666 



Professor Dewar on 



[Jan. 18, 



unexcited sulphide from " absorbing " light energy that can be evolved 

 at higher temperature. By means of liquid air we can now cool 

 substances to temperatures ranging from — 180° to — 200°. Under 

 such conditions all known organic compounds are solids, and this 

 condition of matter is specially favourable to phosphorescent phe- 

 nomena. Moreover, the list of truly phosphorescent bodies has been 

 greatly extended, and knowledge of their peculiarities in this direction 

 increased. 



The effect of temperature on phosphorescence is easy to observe 

 by taking two portions of the same substance placed in similar very 

 thin test tubes, cooling one of the specimens in liquid air, and then 

 quickly exposing both samples side by side to the same light stimu- 

 lation. The form of apparatus used is shown in Fig. 1. A is a 

 powerful electric lamp in a lantern, the latter carrying a fitting 

 whereby the light is screened from the eye of the observer. E is 



^ 



0- 



— F 



Fig. 1. 



a double vacuum vessel containing liquid air or oxygen. The 

 substance to be examined is plunged below the surface of the liquid 

 air, as at F. When it is thoroughly cooled, it is withdrawn from 

 the liquid and exposed for a few seconds to the full light of the arc 

 in the horizontal part of the tube. It is then quickly withdrawn 

 and examined for phosphorescence in the darkened room. The fitting 

 B is another modification of the apparatus for experimental purposes, 

 and consists of a slit and suitable lens and prism, whereby the 

 spectrum can be thrown on to a table, as at D 0, and cooled bodies 

 examined in various parts of the spectrum. If, duriDg the light 

 excitation caused by burning magnesium or a flash of the electric 

 light, the eyes are carefully covered, then the comparative phos- 

 phorescence, if any, of the cooled and uncooled substance can be 

 observed. In this mode of working the action of the very short 

 wave-lengths of light are stopped by the opacity of glass, but 

 the solid condition of all substances at the low temperature enables 



