Detection of Induced Beta-ray Emission. 
299 
end of the plate, in which region the red-lead surface was at an approximate 
distance of 2 cm. from it. Particles must therefore have traversed this 
distance in normal atmospheric air. Part of the impression must be due to 
/c^-radiation from the air itself, although fig. 2 shows that it cannot be very 
much. The limiting radiation from the Rontgen-ray tube will be tungsten 
" K " radiation of wave-length about 0'2 x 10~^ cm. Using this value and 
a rough average value, say 1 "5 x 10**^, for Whiddington's absorption constant 
" the value of d, the maximum range in air of the /:/-particles should be 
about 3' 3 cm. As a matter of fact it is not easy to obtain the limiting rays 
from the tungsten anti-cathode, and I do not think they were present in the 
mixed beam falling upon the red-lead. 
Figs. 5 and 5a are perhaps of more general interest. In this and in 
several other similar experiments it was found that if the red-lead surface 
be replaced by a photographic plate the silver salts themselves give rise to a 
copious supply of electrons in the same manner as the lead, affecting 
another photographic plate at a considerable distance from the first. Fig. 5 
shows the direct impression of the beam taken in the same manner as fig. 4, 
and fig. 5a shows the effect on the opposite plate upon which no direct 
Eontgen-rays fell. In fact, a similar effect would be given by any element 
of high atomic weight. 
It IS interesting to note that Sir W. H. Bragg * states that it is these 
electrons with which I have been dealing which cause the chemical action in 
the emulsion of a photographic plate when an ordinary X-ray photograph is 
taken in the usual manner. The number of particles and therefore the 
photographic effect can be increased by loading the film with a lead 
compound. Some experimenters have laid a thin sheet of lead on the film 
so that electrons generated in the considerable absorption of X-rays by the 
lead may strike back on to the photographic plate. These particles will 
traverse only the thinnest films of solid, and we are bound to conclude from 
the study of these photographs when one examines the relative impressions 
due to /:?-particles and Eontgen-radiation from the same mass of substance, 
the latter being comparatively non-existent, that the latent image on a 
photographic plate, whether caused by ordinary light or X-rays, is inseparably 
associated with the ionisation of the molecule. By ionisation is meant the 
separation of one electron from the molecule. X-rays are ineffective on a 
photographic plate until the energy has been converted in the production of 
electrons ; these are the effective agents. When X-rays traverse a gas, the 
first act is to produce comparatively high-speed electrons which spend their 
energy in ionising local molecules. There is no reason to suppose that 
the process is different when X-rays are absorbed by a solid, such as, for 
example, the photographic film, except in that the range of action of the 
electrons is more limited. 
* * Trans. Faraday Soc./ xv, pt. 2, 1919 ; " Symposium on RadiometallogTaphy," p. 30. 
