322 Mr. W. Wilson on the Radio-actire 



decays as if the disintegration products, both of the emanations 

 of radium and of thorium, were present. He found that for 

 a twelve hours' exposure the activity due to the thorium was 

 about 1.5 per cent, of the whole. Blanc *, working in Rome, 

 made a series of experiments to determine the ratio of the 

 activities due to the two deposits, and found that for a long 

 exposure (3 days) the activity due to the thorium deposit was 

 60 per cent, to 70 per cent, of the whole activity obtained. 



Since it is of some importance to know the relative amounts 

 of these substances at different parts of the earth's surface, 

 the following experiments have been made at Manchester 

 during the past year. 



A copper wire 36 feet long, maintained at a negative 

 potential of from 8000-12,000 volts by means of a Wimshurst 

 machine, was suspended from an insulated support out of an 

 upper window of the University Physical Laboratory. The 

 potential was measured by means of a Kelvin Static Volt- 

 meter. After the wire had been exposed to the atmosphere 

 for a sufficient time (3 to 8 hours) it was removed and 

 coiled round a square brass cage, which was then placed in 

 an electroscope by means of which the activity could be 

 measured from time to time. The natural leak of the instru- 

 ment was determined before and after the experiment by 

 taking a number of readings with a " clean " wire coiled 

 round the cage. The wire when drawn in was always covered 

 with soot particles. 



It was found that the activity of the wire fell off with the 

 iime as if products of radium and thorium were present. 

 The readings were continued for about 48 hours. 



The whole decay curve of the thorium deposit could be 

 deduced from the later readings, since the activity due to the 

 radium deposit has practically disappeared after four hours. 

 The decay curve of the radium deposit could then be obtained 

 by subtraction. 



The ratio which the activity of the thorium deposit would 

 bear to that due to the radium, if the wire were exposed till 

 equilibrium was obtained, was calculated for each experi- 

 ment, and the mean value found was 1*64, or 62 per cent, of 

 the initial activity is due to the deposit from thorium. The 

 extreme values obtained were 59 per cent, and 69 per cent., 

 but most were within 2 or 3 per cent, of the mean. 



In a recent paper Dadourian f has calculated the relative 

 amounts of radium and thorium present in the atmosphere 

 near the earth's surface, on the assumption that the active 



* Blanc, Phil. Mag-. May 1907 ; Phys. Zeit. May 1908. 

 t Dadourian, Le Radium, April 1908. 



