120 RADIO-ACTIVITY 



his hand between the screen and the tube, he saw, for the first 

 time, the now familiar sciagraph of the bones of the hand. The 

 X- or Roentgen rays originate from the place where a kathode 

 ray strikes, from the walls of the tube, in the first instance, or in a 

 focus tube from the piece of platinum (anti-kathode) upon which 

 the kathode rays are focussed. They issue equally in all directions 

 and travel in straight lines. For any tube, the power of pene- 

 tration of the X-rays is inversely proportional to the density of. 

 the substance penetrated. The higher the degree of exhaustion 

 of the tube the greater the penetrating power of the rays produced. 

 In a " hard " tube the vacuum is so good that a very great differ- 

 ence in potential between the electrodes is necessary to force the 

 discharge through. The kathode rays therefore attain a high 

 velocity and the X-rays they produce on impact with the anti- 

 kathode have a high penetrating power. On the other hand, if 

 the tube is not well " exhausted," the X-rays evolved are easily 

 absorbed. Such a tube is termed " soft." Unlike the kathode 

 rays they are not affected by the most powerful magnetic field. 

 Like the kathode rays they excite fluorescence, act on sensitised 

 photographic plates and ionise gases, i.e. they 'make air, or other 

 gas through which they pass and which under ordinary circum- 

 stances are practically insulators, capable of conducting limited 

 quantities of either positive or negative electricity. 



Poincare suggested that the production of X-rays might be an 

 effect common to all fluorescence. In 1896, Becquerel, acting 

 on this idea, examined some fluorescent salts of uranium. He 

 found that the double sulphate of uranium and potassium exposed 

 to sunlight could affect a sensitised plate even when the plate was 

 protected by a layer of copper or aluminium foil. This metallic 

 layer excluded the possibility of action by ultra-violet light or by 

 chemical vapours emitted by the salt. Further investigation 

 showed that the phenomenon was exhibited by uranous salts 

 which are not fluorescent as well as by the fluorescing uramc 

 salts. Both are active in proportion to the amount of uranium 

 they contain. That is, the continuous emission of these rays is 

 a specific property of uranium now generally termed Radio- 

 activity. 



The characteristics of the radiation from uranium are very 

 similar to those of the X-rays. They are found to consist of three 

 very distinct types of rays, differentiated in the first instance by 

 their power of penetrating matter. They have been termed by 

 Rutherford a, {3 and y rays. The a rays are particles of the gas 



