In the case of the absorption of radiations by solid screens, we find another fundamental difference between radium and polonium. Radium emits rays capable of penetrating great thicknesses of solid matter, e.g., several centimetres of lead or of glass. The rays which have passed through a great thickness of a solid body are extremely penetrating, and it is practically impossible to absorb them entirely by any material whatever. But these rays form only a small fraction of the total radiation, the greater part of which is absorbed by a slight thickness of solid matter.
Polonium emits rays which are readily absorbed, and which can only pass through extremely thin screens.
The following are figures showing the absorption produced by an aluminium lamina of thickness 0·01 m.m. This lamina was placed above and almost in contact with the substance. The direct radiation and that transmitted by the aluminium were measured by the electrical method (apparatus of Fig. 1); the current of saturation was practically obtained in every case. I have represented the activity of the radiating body by , that of uranium being unity.
| Fraction of radiation transmitted. | ||
| Chloride of barium and radium | 57 | 0·32 |
| Bromide "" | 43 | 0·30 |
| Chloride "" | 1200 | 0·30 |
| Sulphate "" | 5000 | 0·29 |
| """ | 10,000 | 0·32 |
| Metallic bismuth and polonium | — | 0·22 |
| Compounds of uranium ...... | — | 0·20 |
| Compounds of thorium in a thin layer............ |
— | 0·38 |
We see that radium compounds of different nature and activity give very similar results, as I have already pointed out in the case of uranium and thorium compounds at the beginning of this work. We see also that, taking into account the whole of the radiation, and with a given absorbent screen, the different radio-active bodies can be arranged in the following decreasing order of penetrating; power:—Thorium, radium, polonium, uranium.
These results are similar to those which have been published by Mr. Rutherford.
Mr. Rutherford also finds that the order is the same when air is the absorbent substance. But it is probable that this order has no absolute value, and would not be maintained
