H. A. Bumstead — Atmospheric Radio-activity. 11 



was too small for accurate observations to be made. The rate 

 appeared to be somewhat slower than that of the excited 

 activity due to radium but no great confidence can be placed 

 in the result. It is likely that a larger cavity in the ground 

 would give more definite results but I have not yet been able 

 to try this. 



Conclusions. 



1. The radio-activity acquired by a negatively charged wire 

 exposed in the open air (at least as observed in New Haven) is 

 mainly, if not wholly, clue to the excited activities of radium 

 and thorium. With a three-hour exposure, 3 to 5 per cent of 

 the total initial effect is due to the thorium activity, the pro- 

 portion depending apparently upon the greater or less ease 

 with which the emanations escape from the soil. With a 

 twelve-hour exposure the thorium activity is sometimes 15 per 

 cent of the whole, and with a long wire, its decay may be fol- 

 lowed for several days. There is some evidence that a small 

 quantity of a more rapidly decaying activity is present in addi- 

 tion, but these experiments do not definitely establish this. 



2. The radio-activity of rain and snow is probably due to 

 radium-excited activity, the absence of the thorium effect being 

 accounted for by the fact that the rapid decay of the thorium 

 emanation prevents its reaching, in appreciable quantities, the 

 height at which the rain-drops are formed. 



Sheffield Scientific School of Yale University, April, 1904. 



