466 Prof. J. C. Bose. On Electric Touch and the Molecular 



receiver is thus absolutely free from all effects, except those which are 

 to be observed — viz., the effects due to continuous action of radiation. 

 The time of exposure is accurately measured by counting the indi- 

 vidual flashes of radiation, due to interruption of the primary current 

 in the Euhmkorff coil by a tapping key. The conductivity of the 

 substance at a given moment is inferred from the deflection of the 

 galvanometer in circuit with the receiver. When feeble radiation is 

 used, it takes an inconveniently long time to obtain reversals ; there 

 is, besides, a tendency of self-recovery in the receiver. In order to 

 expedite the reversals, the incident electric radiation is made very 

 strong. 



Before entering into the detail of the results obtained, I will say a 

 few words about the principal types likely to be met with. We may 

 have the following : — 



I. Substances in which the B state is unstable under the given con- 

 ditions ; the B state will therefore only persist during the action of 

 radiation, the substance relapsing into the original condition on the 

 cessation of radiation. Two cases are possible (i) when the substance 

 is positive, (ii) when the substance is negative. 



The latter case is exemplified by potassium. In the above curve 

 (fig. 2), A and B represent the two molecular states. The substance 

 being negative, A is more conducting ; a represents the conductivity 

 of the fresh specimen ; the thick dots S S' . . . the individual 

 flashes. It is seen that the effect of radiation is to produce a sudden 

 diminution of conductivity, due to the transient formation of B variety 

 with its diminished conductivity. The substance, electrically speak- 

 ing, is highly elastic, and the limit of its elasticity is also very great. 

 With the majority of substances, however, self -recovery is only possible 

 when the narrow limit of elasticity is not exceeded. 



11. In this class the radiation product is somewhat stable ; the 

 successive conversions from A to B and from B back to A are sup- 

 posed to be complete. Probably there is no substance which exhibits 

 this action in a perfect manner, but we have an approximation to this 

 condition in the ca^e of magnesium, which under proper adjustments 

 shows successive complete reversals for a long time. The substance, 

 however, after a time exhibits the effect of fatigue. 



Time of exposure. 



Fig. 2.— Curve for Potassium. 



