from the Dissociation of Water Vapour. 159 



Teissier tubes filled with hydrogen which has been dried 

 with care, we also obtain a faint continuous spectrum on 

 which are bright lines and dark lines. Moreover, what are 

 apparently the strongest bright lines of the dissociation- 

 spectrum of water-vapour are not reversed. There is a 

 selective reversibility which arises at high temperatures. 



This fact seems to me of great importance in the appli- 

 cation of photography to the study of celestial phenomena. 

 Reversal of spectrum-lines does not necessarily indicate re- 

 versing layers of cooler gases, and in certain cases may arise 

 from photochemical action of the silver salt. One imme- 

 diately thinks in this connexion of the phenomenon of dark 

 lightning or the Clayden effect, and of the interesting expe- 

 riments of Professor Mpher *. Spectrum analysis, however, 

 reveals a selective reversibility which must be carefully 

 studied before we can properly interpret the records of 

 photography. There are doubtless many states of vibration, 

 even in the actinic portion of the spectrum, which are not 

 recorded by the silver salt : for this selective reversibility may 

 obliterate or prevent a permanent record. I have obtained 

 this reversing action with different emulsions on glass and 

 also on celluloid films. The strongest reversals are approxi- 

 mately at wave-lengths 4227, 3930, 3965. There is also a 

 faint reversal at wave-length 3953. Reversals are often 

 seen on the negative which disappear in the fixing-batb. 



In this investigation ten thousand cells were employed to 

 charge a glass condenser of *6 microfarad. The charge was 

 sent through Geissler tubes with practically no self-induction 

 in the discharge-circuit. The bore of the capillary tubes was 

 1 mm., and the tubes were filled with apparently dry 

 hydrogen at a pressure of approximately '1 mm. The tubes 

 were also filled with oxygen and also with rarefied air at the 

 same pressure. I have reached a limit in subjecting glass 

 tubes to powerful discharges, and am now turning my 

 attention to obtaining quartz tubes in the hope of securing a 

 more resisting material. 



In a previous paper f I expressed my conviction that the 

 four-line spectrum observed in the protuberances of the sun 

 is an evidence of the presence of water-vapour in the sun's 

 atmosphere, and an evidence, therefore, of the presence of 

 oxygen. In the spectrum (Plate III. fig. 7, B) one sees on 

 the negative two reversed lines which coincide with the great 

 H, H lines of the solar spectrum. These are seen bright in 



* "On Certain Properties of Light-Struck Plates/' Trans. Academy of 

 St. Louis, Mo. U. S. vol. x. Xo. 0. 

 t Phil. Mag-. [5] vol. 1. p. 338 (1900). 



