Page:Radio-activity.djvu/373

 contained less than the equilibrium amount of Ur X. After falling to a minimum, the β ray activity, in consequence, slowly rose again to the equilibrium value.

These effects exhibited by uranium are of great interest, and illustrate in a striking manner the difference in properties of Ur X and the uranium. The gradual diffusion of the Ur X throughout the mass of crystals is noteworthy. By measurements of the variation with time of the β ray activity, it should be possible to deduce its rate of diffusion into the crystallized mass.

Transformation products of Thorium.

207. Analysis of the active deposit. The radio-active processes occurring in thorium are far more complicated than those in uranium. It has already been shown in chapter vi that a radio-active product Th X is continuously produced from the thorium. This Th X breaks up, 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 completely 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