of curve III express the activity added in consequence of the change of the matter B, present after removal, into the matter C. The matter B present gradually changes into C, and this, in its change to D, gives rise to the radiation observed. Since the matter B alone is considered, the variation of activity with time due to its further changes, shown by curve III, should agree with the curve obtained for a short exposure (see Fig. 87), and this, as we have seen, is the case.
The agreement between theory and experiment is shown in the following table. The first column gives the theoretical curve of decay for a long exposure deduced from the equation
taking the value of λ2 = 5·38 × 10-4 and λ3 = 4·13 × 10-4.
| Time in minutes | Calculated values | Observed values |
|---|---|---|
| 0 | 100 | 100 |
| 10 | 96·8 | 97·0 |
| 20 | 89·4 | 88·5 |
| 30 | 78·6 | 77·5 |
| 40 | 69·2 | 67·5 |
| 50 | 59·9 | 57·0 |
| 60 | 49·2 | 48·2 |
| 80 | 34·2 | 33·5 |
| 100 | 22·7 | 22·5 |
| 120 | 14·9 | 14·5 |
The second