658 Production of Fluorescent Rontgen Radiation. 



smallest extent an effect o£ /3-ray activity, his argument is — 

 so it seems to me — inconclusive. It is to be remembered, 

 also, that Beatty has shown that homogeneous radiations are 

 produced by cathode rays of sufficient individual energy. 



Mr. Porter and I have been trying for some time to solve 

 this problem, using homogeneous, not heterogeneous, X-rays. 

 The method is more difficult because the radiation employed 

 is much weaker, but I believe it is by no means impossible 

 to follow it to the end. At any rate, we have made some 

 progress. We had not intended to publish any of our results 

 until they were more fully advanced ; yet, in view of 

 Mr. Chapman's letter, it seems better to do so, with the re- 

 servation that we do not ourselves consider them as final. 



If cathode rays do intervene between the exciting X-rays 

 and the homogeneous excited rays, the effect should be more 

 obvious in cases where it is known that the energy of the 

 excited rays forms a reasonable fraction of the energy of the 

 exciting X-rays. These occur when the exciting rays are 

 homogeneous X-rays emitted by atoms of somewhat greater 

 weight than those in which the rays are to be excited. For 

 that reason Mr. Porter and I have been investigating the 

 action of Mo X-rays on ethyl bromide, sulphur dioxide, and 

 air, comparing these results with the action of Se X-rays on 

 the same gases. It will be observed that Mo and Se lie on 

 either side of Br as regards atomic weight. Mo X-rays can 

 excite Br X-rays, but Se X-rays cannot. If there is truth 

 in the hypothesis of the intermediary cathode ray, three 

 results may be expected and looked for : — 



(1) The cathode ray produced by the Mo X-ray should 



show some deficiency in penetrating power in ethyl 

 bromide, since it is spending energy in a special 

 fashion. 



(2) The same cathode ray should be deficient in ionizing 



power in ethyl bromide, since it is spending energy 

 in other ways. In the same way there is a sharp 

 contrast between the action of Mo X-rays and Se 

 X-rays on ethyl bromide and S0 2 . The former rays 

 produce even a little less ionization in ethyl bromide 

 than in sulphur dioxide, while Se rays produce far 

 more. It is apparently easier to ionize ethyl bromide 

 than sulphur dioxide, and the defective action of the 

 Mo X-rays is to be ascribed to their being spent 

 somewhat in producing Br X-rays. 



(3) The most direct result, but the most difficult to 



measure, should, of course, be the production of 

 secondary Br X-rays by Mo cathode rays. 



