390 Mr. P. W. Burbidge on the Absorption 



from the greater value obtained by passing a very thin pencil 

 o£ the same radiation through the same medium, the dif- 

 ference would approximate to the coefficient ©f scattering in 

 the medium*. Such an ideal comparison is experimentally 

 difficult, but it is possible to compare the absorption of a wide- 

 angle beam with that of a very narrow one, and so detect 

 any very large small-angle scattering. 



Experimental Arrangements, . 



The source of X-rays was a palladium bulb run under 

 constant conditions (i. e., speed of mercury break, currents in 

 primary and secondary of the coil, alternative spark-gap 

 [1 cm. between 1 cm. spheres, corresponding to 27,000 volts]) 

 to give the maximum of palladium radiation,' which was then 

 filtered through silver foil to transmit mainly the palladium 

 K« linef. This gave a fairly homogeneous primary radiation 

 (see figure) the absorption of which in sulphur dioxide was 

 measured by a ratio method, with reference to an unaltered 

 beam of rays in the manner described in the previous paper J. 



For the absorption chamber, a wide brass cylinder was used, 

 29'7 cm. long by 11 cm. in diam., lined with aluminium 1 mm. 

 thick. In experiments with the wide-angle beam the full 

 aperture of this vessel was used, the ends being of perforated 

 aluminium as described in the previous paper. The narrow 

 beams were obtained by using two sets of horizontal slits, 

 in 3 mm. lead, placed one at each end of the absorption 

 chamber and independent of it ; one set had 4 mm. slits, 

 the other *6 mm. The chamber had special ends, each fitted 

 with a continuous narrow aluminium window ; these special 

 windows and the independence of the slits from, the absorp- 

 tion chamber were necessarv to avoid errors from mechanical 

 displacements consequent on pressure changes in the 

 chamber. 



Two separate ionization vessels were used corresponding 

 to the sections of tlie X-ray beams. Any characteristic 

 radiation (Aluminium K or Lead L) excited in the passage 

 of the palladium rays was reduced to a negligible amount by 

 the time the interiors of the ionization vessels were reached. 

 Owing to the small ionization from the narrow beams, 

 sulphur dioxide was used in the ionization vessel with the 

 4 mm. beam, and methyl iodide vapour mixed with air in the 



* The actual total scattering coefficient would be obtained if a 

 correction could be made in the spherical case for the average increase of 

 path of the scattered rays. 



f Hull, JPhys. Rev., Series 2, vol. x., 1917. 



X See p. 383. 



