giving rise to the radio-active emanation. The emanation produces from itself a type of active matter which is deposited on the surface of bodies, where it gives rise to the phenomena of excited or induced activity. This active deposit possesses some distinctive chemical and physical properties which distinguish it from the emanation and the Th X. We have seen (section 180) that the rate at which the active deposit loses its activity depends upon the time of exposure of the body made active to the emanation. The explanation of the activity curves for different time of exposure will now be considered.
The curve of variation of activity for a short exposure of 10 minutes has already been given in Fig. 65. The activity is small at first but increases rapidly with the time; it passes through a maximum about 4 hours later, and finally decays exponentially with the time, falling to half value in 11 hours. This remarkable effect can be explained completely6 if it be supposed that the active deposit consists of two distinct substances. The matter initially deposited from the emanation, which will be called thorium A, is supposed to be changed into thorium B. Thorium A is transformed according to the ordinary exponential law, but the change is not accompanied by any ionizing rays. In other words, the change from A to B is a “rayless” change. On the other hand, B breaks up into C with the accompaniment of all three kinds of rays. On this view the activity of the active deposit at any time represents the amount of the substance B present, since C is inactive or active to a very minute extent.
If the variation of the activity imparted to a body exposed for a short interval in the presence of the thorium emanation, is due to the fact that there are two successive changes in the deposited matter A, the first of which is a “rayless” change, the activity It at any time t after removal should be proportional to the number Qt of particles of the matter B present at that time. Now, from equation (4) section 197, it has been shown that