apart to absorb all the α rays in the gas, is proportional to the energy of the α particles escaping into the gas.
It has been shown that the minimum activity of radium after removal of the emanation, measured by the α rays, is 25 per cent. of the maximum value. The remaining 75 per cent. is due to the α particles from the other products. Now the activity supplied by radium A and C is nearly the same (section 228). If the emanation is introduced into a cylindrical vessel about 5 cms. in diameter, the activity increases to about twice its initial value owing to the deposit of radium A and C on the surface of the vessel. This shows that the activity of the emanation is of about the same magnitude as that supplied by radium A or C, but an accurate comparison is beset with difficulty, for the emanation is distributed throughout the gas, while radium A and C are deposited on the walls of the vessel. In addition, the relative absorption of the emanation compared with that of radium A and C is not known.
The writer has made some experiments on the decrease of activity of radium immediately after heating to a sufficient temperature to drive off the emanation. The results obtained by this method are complicated by the alteration of the radiating surface in consequence of the heating, but indicate that the emanation supplies about 70 per cent. of the activity of radium A or C.
This points to the conclusion that the α particles from the emanation are projected with less velocity than those from radium C.
The following table shows approximately the activity supplied by the different products of radium in radio-active equilibrium.
Product Percentage proportion of
total activity
Radium 25 per cent.
Emanation 17 "
Radium A 29 "
Radium B 0 "
Radium C 29 "
The products of radium and their radiation are graphically shown later in Fig. 95.
