62 Prof. J. Trowbridge on Spectra of 



when metallic terminals were 1 centimetre apart in lead- 

 glass. Iron terminals gave no iron lines in the quartz tube?, 

 while aluminium lines appeared when aluminium terminals 

 were used instead of iron terminals. 



When the metallic terminals were placed 3 millimetres 

 iipart in the quartz tubes, the light from the tubes was very 

 feeble ; traces of metallic spectra appeared, and the walls of 

 the quartz capillaries were speedily covered with thin films 

 of the metals ; even at the first discharge, before there was a 

 sensible obscuration due to the formation of the films, there 

 was no evidence of gaseous spectra. The main discharge 

 appeared to be carried over by the metallic vapour, and no 

 dissociation of the gas was evident. 



Measured by definite amounts of electrical energy, the 

 rating of the intensity of spectral lines differs totally from 

 existing eye estimates. The lines which coincide with the 

 H.H lines in the spectra of the metals with high melting- 

 points generally came out first on the photographic plate 

 when the method of successive discharges was employed. 



Rarefied nitrogen gave far less light than hydrogen, water 

 vapour, or oxygen. When oxygen was employed charac- 

 teristic groups of doublets were obtained similar to the A 

 and B groups in the solar spectrum. The heads of these 

 groups apparently coincided with the middle of the broad lines 

 shown in fig. 4. The middle of these broad bands or broad 

 lines coincides also with narrow lines usually attributed to 

 silicon. Are certain lines attributed to silicon really oxygen 

 lines ? Salet and also Rowland assign the photometric in- 

 tensity of 4 to the lines 4131'5 and 4126-5, 3 to the lines 

 3905 to 3855-7, and 10 to the line 2881. When the lines 

 given on fig. 4 and those on fig. 7 are photographed on 

 the same plate by the same number of discharges, and are 

 compared in regard to intensity, the rating is completely 

 reversed ; the lines at 4131-5, 4126-5, and 3905 to 3855*7 

 being 10, and the line 2881 being 3 or 4. 



The broadening of what have been considered metallic 

 lines in rarefied gases I consider a most interesting pheno- 

 menon. Only the strong lines of the spark-spectrum of the 

 metal in air seem to be reversed under the effect of powerful 

 discharges in rarefied gases. This broadening appears to be 

 the evidence of reactions between the vapour of the metal 

 and the surrounding gases. In this connexion it is well to 

 bear in mind the fact that metals continue to give off gases 

 for a long time when submitted to powerful electrical dis- 

 charges in vacuum-tubes. 



These nascent gases are in condition to exhibit complicated 

 reactions with the strongly heated metallic terminals. 



