16 ROYAL SOCIETY OF CANADA 



II. Relation of Activity to Area of Salt Exposed, 



Before proceeding with the examination of the different salts 

 some preliminary measurements were made on the activity of potas- 

 sium sulphate when different areas of a layer of this salt were exposed 

 to the air in the chamber. A layer of the salt some 6 mm. in thickness 

 was placed in the tray mentioned above, which was 35 cms. long and 

 18 cms. wide, and then covered with a thick plate of metal divided 

 into sections 18 cm. long, and 5 cms. wide. These sections were, one 

 after another, removed from the tray, so that larger and larger areas 

 of the salt were left exposed to the air in the chamber. The satura- 

 tion current corresponding to each area was measured, and the values 

 which are given in Table I, and plotted in Fig. 2, shew that the 

 saturation currents were directly proportional to the areas of the salt 

 exposed. From this result it was evident that the substance which 

 constituted the source of the radiation was uniformly distributed 

 throughout the mass of the salt. 



III. Relation Between Thickness of Salt Layer and Activity. 



Some additional measurements were made to ascertain the rela- 

 tion of the activity of a number of the salts to the thickness of the 

 layer of salt exposed. In making these measurements the salts were 

 ground to a fine powder and then sifted as uniformly as possible on 

 the bottom of a shallow tray which had a surface area of 228 square 

 cms. This tray was then placed in the ionising chamber and the 

 saturation currents measured for each layer as before. 



