The activity falls off according to an exponential law with the time, and decays to half value in 3·71 days. With the usual notation
I_{t}/I_{0} = e^{-λt},
the mean value of λ deduced from the results is given by
λ = 2·16 × 10^{-6} = 1/463000.
P. Curie determined the rate of decay of activity of the emanation by another method. The active matter was placed at one end of a sealed tube. After sufficient time had elapsed the portion of the tube containing the radium compound was removed. The loss of activity of the emanation, stored in the other part, was tested at regular intervals by observing the ionization current due to the rays which passed through the walls of the glass vessel. The testing apparatus and the connections are shown clearly in Fig. 53. The ionization current is observed between the vessels BB and CC. The glass tube A contains the emanation.
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Fig. 53.
Now it will be shown later that the emanation itself gives off only [Greek: alpha] rays, and these rays are completely absorbed by the glass envelope, unless it is made extremely thin. The rays producing ionization in the testing vessel were thus not due to the [Greek: alpha] rays from the emanation at all, but to the β and γ rays due to the excited activity produced on the walls of the glass tube by the emanation inside it. What was actually measured was thus the decay of the excited activity derived from the emanation, and not the decay of activity of the emanation itself. Since, however, when a steady state is reached, the amount of excited activity is nearly proportional at any time to the activity of the emanation, the rate of decay of the excited
