156 Prof. J. Trowbridge on Spectra arising 



of single atoms, the work of §§ 27, 28 remains valid ; and 

 thus we arrive at the very important and interesting conclusion 

 that when there is repulsion between nearest nets, attraction 

 between next-nearests, and no force between next-next- 

 nearests or any farther, the disturbance from homogeneous- 

 ness in the neighbourhood of the bounding plane consists in 

 alternate diminutions and augmentations of density becoming 

 less and less as we travel inwards, but remaining sensible at 

 distances from the boundary amounting to several times the 

 ■distance from net to net. 



XIII. On Spectra arising from the Dissociation of Water 

 Vapour and the Presence of Dark Lines in Gaseous Spectra. 

 By John Trowbridge *. 



[Plate III.] 



IN passing from the study of the light emitted by gases 

 under the effect of electrical discharges to the investi- 

 gation of the light produced by discharges of great quantity, 

 one enters a new field of research. In previous papers on 

 the spectra of hydrogen I have stated my convictions of the 

 importance of the role played by water-vapour in glass 

 spectrum-tubes. The results'of further study emphasize these 

 convictions. With powerful discharges in hydrogen, oxygen, 

 and rarefied air, even when these gases are dried with the 

 utmost care, I always obtain the same spectrum, which I 

 regard as that arising from the dissociation of water-vapour 

 which is always present in glass tubes. The bright-line 

 spectrum, moreover, at high temperatures is accompanied by a 

 faint continuous spectrum on which are dark lines which 

 indicate a selective reversibility in the silver salt. This re- 

 versibility, it seems to me, is of great significance in the 

 application 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' vapour of a metal under the excitation of heat ; 

 and when the improvements in photography enabled us to 

 obtain permanent records of the spectra of gases, it was sup- 

 posed that we had a means of escaping from the fallacies of 

 eye-observations which arose 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 



* Communicated by the Author. 



