74 



Profs. G. D. Liveing and J. Dewar. [Mar. 8, 



angles to one another. It seems to follow from this that the full 

 photographic effect on the dry gelatine plates used by us ensues when 

 the intensity of the light reaches a certain limit, but that for intensities 

 of light beyond that limit there is no sensible increase in the effect 

 until the stage of solarisation is reached. 



II. " Note on the Eeversal of Hydrogen Lines ; and on the Out- 

 burst of Hydrogen Lines when Water is dropped into the 

 Arc." By G. D. Liveing, M.A., F.R.S., Professor of 

 Chemistry, and J. Dewar, M.A., F.R.S., Jacksonian Pro- 

 fessor, University of Cambridge. Received March 1, 1883. 



The concentration of the radiation of hydrogen in a small number 

 of spectral lines would lead us to expect that the absorption of light 

 of the same refrangibility as those lines would, at the temperature of 

 incandescence, be correspondingly strong, and that therefore the 

 hydrogen lines would be easily reversed. The mass of hydrogen 

 which we can raise to a temperature high enough to show the lines 

 is, however, so small, that notwithstanding the great absorptive power 

 of hydrogen for the rays which it emits, the reversal of the lines has 

 not hitherto been noticed. We find, in fact, that the lines are very 

 readily reversed, and the reversal may be easily observed. 



When a short induction spark is taken between electrodes of 

 aluminium or magnesium in hydrogen at atmospheric pressure, a 

 large Ley den jar being connected with the secondary wire of the coil, 

 the hydrogen lines show no reversal ; but if the pressure of the 

 hydrogen be increased by half an atmosphere or even less,* the lines 

 expand and a fine dark line may be seen in the middle of the F line. 

 As the pressure is increased this dark line becomes stronger, so that 

 at two atmospheres it is very decided. As the F line expands with 

 increase of pressure the dark line expands too and becomes a band. 

 It is best seen when the pressure is between two and three atmo- 

 spheres. When the pressure is further increased the dark band 

 becomes diffuse, and at five atmospheres cannot be distinctly traced. 

 No definite reversal of the C line was observed under these circum- 

 stances. The dispersion used, however, was only that of one prism. 



By using a higher dispersion the reversal of both the C and F 

 lines may be observed at lower pressures. For this purpose we have 

 used a Pliicker tube, filled with hydrogen and only exhausted until 

 the spark would pass readily when a large jar was used. 



* The pressures here mentioned are only measured by a metallic gauge attached 

 to the Cailletet pump employed, and must therefore be only taken as approximately 

 correct. 



