34 Mr. J. Satterly on the Amount of Radium 



been reached by Cooke*, who got 9*1 in a brass vessel 

 screened by large masses of lead, and by Wright f, who got 

 down to 6*0 in an unscreened zinc vessel on the surface of 

 Lake Ontario, and to 8'6 at the same time in a similar lead 

 vessel. In the calculation of their results all observers 

 have taken the old and small value of e, the electronic charge. 

 Wright took 6 = 3*4 x 10~ 10 E.s.u. If we substitute the 

 correct value 6 = 4*65 X 10 -10 E.s.u., Wright's figures for 

 the number of ions become 4*4 and 6*3 respectively. Wright 

 also found 8'2 and 9*8 for the same two vessels in the open air 

 on the ground near the newly erected Physical laboratory at 

 Toronto, and 9*9 and 10*3 in a room within the laboratory. 

 In each case the vessel had been cleaned and filled with 

 filtered air just before the readings were taken, and was 

 unscreened. 



McLennan has shown that the water of Lake Ontario is 

 quite free from radioactive matter {, and Wright has shown 

 that if the depth of water is greater than four metres it 

 absorbs all the penetrating radiation from the earth below 

 the lake. Hence it follows, considering the zinc vessel alone, 

 that the diminution from 8*2 to 44 was due to the cutting 

 off of the penetrating radiation from the earth. Allowing 

 1 ion per c.c. per sec. to be due to the radium emanation in 

 the air in the vessel, it follows that the remainder must be 

 due to an intrinsic radiation from the walls of the zinc vessel 

 itself. Of the total number 8 produced on the ground near 

 the laboratory, we may therefore say that (a) 4 are due to 

 the penetrating radiation from the ground (including any 

 secondaiy effect this may produce), (b) 3 are due to a radia- 

 tion from the walls of the zinc vessel itself, and (c) 1 is due 

 to the radium emanation in the air. In free air (a) would 

 be reduced to 1 (see Wright, loc. cit, p. 317), (b) would be 

 absent, and (c) would be raised to 2 by the presence of the 

 thorium products, giving a total of about 3 ions per c.c. 

 per sec. in free air. 



Volume of Radium Emanation in the Air. 



Rutherford § has shown that the volume of radium emana- 

 tion in equilibrium with 1 gm. of radium is "585 cub. mm. 



* Phil. Mag. Oct, 1903. t Phil. Mag. Feb. 1909. 



% Eve (Phil. Mag. July 1909) has shown that the radium content of 

 the St. Lawrence ac Montreal is *2ox 10~ 12 gm. of radium per litre of 

 the water. 



§ Proc Roy. Soc, Aug. 1908. 



