76 LIFE AND WORK OF SIR JAGADIS C. ROSE 



The invisible molecular change may thus be detectible 

 by this subtle electric test, with its great advantage 

 over chemical estimation, which requires large quantities, 

 and long hours of analysis, during which the substance 

 may have automatically returned to its primitive state. 



That allotropic transformation may electrically be 

 detected, is also seen in other ways. In a selenium cell 

 the incidence of light causes an increase of conducting 

 power ; and removal of light is attended by self -recovery. 

 If the stress has been moderate the recovery is quick ; if 

 very great, as by strong light, the recovery is very much 

 protracted. Nor is light the only agent of such allotropic 

 change ; heat-rays may also produce it. Thus ordinary 

 iodide of mercury spread out in a thin film is practically 

 a red pigment, but when exposed to heat radiation it 

 becomes transformed into a yellow allotropic variety. On 

 removal of the radiation the substance recovers, the recovery 

 being hastened by a mechanical scratch, and the thin 

 film becomes red once more. Bose found that this visible 

 change had as a concomitant, a change of electric resistance. 



In summary, then, we find that in iron-like substances 

 with the positive ' touch ' the transformation is towards 

 an increase of conducting power ; and in potassium-like 

 substances, it is towards diminution. Just as all substances 

 as regards their magnetic properties fall into two classes, 

 para-magnetic or dia-magnetic, so all substances are 

 divisible into two classes, one exhibiting a positive and 

 the other a negative ' touch.' 



Bose had here discovered new classes of electric 

 phenomena ; and these two classes of conducting bodies 

 (for only with conductors was experimentation found 

 possible) we may characterise as ' contact-positive ' and 

 ' contact-negative.' In at least one almost neutral yet 

 slightly positive substance silver Bose succeeded in 

 producing by chemical means a negative variety, which 

 gave the negative response of diminished conductivity. 

 This variety was found Jess stable, since heating restored 



