1898.] on Liquid Air as an Analytic Agent. 829 



Further trials were made by bringing the cooled plate nearer to the 

 source of light until finally a position was found where the very 

 feeble photographic impression that appeared on both plates had the 

 same density. In this position the relative distances of the plates 

 from the source of light were measured. This mode of conducting 

 the photographic comparison of the hot and cold plates gets over the 

 difficulty of variation in the intensity of the source of light. From 

 these experiments it would appear that when cooled to the tempera- 

 ture of liquid air both the incandescent lamp and the Eontgen radia- 

 tion were reduced to 17 per cent, of their photographic action at the 

 ordinary temperature ; whereas the ultra-violet radiation was reduced 

 to about 6 per cent. This marked increase in the inertia of the photo- 

 graphic plate at low temperatures for the short wave-lengths cannot 

 be explained by the absorption of liquid air, for such radiation as 

 this is small for a thickness of 10 to 20 mm. of the liquid. It is 

 possible that the ultra-violet radiation is dissipated by the photo- 

 graphic film at low temperatures to a greater extent than with ordi- 

 nary light, through absorption and subsequent emission as a phos- 

 phorescent glow. It would seem probable that if the plate could be 

 developed at these low temperatures no action would be apparent, 

 and that it is during the heating up after the low temperature ex- 

 posure that the photographic action on the film takes place through 

 an internal phosphorescence. This possibility must make us cau- 

 tious in drawing inferences as to possible chemical action at low 

 temperatures. 



A more elaborate study of photographic and phosphorescent effects 

 at low temperatures would add much to our knowledge of the 

 chemical and physical actions of light. 



Vol. XV. (No. 02.) 3 i 



