PHYSICAL RESEARCHES CONTINUED 75 



is transmuted into the red variety, less dangerous because 

 no longer liable to that rapid oxidation in the atmosphere 

 which may readily set the familiar variety on fire. Sulphur 

 exposed to light is not changed to the eye, but treat- 

 ment with bisulphide of carbon, so convenient a solvent of 

 common sulphur, proves that light has somehow rendered 

 it insoluble. To this phenomenon of ' allotropism ' we shall 

 return later : it is enough at first to note that the action 

 of light on bodies, though sometimes within our direct 

 observation, need not necessarily be so, yet may none the 

 less be real and profound. 



But how shall we proceed to the investigation of these 

 changes ? how detect changes if they take place ? and 

 how discriminate between the exposed substances and 

 the unexposed ? The photographic plate is the familiar 

 instance in point ; but though chemists have endeavoured 

 to explain what happens (as by reduction of silver chloride, 

 and the reduction of this to metallic silver), the amount 

 of material altered is too small to admit of analysis 

 and verification. Bose showed how it can be detected 

 electrically, for which the galvanometer is sensitive to a 

 degree incomparably beyond that of chemical estimation. 



Now the allotropic variation or change in molecular 

 aggregation in a substance must, according to Bose, change 

 more or less all its properties physical and chemical e.g. its 

 solubility, its density, its chemical activity, and its position 

 in the voltaic series in consequence of which a current flows 

 from electro-positive to the relatively electro-negative ; it 

 would also change its power of electric conduction. Note in 

 this connection the familiar difference of conducting power 

 in the three allotropic modifications of carbon. As charcoal, 

 its conducting power is high ; as graphite, its conducting 

 power is only moderate ; while diamond is practically a 

 non-conductor. Let us call these A, B, and C. If we 

 could produce any transformation of graphite (B) towards 

 charcoal (A), it would be detected by increase of con- 

 ducting power, and if towards diamond (C) by decrease 



