908 Mr. F. P. Slater on Excitation of <y Radiation 



anhydride, and could be lined with sheets of the matter to be 

 bombarded by the a particles. Phosphoric anhydride wa& 

 found to be the only reliable grease for the ground-glass 

 joint, as 80 per cent t of the emanation could be recovered 

 after completing the experiment. A soft wax seal wa& 

 placed at L to ensure a sound joint. Various tap-greases 

 were tried, but with the best it was never possible to get back 

 more than 50 per cent, of the emanation, which is very easily 

 absorbed in the ordinary greases used. 



The whole of this apparatus was then placed under the 

 electroscope, as shown in fig. 1, and an electric field of 

 110 volts put on the electrodes in B, to collect and hold the 

 active deposit. Tap $ was then opened and then tap T, 

 which admits the emanation to the examination tube A, the 

 precise instant being recorded. The constriction at C was 

 then rapidly sealed off, the electric field being kept on until 

 this was completed, and the whole of the apparatus, from 

 to the right, removed as quickly as possible to a distant spot r 

 from which it could exert no effect on the electroscope. 

 The rise of the activity was then taken, the times being- 

 recorded on a chronograph tape machine. Reliable readings 

 could be taken within 30 sees, of the time of introduction of 

 the emanation into the tube A. The maximum to which the 

 activity rises was sometimes as much as 10,000 divisions a 

 minute, and could only be estimated by comparison with 

 a standard source of 0*38 milligram of Ra Br 2 on a standard 

 electroscope. Carves were then obtained with various sub- 

 stances, paper, tin, aluminium, and lead, lining the tube A, 

 showing the percentage of the maximum activity at times 

 from the moment of introduction of the emanation. 



Discussion of the Results, 



Provided no active deposit is carried through with the 

 emanation the rise curve should start from zero. Moseley 

 and Makower *, who used a somewhat similar method for 

 the determination of the absorption coefficient of the 7 radia- 

 tion from radium B through various thicknesses of lead,, 

 remark, with surprise, that the amount of contamination 

 is negligible and at most cannot exceed 0*01 per cent, of the 

 maximum. 



Fig. 2 shows the rise curves obtained when the tube A is 



lined with lead foil rolled down until of sufficient thickness 



to just more than stop the « particles of longest range — 



i. e., those from radium C. Curves I, If, III, and IV are 



* Moseley and Makower, Phil. Mag. xxiii. p. 302 (1912). 



