Secondary Rontgen Radiation jrom Carbon. 773 



to the degree o£ softness attained, the lower ratio being 

 obtained with a very soft bulb. With harder rays, when the 

 secondary beam was cut down in each case to about two-thirds 

 o£ its original value, the ratio was considerably increased, the 

 average of a number of values being 1*69. The increase in 

 the ratio for the harder portion of the secondary beam was 

 confirmed by a large number of experiments. The result 

 may be explained by the fact that when a bulb is most com- 

 pletely polarized, the harder portion of it is more completely 

 polarized than the softer portions. This, however, would 

 have to be very pronounced to produce so large an effect on 

 the ratio. A second and more probable explanation is that 

 the specimen of carbon used which was supplied as pure 

 contains a very slight proportion of iron. Let us suppose 

 that the proportion of iron radiation in the direction a — 90° 

 is 10 per cent, of scattered radiation, then the ratio observed 



1*85 



would be yTjA =l"fi8 instead of the value 1*75. The sheet 



of aluminium of thickness 0015 cm. used to cut down the 

 secondary rays would cut more than 90 per cent, of the iron 

 radiation, and so the ratio would jump up to 175. Now 

 only a very small portion of iron will be necessary to produce 

 this percentage of the ionization — something of the order 

 0*1 per cent. A numerical example, giving an extreme case, 

 will make this clearer. Suppose we have incident on a very 

 thin carbon radiator the secondary radiation from nickel ; 

 let the amount of iron be w grs. per unit mass of the carbon. 

 The proportion of energy in the scattered beam to the iron 



fluorescent beam is *2 : -——w=. ——- w. 

 7*7t> o7*b 



*2 = the coefficient of scattering. 



526 is the transformation coefficient of nickel radiation 



incident on iron and 7*76 is the density of iron. The ratio 



of the ionization these two radiations would produce in a 



•2 239 



thin layer of air is „„» w x T^ry equals approximately lOOiv. 



•239 and 159*5 are the absorption coefficients of iron and 

 nickel radiation respectively in aluminium. 



Hence if the ionization due to the iron is about 10 per cen". 

 of that due to the carbon, then there must be present in the 

 carbon 



0*0002 gr. of iron per gram of carbon. 



Now the specimen of carbon used contained a small amount 

 Phil. Mag. S. 6. Vol. 24. No. 143. Nov. 1912. 3 E 



