162 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Apparatus. 



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

 which enables one to use chemically pure electrodes having little or no 

 chemical reaction with the gas employed. In this arc, then, one may 

 expect the gas to give off its characteristic radiations with greater in- 

 tensity than in one where the gas may enter into chemical compounds 

 before a temperature is reached at which it becomes luminous. This arc 

 enables one also to select such metals as do not have strong lines in the 

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

 arc there are few sjjaces not already occupied by lines of carbon or of an 

 impurity. 



In the rotating arc, one electrode, either a disc or a rod of metal, 

 rotates upon an axis, making about 700 rotations per minute, while the 

 other electrode has a slow movement of translation toward the axis of 

 rotation. The rotation not only prevents the excessive heating and 

 welding together of the electrodes, but it throws the hot gases to one 

 side, so that the arc has the appearance of a small fan. The part of the 

 flame thus separated from the poles is very free from continuous 

 spectrum. 



In the apparatus used in these experiments the arc is enclosed in a 

 brass box, or " hood," having a volume of about 1^ litres and being com- 

 paratively gas-tight. The light from the arc issues through a long brass 

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

 wall of the hood, but is so far removed from the arc that it receives com- 

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

 hence remains clean. 



A stream of gas enters the hood at one stoi^-cock and leaves it at 

 another ; a third cock is provided for attachment to a manometer. Al- 

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

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

 sure inside the hood above that outside, and partly by the fresh supply of 

 pure gas constantly running through the hood. The hydrogen used was 

 generated electrolytically, and varied in quantity from 10 to 15 litres 

 per hour. 



The spectra have been examined both visually and photographically 

 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). 



