668 



Professor Dewar on 



[Jan. 18, 



because it disappears on ignition. If the metal is subsequently 

 touched, the phosphorescence re-appears. 



So far as the examination has been carried, the two most remark- 

 able classes of substance for phosphorescence are the platinocyanides 

 amongst inorganic compounds, and the ketonic compounds, like 

 acetophenone and ethyl phenyl ketone, and others of the same type, 

 amongst organic. When ammonium platinocyanide is cooled with 

 liquid air and maintained at this temperature by being immersed in 

 the liquid while stimulated by exposure to a beam of the electric arc, 

 it continues to glow in the dark with a feeble emission as long as 

 the temperature is kept about — 180°. On pouring off, however, the 

 liquid air from the crystals so that the temperature may rise, then 

 the interior of the test tube glows like a lamp from the sudden 

 increase of light emissivity as the temperature rises. It seems clear 





~~C 

 — B 



Fig. 2. 



Fig. 3. 



from this experiment that similar initial light intensities being used 

 for stimulating, the substance at this low temperature must have 

 acquired increased power of absorption, and it may be that at the 

 same time the factor of molecular friction or damping may have 

 diminished. That the absorptive power of substances for light is 

 greatly changed at low temperatures is proved by the change of 

 colour in substances like oxide, iodide, and sulphide of mercury, 

 chromic acid, &e., when cooled. Many quantitative photometric 

 measurements must be made before the actual changes taking place 

 in the conditions governing the phenomena can be definitely stated. 



Along with these experiments on phosphorescence, a number of 

 photographs have been taken at — 180°, using various sensitive 

 plates and films, and these have been compared with similar photo- 

 graphs taken at the same time under similar conditions at the ordi- 



