648 Prof. C. G. Barkla on the 



Conclusions can be drawn similar to those indicated in the 

 previous case considered. The introduction o£ acceleration 

 into the motion of the electrified sphere, which was previously 

 uniform, results in a small disturbance of the initial uniform 

 distribution. The disturbance and rearrangement of the 

 charge give rise to its oscillation which sends out the damped 

 wave-train into the sether, but the oscillation and sether dis- 

 turbance soon die away, and the system settles down into a 

 steady state of motion with a uniform acceleration. The 

 electrical reaction to the starting of the accelerated motion 

 is initially zero, but rapidly assumes its steady value. 



The case of a rigidly charged dielectric sphere possesses 

 some additional characteristics, and I shall reserve the dis- 

 cussion of it for a future paper. 



Sheffield, February 1911. 



LXXYI. Note on the Energy of Scattered X-radiation. 

 By Charles (i. Barkla*. 



IN 1904 the writer made an experimental determination 

 of the energy of Rontgen radiation scattered by light 

 elements (Phil. Mag. May 1904), and applied the result to 

 calculate the number of scattering electrons in a known 

 quantity of matter, on the theory of scattering given by Sir 



e 

 J. J. Thomson f. With the data for — and e of an electron 



m 



at that time available, the number of electrons in a cubic 

 centimetre of air under normal conditions of pressure and 

 temperature was found to be 6 x 10 21 , or between 100 and 

 200 per molecule of air. Using the more recently deter- 



mined values of — , e, and n (the number of molecules per 



cubic centimetre of gas) J, the calculation gives the number 

 of scattering electrons per atom as about half the atomic 

 weight of the element. 



In a recent paper, however (Proc. Roy. Soc. A. lxxxv. 

 pp. 29-44), Mr. Crowther has determined the energy of the 

 scattered radiation from experiments on aluminium. He 



* Communicated by the Author. 



f The theory was first given, together with the expression for the 

 energy of the scattered radiation, in the First Edition of ' Conduction 

 of Electricity through Gases.' 



\ — =r73x!0 T e.rn.u. (Bucherer) ; e = T55x 10~ 23 e.m.u. (Ruther- 

 ford & Geiger) ; rc = 2"8xlC 19 (Rutherford;. 



