234 Mr. D. L. Webster on the 1 heory of the 



due to its ionization, but rather to an atomizing effect of the 

 cathode rays on the gas — the active product being monatomic 

 nitrogen, and the total number of modified gas molecules 

 showing an exact proportionality to the ionization in the gas 

 by the rays. 



II. The ionization in nitrogen by cathode rays has been 

 shown to vary inversely as the kinetic energy of: the rays, for 

 rays of velocities ranging from 2*92 x 10 9 cm. per sec. to 

 4*76 x 10 9 cm. per sec. 



The ionization also varies linearly with the pressure over 

 the range "083 mm. to *025 mm. of mercury. 



My best thanks are due to Prof. Sir J. J. Thomson for his 

 kind interest in this research. 



Cambridge, 

 Dec. 1912. i , 



XXVIII. The Theory of the Scattering of Ront a en Radiation* 

 By D. L. Webster *. 



IN a recent paper by Mr. J. A. Crowther f , are described 

 some experiments on the scattering of Rontgen rays by 

 disks of aluminium or paraffin paper, made to test the formula 

 that gives the intensity of the rays scattered to an angle 6 as 

 proportional to (l-fcos 2 #). This formula may readily be 

 obtained from the expressions for the radiated electric and 

 magnetic vectors given by Prof. Sir J. J. Thomson in his book, 

 ' The Conduction of Electricity through Gases/ with the 

 assumption that each electron in the radiator scatters the same 

 amount of energy that it would scatter if it were alone. 



The observed radiation is found to fit the formula well at 

 angles greater than about 60°, but to be much in excess of the 

 calculated value at small angles. The properties of this 

 excess radiation are summarized in a second paper by 

 Mr. Crowther J, as follows : — 



" (1) For a given radiator and for primary rays of given 

 hardness the excess radiation in any direction is directly 

 proportional to the thickness of the radiator. 



" (2) The total excess radiation round the radiator decreases 

 as the primary beam becomes harder, and increases with the 

 atomic weight of the radiator. 



" (3) Within the limits of experimental error the hardness- 



* Communicated by Dr. Theodore Lyman. 

 t Proc. Roy. Soc. A. lxxxvi. pp. 472-494. 

 t Proc. Camb. Phil. Soc. xvi. 6, pp. 534-539. 



