220 



Mr Townsend, Secondary Rontgen Rays. 



We do not therefore consider that the radiation which extends 

 to a distance is affected by the nature of the surface and the 

 following experiment confirms this view. 



With the same apparatus a set of experiments were made with 

 paraffin to see whether a thin layer would give out the same 

 amount of radiation as a block. The surface of a thin sheet of 

 aluminium was covered with a thin layer of paraffin, and used at 

 D as a radiator. 



The thin sheet of aluminium coated with paraffin gave out a 

 radiation which was only one sixth as effective in producing 

 ionization as the radiation from a paraffin block. 



We see therefore that the secondary radiation which extends 

 to a distance from the body which emits it does not come directly 

 from the surface, but must be considered as emanating from the 

 substance of the material. 



3. In addition to the radiation which extends to a distance 

 there is also a radiation which is rapidly absorbed by the gas near 

 the surface of the radiating body. In order to examine this 

 radiation the apparatus shown in Fig. 2 was used. 



It was arranged to find the maximum conductivities between 

 the two circular plates A and B, 4 - 8 centimetres radius, for differ 



ent distances, X, between the plates. The lower plate B was of 

 aluminium and its upper surface was turned plane. It rested on 

 a sheet of lead L and was screwed through the lead to a wooden 

 base W, which rested on the top of the box containing the bulb 

 and the Ruhmkorff coil. The apertures, M and N, in the lead, 

 through which the rays from the bulb passed were 1*4 centimetres 

 radius. After traversing B and the air space, X, the rays fell on 



