190 



Prof. J. C. Bose. 



[Apr. 18, 



to the light, there is either no hydrochloric acid or only a very small 

 quantity formed in the first moment ; but the rate of formation in- 

 creases so that the quantity formed in a given time, e.g., a minute, 

 continues to increase until it attains a maximum value. Bunsen terms 

 the gradual increase in the action induction. If the gaseous mixture has 

 been once exposed to the light, it will retain in the dark, for about 

 half an hour, its capacity for forming HC1 in the light. If the gas has 

 remained in the dark for a short period and is again brought into the 

 light, it requires a very short period of induction ; but the period 

 of induction will be lengthened by keeping the mixture in the dark 

 for a long time. [This is evidently due to self-recovery. — J. C. B.] 

 Exposure to the light renders the gaseous mixture capable of entering 

 into combination, but it does not bring about combination itself."* 



The latent period of the curve, due to molecular inertia, would thus 

 appear to offer an explanation of induction. In connection with this 

 it is interesting to note the well-known fact that a very slight pre- 

 liminary exposure of the photographic plates considerably enhances 

 their sensitiveness. 



It would also appear from the inspection of the curve, that the 

 general law of photo-chemical action, which regards the total action as 

 proportional to the product of the light intensity multiplied by the 

 time of exposure, is subject to several modifying conditions. During 

 the latent period, this cannot hold good in the first part, nor can it 

 be true after the maximum is reached. It can hold good only in the 

 second stage when the action proceeds uniformly. 



7. The Effect of Intermittence in Modifying the Law of Photo-chemical 



Action. 



But even after the substance has arrived at the second stage of 

 uniform action, there may still be deviation from the above law. If 

 in one case light be intermittent, and in the other continuous, the 

 effects may be quite different, though the total durations be equal. 

 For in the former case, during the continuation of light we may have 

 distortion or molecular swing proceeding in a given direction, but on 

 the stoppage of light, the swing stops too, sooner or later (sooner if 

 the distortion has been considerable, when the force of restitution 

 becomes great), and owing to self-recovery may even become reversed. 

 After an interval, when the light is again allowed to act, it has not 

 only to overcome the molecular inertia, but may have in certain cases to 

 reverse the negative swing. In the case of continuous radiation, on 

 the other hand, the molecular action proceeds without hindrance. 



This is very well seen in the curve given below, which shows the 

 difference in the extent of molecular effects produced in an AgBr 



* Meyer, ' Modern Theories of Chemistry,' p. 507. 



