres and being corn- 

 el ivly .as-tight. The light from the arc issues through a long bras 

 LTdosed with a lens at the outer end ; the lens thus forms part of the 

 S of the hood, but is so far removed from the arc that it receives »m- 

 parativel, little of the deposit sometimes formed inside the hood, and 



hence remains clean. lnnT , 0[! - f _ t 



A stream of gas enters the hood at one stop-cock and leaves ,t at 

 another- a third cock is provided for nttachment to a manometer A - 

 1th the hood is not absolutely gas-tight, the purity of the gas inside 

 preserved in these experiments, partly by the small excess of pres- 

 Tre i id tie hood above that outside, and partly by the fresh supply of 

 ; u e gas constantly running through the hood. The hydrogen used w 

 generated electrolytically, and varied in quantity from 10 to lo hues 



^ThTspectra have been examined both visually and P» b ical,y 

 by means of a small plane grating spectroscope and by means of a large 

 concave grating spectroscope. 



* Crew and Tatnall, Phil. Mag., 38, 379 (1894). 



