696 Mr. E. Marsden and Dr. P. B. Perkins on tie 



electroscope W. The current observed in the latter was 

 thus due to the ionization produced by the a particles in the 

 shallow space between G 2 and D, which was 3 mm. in depth, 

 and by altering the distance of the source an ionization curve 



Fig. 4. 



Manometer. 



£Af?TH 



7 



could be obtained. The electrode D had a diameter of 6 cm. 

 and the brass tube T an internal diameter of 10 mm. The o-lass 

 tube B was waxed air-tight to the brass cylinder so that the 

 whole apparatus could be exhausted and any desired pressure 

 used, the source being introduced through a ground-glass 

 plate P. For radium C, actinium C, and thorium C, small 

 well-polished metal disks of diameter about 4 mm. were 

 exposed to the respective emanations; as source of polonium 

 an equal-sized sheet of platinum was used, which had been 

 activated with radium (D + E + F) by repeated exposure to 

 large quantities of radium emanation about two years ago. 

 The surface of the platinum was bright and not contaminated 

 in any way. 



In the experiments with the various substances the pressure 



