160 Prof. J. Trowbridge on Spectra arising 



B, fig. 4. One also sees a strong reversed line at approxi- 

 mately wave-length 4227 ; and there is a reversed band 

 coinciding with the solar region of reversed lines between 

 wave-lengths 4315-4285. These regions in the sun are doubt- 

 less composite photographs of reversals of many elements. 

 I believe that there is a basis of reversal due to the disso- 

 ciation of water- vapour. 



The nomenclature of the stars in regard to their types of 

 spectra may need revision. The higher temperature I get 

 the more dark lines I obtain. 



An excess of dissociation of water-vapour may suffice to 

 give at a comparatively low temperature the bright-line 

 spectrum of hydrogen. At higher temperature the disso- 

 ciation of this vapour in the presence of atmospheric air may 

 give dark lines. The intense light due to the dissociation of 

 water- vapour under the effect of powerful discharges is the 

 nearest approach to sunlight which I have been able to 

 produce. Its actinic effect is greater than that of magnesium, 

 zinc, or aluminium. It may be that the variability of certain 

 stars is clue to a variability in the amount of water-vapour 

 which is being dissociated ; and one is led to conjecture 

 whether the light of the sun's atmosphere may not be due to 

 an electrical dissociation. The selective reversibility of the 

 silver salts seems to me, therefore, of great interest in the 

 subject of astrophysics ; for we can have reversible effects 

 on the photographic plates which are not due to the reversing 

 effect of colder layers of gases. In other words, we have 

 actions recorded which are on the plates and not in the 

 heavens. The intense light due to the dissociation of water- 

 vapour may entirely mask the fainter light of the metallic 

 lines in stars which show only gaseous spectra, especially 

 when we consider the varying distances of the stars. I have 

 employed electrodes of platinum, copper, silver, aluminium, 

 iron, and found no trace of the lines of their vapour in the 

 spectrum of the dissociation of water- vapour. Even when 

 caustic soda is present in the tubes, although it fills the tube 

 with a brilliant yellow light with comparatively low discharges, 

 no trace is seen of it when the tube is excited with powerful 

 discharges. Then we have the brilliant white light of the 

 water-vapour spectrum. 



The silver salt, therefore, does not respond to all rates of 

 vibration ; or if it does respond, the molecular action is 

 unstable and there is no resultant reaction which is evidenced 

 by a photographic image. There may be spectra at very high 

 instantaneous temperatures which we cannot photograph. It 



