126 Trowbridge — Spectra of Gases. 



lary a continuous spectrum, and where the fine bright line 

 crossed this continuous spectrum, it was reversed. 



Kirehhoff's law of radiation thus applies to the radiation in 

 Geissler tubes, and Pringsheim's contention is justified. If 

 the solar corona is an electrical phenomenon of the nature of 

 luminescence, it can exhibit either bright lines or dark lines 

 according as it is hotter or colder than the background. 



In this study of the upper limit of temperature which one 

 can reach by electric discharges through rarified gases, we per- 

 ceive that spectrum analysis is one of the most difficult analyses 

 which modern science has revealed. There are a few broad 

 facts such as Doppler's principle and the reversal of spectral 

 lines according to Kirehhoff's law ; on the other hand, there is 

 ionization, dissociation, adsorption and absorption, all modified 

 by the glass or quartz vessels which must be employed. 



M. Cantor calls attention to the fact that Hittorf failed also 

 to observe reversals of spectral lines in the case of electric dis- 

 charges in Geissler tubes. Hittorf speaks of a first series of 

 hydrogen lines which are seen with feeble discharges. This 

 feeble spectrum with its bands seems to be a peculiarly lumi- 

 nescent effect in which any translatory or colliding effect of 

 the molecules is a minimum. The new theories in regard to 

 the composite nature of the atom seem to demand an exten- 

 sion of our views in regard to the nature of the light emitted 

 by atoms and their aggregates under the stimulus of an elec- 

 tric discharge. The phosphorescent and fluorescent light of a 

 gas under this stimulus may arise from the mechanism of the 

 atom and therefore may not give sensible heat. The combina- 

 tion of' atoms into molecules, and their dissociation and forma- 

 tion of new combinations, may give the spectra we usually 

 observe under the effect of fairly strong electric discharges, 

 and provide the sensible heat which can be measured by the 

 bolometer or the -thermal junction. 



Spectrum analysis of the future thus becomes more and 

 more difficult of application, and one of its most important 

 fields is in the study of phosphorescent and fluorescent light 

 emitted by gases. We seem to be on the point of regarding 

 the light and heat of the sun more from the electrical stand- 

 point. And the study of discharges of electricity in rarified 

 gases assumes a great importance. 



