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Mr. J. A. Crowther on the Coefficient of 



on passing through matter, there should be a marked increase 

 in the coefficient of absorption as the thickness of matter 

 traversed was increased. No such effect was observed, and 

 hence we may assume that, at any rate until the energy of 

 the rays is greatly reduced, there is no appreciable change in 

 their velocity. 



As the plan of the experiments included the investigation 

 of elements, many of which could only be obtained in small 

 quantities, the apparatus had to be kept quite small. Its 

 final form was as follows : — The uranium oxide was contained 

 in a depression, 2 mms. deep and 4*5 cms. square, in a lead 

 plate A (fig. 1), and covered in with a sheet of aluminium,. 

 O'l mm. in thickness, in order to cut off all the a radiation. 



Fie-. ] 



To POTEA'T/OMETEff. 



To electroscope: 



TO CELLS 



The rays then passed up into the chamber B, ionizing the 

 air between two plates C and D, 4'2 cms. apart. C was con- 

 nected to a battery of small storage-cells, and raised to a 

 sufficiently high potential (360 volts) to produce the satura- 

 tion current through the vessel when the radiation was 

 a maximum; D, 4 cms. in diameter, and surrounded by a 

 guard-ring, was connected to a Wilson inclined gold-leaf 

 electroscope through a mercury key E, which could be 

 operated from a distance and by means of which the gold-leaf 

 system could be earthed,' left insulated, or charged to a definite 

 potential. The insulations in the gold-leaf system were of 

 sulphur, and the whole was screened from induction effects 

 by earthed metal tubes. In its sensitive position the electro- 

 scope was capable of giving 30 scale-divisions per volt, and 

 could be read to one-fifth of a division. 



The rate of leak of the electroscope measured the ionization 



