1901.] Effect of Light and Electric Radiation on Matter. 157 



and diamagnetic classes we also come across characteristic differences, 

 though these divisions do not coincide with the classes giving the 

 positive and negative electric touch. There is, however, one simi- 

 larity ; for just as the paramagnetic effect is more intense, so is the 

 conductivity variation associated with the positive, generally speak- 

 ing, much stronger than the effect on the negative class. From the 

 strongly positive substance like Fe Mg to the pronounced negative 

 like K there are numerous gradations. In silver we have a material 

 which is almost on the line of demarcation ; it passes easily from one 

 condition to another under the influence of external circumstances. 



2. External Influences. 



If the response, positive or negative, is really an expression of some 

 changed molecular condition, we may expect it to be modified not 

 only by the chemical nature of the substance, but also (1) by the 

 previous history of the substance, (2) by the temperature, (3) by 

 pressure. 



Influence of 'previous History. — As regards the first, I have already 

 shown in my previous paper* that a substance strained by radiation 

 often exhibits opposite or reversal effects. Freshly powdered particles 

 often show erratic results, but the effects become consistent after 

 annealing ; this often increases the sensibility also, by increasing the 

 molecular mobility. Fresh particles are sometimes found to exhibit 

 very little sensitiveness. At first I thought that this might be due to 

 some kind of fatigue; the following, however, showed that such 

 could not be the case. I found that in these cases the sensibility was 

 increased by subjecting the substance to strong radiation or even by 

 passing a few induction shocks. The increase of sensitiveness thus pro- 

 duced appears to be due to the removal of molecular sluggishness. The 

 improvement in sensitiveness often obtained by shaking of the particles 

 is no doubt due to the same cause. In the various types of molecular 

 receivers, whether responding to electric radiation, light, or mechanical 

 vibration, the same effect is noticed. Generally speaking, the receiver 

 in the first place improves gradually with working. But as it gets 

 overstrained it exhibits fatigue. 



Influence of Temperature. — As regards temperature, I have in many 

 instances found that on excessively cold days some receivers exhibit 

 a diminution of sensibility, removed by warming. Several effects 

 which were very strongly exhibited in the warm climate of India, I 

 found to be much diminished here. Cautious application of heat often 

 increases, not only the sensibility, but also the power of self-recovery. 

 But excess of temperature produces erratic behaviour by causing 

 violent molecular disturbance. 



VOL. LXX. 



* hoc. cit. 



M 



