1 903.] Luminosity and its Correlation with Radio- Activity. 263 



intramolecular wobble. To us it seems likely that the change is con- 

 ditioned by a catalyst and that it occurs within a complex electrolytic 

 circuit. From this point of view the fluorescence of uranium glass 

 is noteworthy, as showing that changes in molecular structure may 

 go on in a solid viscous medium. It will be desirable to ascertain 

 whether such glass is fluorescent at low temperatures at which it 

 ceases to be viscous. 



PJwsphorescence. 



The phenomena of phosphorescence need to be considered with 

 reference both to cases in which the manifestation attends oxidative 

 or other kinds of chemical change (the glow of phosphorus, the glow- 

 worm, phosphorescent bacteria) and to those in which it is induced 

 by exposure to light (luminous sulphides). The former might well 

 almost be regarded as cases of fluorescence, as a continual supply of 

 energy is derived from the continued occurrence of a chemical change 

 involving loss of energy. With regard to the latter, it would seem 

 that it is not a property of pure substances. Thus it is known 

 that the production of sulphides which can be rendered luminous by 

 exposure to light is dependent on certain special conditions being 

 fulfilled, that, for example, barium sulphide, which is not sensitive 

 per se, becomes highly so when it is associated with a minute 

 proportion of bismuth.* 



The phosphorescent medium may be pictured as a complex system 

 capable of undergoing " electrolytic " deformation under the influence 

 of light of high refrangibility : as the changes thus induced are 

 reversed, the energy stored up during insolation becomes liberated 

 and the persistence of the effect is but a consequence of the fact 

 that the change takes place under restraint, in a viscous medium. 



Dewar's remarkable observations on phosphorescence at low 

 temperatures clearly foreshadow the conclusion that the property is 

 to be correlated with structure. The two most remarkable classes 

 of substances, he states, are the Platinocyanides amongst inorganic 

 compounds and the Ketonic compounds amongst organic. But these 

 latter are precisely those which are most prone to undergo conversion 

 into isodynamic forms. It is very noteworthy that, according to 

 Dewar, "water when pure is only feebly phosphorescent but 

 remarkably so when impure." 



* L. E. O. de Yisser, ' Eec. Trav. Chim.,' 1901, vol. 20, p. 435 ; 1903, vol. 22, 

 p. 133. In the case of calcium sulphide, the maximum effect is produced when 

 about 1 atom of bismuth is present to 50,000 of calcium. According to the later 

 paper, bismuth alone does not confer the property, but the presence of a sodium 

 salt in addition makes the mixture sensitive. 



