Frequencies in the Spectra of Various Elements. 463 



was sealed on the end B. Connected to this iron tube, a 

 small iron rod passed along S. At the farther end it had 

 a ring from which a small chain dipped into the mercury. 



ID 



FJG.l. 



This was to keep the mercury in electrical contact with the 

 iron electrode. The tube, was then highly exhausted by a 

 Gaede pump through a brass tube sealed on M N, and when 

 a low vacuum was reached the metal was vaporized by 

 heating the receptacle L with a Bunsen burner. When the 

 tube was in operation, the terminals o£ an auxiliary heating 

 circuit were attached at H and K. The impact voltage was 

 applied between K and B, the latter being the positive 

 terminal. In taking photographs, the tungsten was brought 

 to incandescence by means of the auxiliary heating circuit, 

 the metal in L was heated by the flame of a Bunsen burner 

 to produce vapour of the metal, and the collimator of a small 

 quartz Hilger spectrograph of type A was lined up with the 

 arm S in front of the window at the end B. 



(a) Mercury Vapour. 



With mercury vapour, the results obtained were no better 

 than those published originally by McLennan and Henderson. 

 With an impact voltage of about 5 volts it was found an easy 

 matter to obtain the monochromatic radiation of wave-length 

 X = 2536"72 A.U. With still higher impact voltages, no 

 trace of shorter wave-lengths even with long exposure was 

 obtained until the impact voltage was sufficiently great to 

 cause the arc to strike. Reproduction No. 2, fig. II. (PI. XIII.) 



212 



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