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 increases 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 HCl 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."[1]
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 preliminary 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 Modifyying 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.
