350 Sir E. Rutherford, Prof. Barnes, and Mr. Richardson: 



very similar, and it was difficult to distinguish one from the 

 other. 



Russ (Joe. cit.) had observed that the proportion of pene- 

 trating radiation increased with increase of current through 

 the bulb. This, if true, is a very important observation, for 

 it would indicate that the quality of the rays depends not 

 only on the velocity of the electrons but also on their 

 number. In our preliminary examination of this point, 

 results were obtained in general agreement with those of 

 Russ, but the difference between the absorption curves was 

 finally traced to another cause. When working with very 

 intense radiations, it was necessary to nearly close the 

 opening in the ionization vessel by means of the lead slides. 

 Some radiation was found to enter the vessel by scattering 

 from one lead plate to another or by the excitation of charac- 

 teristic radiations. When the front of the ionization vessel 

 was covered with a thick lead sheet and the rays allowed to 

 enter through a small opening, the disturbance was elimi- 

 nated, and the absorption curves were found to be indepen- 

 dent of the current through the bulb over a wide range.. - 



Since a much greater intensity of radiation could be 

 excited by the use of the coil, the absorption curve could be 

 obtained over a greater range of thickness of aluminium. 

 In such cases, the penetrating power of the "end " radiation 

 appeared to be slightly greater than that observed with the 

 Wimshurst. On account, however, of the uncertainty as to 

 the equality of the maximum potential in the two cases, not 

 much stress can be laid on this difference ; for it is probable 

 that the minimum voltage required to produce a given 

 length of spark is greater for the coil than for the Wims- 

 hurst, on account of the rapid variations in the potential of 

 the former. 



Since for the "end" radiation, the coil gave about the 

 same value as the Wimshurst, the former was employed to 

 determine the penetrating power of the radiation between 

 110,000 and 175,000 volts. For such high voltages, the end 

 radiation is absorbed nearly exponentially over a wide range. 

 On account of the danger that the bulb might break down 

 under such high voltages, the experiments were confined to 

 an examination of the end radiation alone. In order to 

 detect small variations in the absorption of the rays, the 

 experiments for 125,000, 145,000, and 175,000 volts were 

 made in the following way. Two sheets of lead each of 

 thickness '62 mm. were placed in front of the ionization 

 vessel, and the current determined with the fixed capacity 

 in parallel. A thickness of aluminium 3*24 cm. was. 



