PHYSICAL METABOLISM 241 



by noting the light in the direction As, when A is 

 screened from the illumination of the spark, but 

 exposed to that from B. 



This effect is of the same order in both cases. 

 The point in this last experiment may not be 

 very clearly understood without a few remarks 

 about the experiments of Becquerel and Stokes 

 upon this property which the less refrangible rays 

 of the spectrum possess of discharging the phos- 

 phorescence when acting in consort with the 

 exciting rays. Becquerel '^ found this in a very 

 remarkable degree, so much so that he has suc- 

 ceeded by this means in studying the infra-red 

 spectrum by the action of the rays upon a 

 phosphorescent screen. These rays appear to have 

 a neutralising action on the effect produced by 

 the more refrangible ones of the spectrum, an effect 

 somewhat similar to the reversal in photography : 

 but the heat due to their absorption may be 

 sufficient to increase the brightness of the phos- 

 phorescent screen and thus diminish the duration, 

 in the affected part, relatively to the other parts of 

 the screen. Thus in the regions of maxima intensity 

 in the spectrum there would be correspondingly 

 dark bands in the phosphorescent screen. This 

 may simply be a case of thermoluminescence. 

 Many substances, when warmed, possess the power 

 of radiating energy which they had previously 

 stored up in some other way : a phenomenon 

 which is known as Thermoluminescence. It is by 

 1 Comptes Bendus, 1882. 



R 



