H. A. Bum stead — Atmospheric Radio-activity. 9 



ties of the radium and thorium emanations. The experiments 

 plotted in I were made when the ground was hard frozen, and 

 had been so (with occasional superficial thaws) for several 

 months ; III is the result of an exposure made a month later 

 when the frost had entirely disappeared from the ground. 

 The decay of the thorium emanation (half-value in one minute) 

 is so rapid, compared with that of the radium emanation, that 

 any delay in its liberation from the ground would considerably 

 diminish the relative amount in the air. According to the 

 accepted theory of radio-activity, due to J. J. Thomson and 

 Rutherford, the disintegration of the gaseous emanation pro- 

 duces a solid material, responsible for the excited radio- 

 activity, which, following Rutherford, we may call emanation 

 X. The particles of emanation X produced by the gas (of 

 either kind) before it leaves the ground would never reach the 

 upper air, since they would not diffuse like the molecules of a 

 gas but would be deposited in the soil ; on the other hand, 

 those produced after the escape of the gas would settle very 

 slowly, on account of their small size, and might be carried 

 considerable distances by currents of air. The view that the 

 smaller proportion of thorium activity is due to the frozen con- 

 dition of the ground, is supported by two other experiments 

 (incomplete and therefore not given in detail), one made while 

 the ground was frozen and the other when it was not ; in the 

 latter case the decay was noticeably slower, indicating a larger 

 proportion of the thorium activity. The smaller total activity 

 observed in III might be thought to negative the above expla- 

 nation ; I think, however, that it was due to another cause. 

 There was a very perceptible haze in the air on the day when 

 this experiment was made and little wind ; the day of the other 

 experiment was exceptionally clear and a good breeze was 

 blowing. The wind would bring more of the particles of the 

 emanation X within the electric field of the wire, and the fact 

 that they were not loaded with drops of water (or with very 

 small ones) would cause them to move with a greater velocity 

 along the lines of force and hence a greater number might be 

 captured by the wire, even if the number in a cubic centimeter 

 of the air were actually less. The only exposure of a wire 

 which I have made on a clear, windy day since the ground 

 thawed, was one of twelve hours, so that the result is not com- 

 parable directly with the three-hour exposure ; moreover, the 

 total activity was not observed, but only the thorium effect 

 remaining after ten hours. Reducing this to its value four 

 hours after exposure ceased, taking account of the difference 

 in the lengths of the wires, and assuming that the relation 

 between the thorium activity and the time of exposure is 



i, =1(1 _.-*), 



