from the Dissociation of Water Vapour, 1(31 



seems reasonable to suppose that the silver molecule is 

 limited in its rate of vibrations, and that the photographic 

 plate as well as the human eye is a limited instrument of 

 research. 



On riate III., A represents the normal solar spectrum in 

 the neighbourhood of the great H. H lines. B represents the 

 gaseous spectra. The photographs were taken with a Rowland 

 concave grating:, ^nd are not enlarged or touched in any way. 

 Unfortunately the reproductions do not give many of the 

 reversals, and some of the bright lines do not appear. This 

 is especially the case in fig. 5. Figs. 1 and 2, B, are spectra 

 of oxygen and traces of rarefied air taken with comparatively 

 low current and voltage at a pressure of i millimetre. Fig. 3 

 is a spectrum of hydrogen under the same conditions. Fig. 4 is 

 the spectrum arising from the dissociation of water-vapour 

 with very powerful discharges. Fig. 5 shows a line of selective 

 reversibility at wave-length 4227. On the negative the two 

 brilliant lines (tig. -4) which closely coincide with the great 

 H. H lines of the solar spectrum are seen to be reversed, and 

 therefore appear as dark lines. This reversal is shown in 

 rig. 7. The gaseous spectrum B of fig. 7 closely resembles in 

 general features the solar spectrum photographed with a wide 

 slit in order to give the general distribution of darkness and 

 light. 



My conclusions are as follows : — 



Dissociation of water-vapour 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 

 pre>ence of this vapour as conclusive as that generally accepted 

 for the presence of sodium in the sun. 



The dissociation of water-vapour by powerful electric dis- 

 charges in the presence of small amounts of atmospheric air 

 re.-ults in the production of argon even in tubes presumably 

 filled with hydrogen. 



Dark line- occur in the spectra of gases which increase 

 with the intensity of the light, and are photochemical in their 

 origin. 



The great brilliancy of the dissociation spectrum of water- 

 vapour, which obscures the presence of metallic spectra, and 

 the pre-ence of dark lines due to photochemical rever>al>. 

 makes one cautious in accepting photographic evidence in 

 regard to the states of development of stars. 



Phil Mag. S. G. Vol. 4. No. 19. July 1902. M 



