THE SPECTRA OF GASES AND METALS AT HIGH 



TEMPERATURES. 



By John Trowbridge. 



Presented.April 8, 1903. Received May 2, 1903. 



In previous papers I have described certain phenomena which arise in 

 the employment of photography in spectrum analysis — especially the 

 phenomenon of the appearance of dark lines instead of bright lines in 

 the spectra of electrical discharges in Geissler tubes. 



The application of photography to spectrum analysis has the great 

 advantage of giving an impersonal record of certain phenomena by sub- 

 stituting a chemical method of investigation for eye observations. 



We thus obviate the personal equation of the observer ; but unfortu- 

 nately we bring in vagaries of the photographic plate. 



The photographic plate can be called an instrument with an infinite 

 number of adjustments. The molecular movements of the silver mole- 

 cules under different degrees of electric stimulation can give us a great 

 number of combinations. In short, the photographic plate does not 

 afford a simple method of observing the effects of different waves of 

 light on the molecules of matter. Its complicated nature is well shown 

 by the records it gives of the intensity of light which provokes its action. 

 It has long' been recognized that the blackness of a negative is no crite- 

 rion of relative intensity of light ; in other words, that the negative 

 cannot be employed as a photometer except in the crudest way. A 

 method of illustrating this fact is given in this paper. 



The photographic method of observation in spectrum analysis, how- 

 ever, is naturally of great use in preserving records of a great multi- 

 plicity of phenomena ; but we have to be on our guard in interpreting 

 these phenomena ; for, unlike the galvanometer or the bolometer, our 

 recording instrument is complex. Furthermore, when we use an electric 

 spark to agitate the molecules of a gas or the molecules of a metal we 

 use one complicated means to study a still more complicated phenomenon. 

 When one asks what is the spectrum of water vapor, one must define 

 the conditions of the electrical stimulus ; for one can, by increasing the 



