The calculated values of It/I₀ for different values of t are shown in the second column of the following table, and the observed values in the third column.
| Time | Theoretical value | Observed value |
|---|---|---|
| 0 | 1·00 | 1·00 |
| ·25 days | 1·09 | — |
| ·5 „ | 1·16 | — |
| 1 „ | 1·15 | 1·17 |
| 1·5 „ | 1·11 | — |
| 2 „ | 1·04 | — |
| 3 „ | ·875 | ·88 |
| 4 „ | ·75 | ·72 |
| 6 „ | ·53 | ·53 |
| 9 „ | ·315 | ·295 |
| 13 „ | ·157 | ·152 |
Fig. 80.
The theoretical and observed values thus agree within the limit of error in the measurements. The theoretical curve is shown in Curve A, [Fig. 80] (with the observed points marked, for comparison). The curve B shows the theoretical curve of the decay of the activity of Th X and the emanation, supposing there is no further change into the active deposit. Curve C shows the difference curve between the curves A and B, i.e. the proportion of the activity at different times due to the active deposit. The activity due to the latter thus rises to a maximum about two days after removal of the Th X, and then decays with the time at the same rate as the Th X itself, i.e. the activity falls to half value every four days. When t exceeds four days, the term
in the theoretical equation is very small.
The equation of decay after this time is therefore expressed by
i.e. the activity decays according to an exponential law with the time.