100 NATURE OF THE RADIATIONS [CH. 
just produce an impression through ‘01 mms. of alumimium. It 
will be shown, however, in section 76, that = is not a constant for 
all speeds, but decreases with increase of velocity of the rays. The 
difference in velocity between the rays is in consequence not as 
great as this calculation would indicate. On examination of the 
rays from uranium, Becquerel found that the radiation is not as 
complex as that from radium, but consists wholly of rays for 
which the value of HR is about 2000. 
72. Examination of the § rays by the electric method. 
The presence of easily deviable rays given off from an active 
substance can most readily be shown by the photographic method, 
but it is necessary, in addition, to show that the penetrating rays 
which produce the ionization in the gas are the same as those 
which cause the photographic action. This can be conveniently 
tested in an arrangement similar to that shown in Fig. 23. 
The radio-active matter A is placed on a lead block B” between 
the two parallel lead plates BB’. The 
rays pass between the parallel plates and al alee 
ionize the gas between the plates PP’ of 
the testing vessel. The magnetic field is sort oe 
apphed at right angles to the plane of E D 
the paper. The dotted rectangle HEEEH ! 
represents the position of the pole piece. ! 
If a compound of radium or thorium is Bl | 
under investigation, a stream of air is 
required to prevent the diffusion of the 
radio-active emanations into the testing 

vessel. When a layer of uranium, thorium b cocececnscosarened 
or radium compound is placed at A, the Fig, 23. 
ionization in the testing vessel is due 
mainly to the action of the a and § rays. The a@ rays are cut 
off by adding a layer of aluminium ‘01 cm. thick over the active 
material. When the layer of active matter is not more than a few 
millimetres thick, the ionization due to the y rays is small com- 
pared with that produced by the @ rays, and may be neglected. 
On the application of a magnetic field at right angles to the mean 
