404 Prof. W. H. Bragg on the Consequences of 



the ft rays in silver was found by placing one, two, four, 

 eight, and twelve silver foils on the side of the card next the 

 chamber and observing the gradual rise in the cathode ray 

 effect : this gave X equal to 3550. The quantity k was not 

 found directty. The absorption coefficient of card was deter- 

 mined by experiment to be 2'28 : card may be taken as 

 cellulose. C 6 Hi O5; and the figures given by Thomson, ' Con- 

 duction of Electricity through Gases/ p. 307, may be used 

 to show that the coefficient of air must be greater than that 

 of cellulose in the proportion of 8 to 7. In this last calcu- 

 lation the absorbing power of H is neglected, which possibly 

 makes the ratio too large ; but there are no data from which 

 to determine the error ; it must be small, This gives k' = 2*61. 

 Lastly D = 3-6x -0012 = -00432. 



Hence TT ^7/ I& N t-v k 



IDA: — — .\.D.~ 



X k 



2-61 

 43-2 



= 79-7 x 3550 x -00432 x 



= 73-8; 



whereas the ionization actually found, when the card was 

 next the chamber and the nine silver foils on the outside of 

 the card, was 70*3 as alread}' stated. In this case therefore 

 the ionization was somewhat overaccounted for. 



Generally the other experiments gave results of much the 

 same kind; it would not be justifiable to expect more accural e 

 confirmation under present conditions. 



The ordinary primary ray which was used in these expe- 

 riments might well be replaced by one of the streams of 

 homogeneous X rays which Barkla has shown us how to 

 obtain from various metals. Recent papers bj BeaUy* and 

 by Sadler f actually give results from which the desired 

 information may be obtained in part, but neither author has 

 had occasion to measure the value of X. Moreover there is 

 no published determination of k f , the absorption coefficient 

 of homogeneous X rays by air. Mr. Sadler has been good 

 enough to tell me that he finds kf = 9 3 for copper rays. 

 Using this value, and taking X in silver to be the same as X 

 in air, though it is probably greater, I find that on Beatty's 

 results about two-thirds of the ionization can be ascribed to 

 cathode rays : the figures of the latter author give a rather 

 smaller proportion. The agreement would be better if a 

 larger value were assumed for \. Moreover these rays are 



* Camb. Phil. Soc. Proc. vol. xv. pi. v. 

 t Phil. Mag-. March 1910. 



