THE 



AMERICANJOURNAL OF SCIENCE 



[FOURTH SERIES.] 



Art. I. — On Spectra Arising from the Dissociation of Water 

 Vapor ^ and the Presence of Dark Lines in these Spectra ; 

 by John" Trowbridge. (With Plate I.) 



In passing from the stncl_y of the light emitted by gases 

 under the effect of electrical discharges of comparatively small 

 quantity to the investigation of the light produced by dis- 

 charges of great quantity, a new field of research is entered. 

 In previous papers on the spectra of hydrogen, I have stated 

 my convictions of the importance of the role played by 

 water vapor in spectrum tubes. The results of further study 

 emphasize these convictions. With powerful discharges in 

 hydrogen, oxygen, and rarified air, I always obtain the same 

 spectrum, which I regard as that arising from the dissociation 

 of rarified water vapor. The line spectrum, moreover, is 

 accompanied by a faint continuous spectrum on which are dark 

 lines, which indicate a selective reversibility in the silver salt, 

 which is of great significance, it seems to me, in the applica- 

 tions of photography to astrophysics. 



It has long been recognized that spectrum analysis is an 

 extremely delicate method of recognizing the presence of a gas 

 or the vapor of a metal under the excitation of heat ; and when 

 the improvements in photography enabled us to obtain per- 

 manent records of the spectra of gases, it was supposed that we 

 had a means of escaping from the fallacies of eye observations 

 which arose necessarily from personal idiosyncracies. If the 

 photographic plate were a perfect instrument for recording the 

 infinite number of vibrations which light can communicate to 

 atoms of matter, we should certainly feel that we had made a 

 great advance in physical science. When we reflect, however, 

 on the belief that emulsions containing silver salts are capable 

 of responding to all wave lengths of light in the portion of 



Am. Joub. Sci.— Fourth Series, Yol. XIV, No. 79.— July, 1902. 

 1 



