6 J. Trowbridge — Spectra from the Dissociation^ etc. 



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

 vibration ; or if it does respond, the molecule is unstable and 

 there is no resultant reaction which is evidenced by a photo- 

 graphic image. There may be spectra at very high instan- 

 taneous temperatures which we cannot photograph. It seems 

 reasonable to suppose that the silver molecule is limited in its 

 rates of vibration and that the photographic plate as well as 

 the human eye is a limited instrument of research. 



On Plate I, A represents tne solar spectrum in the neighbor- 

 hood of the great H. fl. lines. B represents the gaseous spec- 

 tra. The photographs were taken with a Rowland concave 

 grating, and are not enlarged or touched in 2i\\y way. Unfor- 

 tunately the reproductions do not give many of the reversals, 

 Figs. 1 and 2, B are spectra of air and oxygen taken with a 

 comparatively low voltage and amperage at a pressure of about 

 one millimeter. Fig. 3 is a spectra of hydrogen under the 

 same conditions. Fig. 4 is the spectrum arising from the dis- 

 sociation of water vapor with very powerful discharges. Fig. 5 

 shows a selective reversal line at approximately 4227. On the 

 negative the two bright gaseous lines, which in fig. 4 closely 

 coincide with the great H. H. lines, are seen to be reversed 

 and appear as dark lines. This reversal is shown in fig. 7. 

 B of fig. 7 closely resembles in general features the solar spec- 

 trum when the latter is photographed with a broad slit, so as 

 to obliterate details and give broad characteristics. 



My conclusions are as follows : 



Dissociation of water vapor takes place in the atmosphere of 

 the sun ; oxygen, therefore, must be present. From a careful 

 study of my negatives, I regard the evidence for the presence 

 of water vapor as conclusive as that accepted for the presence 

 of sodium in the sun. 



The dissociation of water vapor, under the effect of power- 

 ful electric discharges in the presence of small amounts of 

 atmospheric air, results in the production of argon even in 

 tubes presumably filled with dry hydrogen. 



The spectrum arising from the dissociation of water vapor 

 contains dark lines as well as bright lines. These dark lines 

 are due to a selective reversibility of the silver salt employed 

 in the photographic emulsions, and are not due to the reversing 

 action of a cooler layer of gas. The great brilliancy of the 

 dissociation spectrum of water vapor, which obscures the 

 spectra of metallic vapors : and the presence of dark lines due 

 to photochemical reversals, make one cautious in accepting 

 photographic evidence in regard to states of development of 

 stars. 



Jefferson Physical Laboratory, 

 Harvard University; 



