by Matter of the /3 Particles emitted by Radium. 175 



which escapes at the open end of the cylinder, and the 

 secondary radiation which, impinging on the walls of the 

 glass tube containing the emanation, is there reabsorbed. 

 The radiation escaping at the top of the cylinder was 

 certainly small, and did not amount to one per cent, of 

 the total radiation ; the secondary radiation absorbed by the 

 glass tube must also have been inconsiderable, since the 

 thickness of the glass was too small to absorb much of 

 the radiation falling on it. If, further, we take into con- 

 sideration the small surface of the glass tube, which amounted 

 to only one-twentieth of the area of the brass cylinder, it is 

 evident that errors due to secondary radiation were without 

 effect within the limits of experimental errors. 



The following numbers illustrate the method of mea- 

 surement : — With the emanation from 13*17 milligrams of 

 radium-bromide contained in a glass tube *078 millimetre 

 thick, the electrometer charged up *238 volts per minute 

 when the tube containing the emanation was connected 

 to the positive pole of a battery of 20 storage-cells, and 

 *281 volts when connected to the negative pole. The 

 capacity of the whole system connected to the electrometer 

 was 9150 centimetres. Assuming the charge carried by the 

 particle to be 4*65 x 10~ 10 electrostatic unit, this gives 

 as the number of /3 particles emitted per second by the 

 radium C in equilibrium with 1 gram of radium, the value 

 3*68 x 10 10 . A second experiment with the same glass tube 

 gave 3'41 x 10 10 . Having regard to the easy absorption of 

 the /3 rays from the active deposit from radium for small 

 layers of absorbing material, these numbers are in fair 

 agreement with the number 5*33 x 10 ]0 found by Rutherford 

 when the radiation had to traverse only *053 millimetre of 

 aluminium. 



The Absorption of the /3 Rays by Glass. 



It has already been pointed out that it is necessary to apply 

 a correction to the number of particles emitted by radium 

 given above, on account of the absorption of the rays by the 

 walls of the thin glass tube containing the emanation. In 

 order to determine the magnitude of the correction to be 

 applied, it was necessary to study the law of variation of the 

 charge emitted by the emanation sealed in the glass tube after 

 traversing different thicknesses of glass. It was also thought 

 that experiments on this point might throw light on the 

 interesting question of the nature of the process by which 

 the j3 rays are absorbed by matter. If, as is sometimes 

 supposed, the absorption of the /3rays consists in a scattering 



