the order of absorption by the metals and solid substances examined.
101. Connection between absorption and density. Since in all cases the radiations first diminish approximately according to an exponential law with the distance traversed, the intensity I after passing through a thickness x is given by
where λ is the absorption constant and I₀ the initial intensity.
The following table shows the value of λ with different radiations for air and aluminium.
| Radiation | λ for aluminium | λ for air |
|---|---|---|
| Excited radiation | 830 | ·42 |
| Thorium | 1250 | ·69 |
| Radium | 1600 | ·90 |
| Uranium | 2750 | 1·6 |
Taking the density of air at 20° C. and 760 mms. as 0·00120 compared with water as unity, the following table shows the value of λ divided by density for the different radiations.
| Radiation | Aluminium | Air |
|---|---|---|
| Excited radiation | 320 | 350 |
| Thorium | 480 | 550 |
| Radium | 620 | 740 |
| Uranium | 1060 | 1300 |
Comparing aluminium and air, the absorption is thus roughly proportional to the density for all the radiations. The divergence, however, between the absorption-density numbers is large when two metals like tin and aluminium are compared. The value of λ for tin is not much greater than for aluminium, although the density is nearly three times as great.
If the absorption is proportional to the density, the absorption in a gas should vary directly as the pressure, and this is found to be the case. Some results on this subject have been given