104 THE ROYAL SOCIETY OF CANADA 



was a brass rod 3 mm. in diameter and 20 cm. long. The rod passed 

 through a plug of sealing wax, and carried on its upper end a strip of 

 brass on which was fastened a gold-leaf five centimeters long. The 

 sealing wax was surrounded by a guard ring and both were fastened 

 in a threaded ebonite plug which completely closed the ionization 

 chamber. The gold-leaf was protected by a felt-covered metal case, 

 which contained two small mica windows, through which the move- 

 ment of the leaf was observed. The electrode was charged by touching 

 the back of the brass strip carrying the gold-leaf with a wire which 

 passed through a glass tube in the case. The charging wire and the 

 guard ring (the latter permanently) were connected to the negative 

 pole of a battery of 480 volts. The positive pole of the battery and 

 the ionization chamber were permanently earthed. The movement 

 of the gold-leaf was observed through a fixed telemicroscope with a 

 scale in the eye-piece. 



To calibrate the electroscope the emanation was separated by 

 the Boltwood method - from a weighed amount of a standard specimen 

 of uraninite, which had a uranium content of 73 per cent, and possessed 

 an emanating power at 18°C of 11 per cent. It was immediately 

 transferred through a calcium chloride drying tube to the electroscope 

 and the rate of movement of the gold-leaf in divisions per minute 

 measured at the end of three hours. Taking the value of the ratio' 

 of radium to uranium in primary uranium minerals as 3.35 x 10-'^,^ 

 a movement of one division per minute of the gold-leaf was found to 

 correspond to the presence in the ionization chamber of the emanation 

 in equilibrium with 2.84 x lO-'^g. radium. 



As the heating of a uranium mineral greatly reduces the emanat- 

 ing power it was desirable that the material used for the experiments 

 to be described should have a high uranium content and, what is 

 more important, a large emanating power, otherwise the weight of 

 mineral which would have to be taken for a practical staxidard would 

 be inconventiently large. A specimen of carnotite and one of uraninite 

 were selected for the purpose, and both, when finely powdered, 

 satisfied the requirements mentioned. The carnotite, obtained 

 from Colorado, had a uranium content of 7 . 6 per cent. It was quite 

 porous in structure and wa,s easily ground to a powder. The emanat- 

 ing power, measured at 18°C, was 16.2 per cent. The uraninite used 

 was a specimen of primary uraninite, supposed to have come from 



2Am. Jour. Sc. 18, 379, 1904; Phil. Mag. 9, 599, 1905. 



^Boltwood's value (Am. Jour. Sc. 25, 296, 1908) in terms of the International 

 Standard. 



