2 J. Trowbridge — Spectra from the 



the spectrum considered most actinic, even when the light ex- 

 ceeds a certain intensity, we are conscious that we cannot rely 

 upon an infinite range of photochemical action ; and I shall 

 show in this paper the existence of a selective reversibility 

 produced on the photographic plate by powerful discharges of 

 electricity through capillary tubes. 



Realizing the importance of studying the behavior of gases 

 under different forms of excitation, I have collected in the 

 rooms devoted in the laboratory to spectrum analysis three 

 forms of apparatus : an induction coil actuated by a very effi- 

 cient liquid break, giving a spark of thirty inches in air ; a 

 step-up transformer excited by an alternating current, pro- 

 ducing with glass condensers of about '3 microfarad, dis- 

 charges of an inch in length, of great body ; and a storage 

 battery of twenty thousand cells. A plant of this nature I 

 conceive to be necessary in the present state of spectrum anal- 

 ysis, for molecular motions excited in rarified gases vary greatly 

 with the kind of electrical discharge. In the application of 

 photography to spectrum analysis, the investigator is immedi- 

 ately confronted with the necessity of submitting the gas to a 

 comparatively long electrical stimulus in order to obtain a 

 negative. Even with a concave grating of short focus several 

 discharges are necessary with a narrow slit. Each discharge is 

 capable of modifying the condition of the gas. This fact is 

 well recognized by taking successive photographs of the light 

 emitted by a spectrum tube under different strengths of cur- 

 rent upon the same photographic plate. A simple form of 

 plate holder enables this to be done. One obtains a striking 

 example of the instability of the spectrum tube filled with 

 apparently dry hydrogen when one subjects it first to very 

 powerful discharges from a glass condenser of '6 microfarad, 

 charged by a storage battery of twenty thousand cells, with 

 practically no self-induction in the circuit ; and follows this 

 excitation by the use of an alternating discharge of much less 

 quantity. The powerful discharge gives what I term the water 

 vapor spectrum ; and after a certain number of these dis- 

 charges one obtains with the alternating current discharges the 

 spectra of argon. This results, I suppose, from the oxidization 

 of traces of air in the tube. The hydrogen has disappeared 

 momentarily ; possibly the dissociation of the water vapor 

 resulted in concealing its presence. 



The period of the condenser discharges which I have em- 

 ployed varied from one five hundred thousandth of a second 

 to one millionth. The practically instantaneous current varied 

 from five thousand amperes to ten thousand. The revolving 

 mirror method showed that the pilot spark was mainly effect- 

 ive, and that the subsidiary oscillations were feeble. The 



