10 JET. A. Bumstead — Atmospheric Radio-activity. 



the value obtained indicates that about 75 per cent more 

 thorium activity would have been deposited on wires of equal 

 lengths, in the same time, on the clear day when the ground 

 was not frozen than upon the clear day when it was. 



A similar explanation may be given of the fact that the 

 radio-activity of rain and snow, discovered by C. T. R. Wilson, 

 decays at a different rate from that of the negatively charged 

 wire. Wilson finds that the former falls to half-value in 

 about 30 minutes ;* and this is very near to the final rate of 

 the radium-excited activity. If we assume that some of the 

 drops in the rain clouds are condensed upon the positively 

 charged particles of radium emanation X present in the air, 

 the time occupied by the drops in falling, and in the collec- 

 tion and evaporation of the water, would prevent the earlier, 

 non-exponential decay of this activity from being observed ; 

 all that would remain would be the final, regularly decaying 

 product. The absence of a noticeable amount of thorium 

 activity may be explained by the rapid decay of the thorium 

 emanation ; although the particles of thorium emanation X, 

 present near the ground may sometimes be carried to consider- 

 able heights by the wind, the proportion of radium activity 

 must steadily increase as we go upward, since, in the case of the 

 radium activity, we have not only the particles blown up from 

 near the ground but also those produced by the radium emana- 

 tion in situ ; the slow decay of this emanation allowing it to 

 diffuse to much greater heights than the thorium emanation. 

 It is to be expected that a negatively charged wire suspended 

 several hundred feet above the ground would show a smaller 

 proportion of thorium than one exposed near the earth's 

 surface. 



I have also looked for evidence of the presence of the tho- 

 rium and actinium emanations in the soil but, up to the present, 

 without definite results. It is, of course, useless for this pur- 

 pose to draw air from the ground and introduce it into an 

 electroscope or condenser, as in testing for the radium emana- 

 tion, on account of the rapid decay of the thorium and actinium 

 emanations. A galvanized sheet-iron pipe 15 cm in diameter and 

 2 meters long, with open bottom, was sunk in the ground and 

 a negatively charged wire suspended in it. The top was closed 

 and a gentle current of air was drawn through the cylinder 

 (entering at the open bottom from the ground) by means of a 

 filter pump. The wire did not acquire sufficient radio-activit} 7 

 to enable one to follow its decay for more than two hours, and, 

 even during this time, the ionization produced in the cylinder 



*Proc. Camb. Phil. Soc, xii, p. 17 (1902). 



