by a Particles from Radium Emanation. 905 



J. J. Thomson * has also shown that X radiation is excited 

 when the slow positive rays impinge on matter, though it 

 was of such a soft nature as to be absorbed in the thinnest 

 sheets of paraffin-wax obtainable. 



In order to test whether any penetrating radiation is 

 emitted when the a particles are stopped by matter, it is very 

 essential that a powerful source of a. particles should be 

 employed, since a very small proportion only can be instru- 

 mental in generating penetrating 7 radiation. It is only 

 when the particle comes into close contact with the nucleus 

 of an atom that a 7 radiation, comparable in penetrating 

 power with the 7 radiations from radium B and radium 0, 

 could possibly be produced. These close encounters are 

 known to happen but rarely compared with those of a more 

 indirect nature. Furthermore, the a-ray source used must 

 not emit a penetrating primary 7 radiation, as it is extremely 

 difficult to pick out a small effect from larger simultaneous 

 effects of a similar nature. Radium emanation, when pure 

 and free from active deposit, emits a-rays only, and attempts 

 to discover any /3- and 7-ray activity associated with this 

 product have not been successful up to the present. This 

 product, being easily obtained in considerable quantity, and 

 being gaseous, thus facilitating manipulation, is very suitable 

 as an a-ray source. The product radium A, the element 

 produced by the disintegration of radium emanation, emits 

 «-ra} r s only as far as is known at present. The next 

 products, radium B and radium 0, emit penetrating 

 7 radiation, but the quantity of these substances present at 

 one minute after the introduction of pure emanation, free 

 from active deposit, into a tube, is but 0"27 per cent, and 

 0'0040 per cent, of their equilibrium values respectively, 

 and their effect is comparatively small at this time, in 

 relation to the effects under investigation. 



In the following experiments these facts were utilized 

 the source of a particles being radium emanation, and the 

 apparatus being designed so that the effects produced by 

 the impact of the a particles on matter could be examined 

 within 30 seconds of the introduction oFthe pure emanation. 



Every precaution was taken to avoid active deposit being- 

 carried over by the emanation into the examination vessel. 

 Previous investigators have remarked that, under similar 

 conditions, the amount of active deposit carried over was 

 indetectable. Nevertheless, in order to eliminate as far as 

 possible the variable effects due to contamination, if present, 



* J. J. Thomson, Phil. Mag\ xxviii. v . 620 (1914). 

 Phil. Mag. S. 6. Vol. 12. No. 252. Dec. 1921. 3 O 



