number of β particles N 2 expelled per second by radium E is determined. If Q 2 is the total number of particles expelled during the life of D + E, then Q 2 as before is approximately given by Q 2 = N 2 /λ 2 where λ 2 is the constant of change of radium D. Now we have seen that if each particle of C and of E gives rise to one β particle, it is to be expected that
The ratio N2/N1 was determined by measuring the activity due to the β rays from C and E in the same testing-vessel. Then, since N2/N1 is known, and also the value of λ1, the value of the constant of change, λ2, of radium D is obtained. In this way it was calculated that D is half transformed in about 40 years.
In the above calculations it is assumed, as a first approximation, that the β rays from C and E have the same average velocity. This is probably not accurately the case, but the above number certainly serves to fix the order of magnitude of the period of the product D. This calculation is confirmed by observations to be given later on the amount of D and E in old radium.
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