118 Dr. Jjbu&selVand Mr. Chadwick on the y Rays of 



They were covered with thick cardboard to minimise diffuse 

 reflexion of ft- and <y-rays from them. The electroscope was 

 of the ordinary type, 15 cm. x 10 cm. x 10 cm., and was 

 placed at the end of the pole-pieces. In the side against the 

 pole-pieces was cut an opening £ cm. by 8 cm., which was 

 covered with aluminium foil -0013 cm. thick. The distance 

 of the polonium preparation from this side of the electro- 

 scope was 5 cm. The strength of the magnetic field usually 

 employed to deviate the /3-rays completely was about 1500 

 gauss. Under these conditions the polonium preparation 

 gave a leak of 6 scale-divisions per minute when the magnetic 

 field was applied. As no a-rays could possibly enter the 

 electroscope, and as the preparation was free from other 

 products, this leak must be due to the y-rays of polonium 

 itself. The absorption of the radiation by thin sheets of 

 aluminium was determined, and the curve obtained showed 

 the presence of one type of radiation only. The mean value 



of jt (cm.) -1 — the absorption coefficient divided by the density 



— obtained from several determinations was 230. This value 

 corrected for obliquity of the beam becomes 215. Although 

 the polonium was deposited on several different metals, this 

 type of radiation was always obtained. It cannot, therefore, 

 be due to the impact of the a-particles against the aluminium, 

 and must be due to the polonium atoms themselves. 



If a harder type of radiation is emitted by the polonium, 

 one would expect it to be of about the same penetrating 

 power as the soft y-rays of radium D. In order that this 

 point might be more thoroughly investigated, Professor 

 Rutherford kindly allowed us to use the electroscope filled 

 with methyl iodide described by Rutherford and Richardson *. 

 Some slight evidence of the existence of a harder type was 

 obtained, but its intensity was only about 2 per cent, of the 

 natural leak. This evidence cannot., therefore, be regarded 

 as definite, and if a harder type does exist it must be very 

 small in intensity compared with the type described above, 

 and would require powerful preparations to permit of its 

 analysis. 



When the magnetic field was not applied the leak in the 

 electroscope was increased about 1 division a minute. This 

 difference is due to /3-rays emitted probably by polonium, 

 for different preparations of polonium gave a /3-radiation 

 proportional in amount to the y-radiation. It is possible,, 

 but unlikely, that the same proportion of radium D and E 

 would be present in preparations of polonium which had been 



* Phil. Mag-, vol. xxvi. p. .324 (1913). 



