670 Professor Dewar on Phosphorescence and Photographic Action, 



was taken at ordinary temperature. By immersing the photographic 

 plate to different depths in the liquid oxygen or air, the comparative 

 opacities of the liquids could be observed. After exposure, the two 

 plates were removed from the supports and developed together in the 

 same solution, when similar results were obtained as in the previous 

 experiments. The photographs have been examined by Captain 

 Abney, who reports that the photographic action has been reduced 

 by 80 per cent, at the temperature of — 180°. If the photographic 

 action is brought about by a chemical change, then it appears to be 

 the only one that can be traced under such conditions, as substances 

 having the most powerful affinities have no action on each other, 

 and all voltaic combinations cease to give a current at such low 

 temperatures. It is certain that the Eastman film, cooled to — 200° 

 by the evaporation of air in vacuo is still fairly sensitive to photo- 

 graphic action. Much further work, however, will be required to 

 reach a definite conclusion as to what is taking place when substances 

 sensitive to photographic action are subjected to such conditions of 

 temperature. 



