208 Hon. R, J. Strutt. [Aug. 5, 



" On the Intensely Penetrating Kays of Radium." By HON. R. J. 

 STRUTT, Fellow of Trinity College, Cambridge. Com- 

 municated by LOED RAYLEIGH, F.R.S. Received August 5, 

 1903. 



Radium is known to emit three types of radiation. These are 



(1) The a rays, very easily absorbed by solids, and carrying a 

 positive electric charge. 



(2) The /? rays, more penetrating than these, and negatively 

 charged. 



(3) The 7 rays, intensely penetrating, and not conveying an electric 

 charge at all. 



In a paper published in the 'Phil. Trans.' for 1901, I investigated 

 the relative ionisations of gases by the a and ft rays. The present 

 communication may be regarded as a sequel to that one, and deals 

 with the 7 rays. 



The radium employed was of activity 1000 (uranium = 1), and 

 was contained in a glass cell, over which was cemented a piece of thin 

 aluminium. The cell was placed in a cavity in a block of lead, and 

 over it was placed a disc of lead 1 cm. in thickness. This it was 

 considered would suffice to suppress all but the 7 rays, which are 

 much the most penetrating. 



In measuring the electrical leakage, the electroscope method was 

 employed. The apparatus was that described in a paper published 

 in the 'Philosophical Magazine' for June, 1903, p. 681. 



The radium, covered by the thick lead, was placed under the 

 apparatus, and the rate of leak determined when the different gases 

 filled the testing vessel. 



The conditions were, of course, arranged so as to use a saturating 

 E.M.F. The 7 rays are so penetrating that there can be no question 

 of their being appreciably absorbed in a moderate thickness of gas. 



For the methods of preparation of the gases I must refer to the 

 former paper.* 



The results were as follows ; the rates of leak are given in scale 

 divisions per hour, and are corrected to 30 inches pressure : 



' Phil Trans.,' A, vol. 196, 1901, p. 508. 



