The agreement between the results of the theoretical equation and the observed values is shown in the following table:
| Time in minutes | Theoretical value of I t / I T | Observed value of I t / I T |
|---|---|---|
| 15 | ·22 | ·23 |
| 30 | ·38 | ·37 |
| 60 | ·64 | ·63 |
| 120 | ·90 | ·91 |
| 220 | 1·00 | 1·00 |
| 305 | ·97 | ·96 |
After 5 hours the activity decreased nearly exponentially with the time, falling to half value in 11 hours.
It is thus seen that the curve of rise of activity for a short exposure is explained very satisfactorily on the supposition that two changes occur in the deposited matter, of which the first is a rayless change.
Further data are required in order to fix which of the time constants of the changes refers to the first change. In order to settle this point, it is necessary to isolate one of the products of the changes and to examine the variation of its activity with time. If, for example, a product can be separated whose activity decays to half value in 55 minutes, it would show that the second change is the more rapid of the two. Now Pegram7 has examined the radio-active products obtained by electrolysis of thorium solutions. The rates of decay of the active products depended upon conditions, but he found that, in several cases, rapidly decaying products were obtained whose activity fell to half value in about 1 hour. Allowing for the probability that the product examined was not completely isolated by the electrolysis, but contained also a trace of the other product, this result would indicate that the last change which gives rise to rays is the more rapid of the two.
This point is very clearly brought out by some recent experiments of Miss Slater8, who has made a detailed examination of the effect of temperature on the active deposit of thorium.
A platinum wire was made active by exposure for a