of Radioactive Products present in the Atmosphere. 823 



for an infinitely long exposure, could be easily deduced with 

 fair accuracy from the decay-curves. 



The experiments were commenced in April and finished 

 in December 1910*. Some of the results are given in 

 Table I. (p. 824). The meteorological data in" the table are 

 taken from ' Monthly Report of the Central Meteorological 

 Observatory of Japan.' The observatory is situated about two 

 miles south of our laboratory. 



In this series of experiments the time of exposure was 

 4 hours, ?'. e. from 9 a.m. to 1 p.m., except in the second 

 observation on the 19th, in which it was from 2 to 6 p.m., 

 and the corresponding values in Column X. must be under- 

 stood as those at 3 and 5 P.M. 



(3) The Table shows that, in this series of experi- 

 ments : — 



(a) the activities due to the radium C in equilibrium were 

 3*14 and 3*33 respectively for the upper and lower wires, 

 on the average (it may be added that the average values in 

 all the series of our experiments were 4*06 and 4*28 respec- 

 tively for the upper and lower wires) ; 



(6) their extreme values were as 3 : 1 (those in all the 

 series of the experiments were as 9:1); 



(c) the ratio of the amount of the radium C on the upper 

 and lower wires was nearly constant and equal to *95, on 

 the average ; 



(a) the activities due to the thorium deposits were 1*34 

 and 2*62 respectively for the upper and lower wires, on the 

 average (those in all the series of the experiments were 1*08 

 and 2*20 respectively) ; 



(e) the relative amounts of the thorium deposits on the 

 upper and lower wires were by no means constant, but 

 varied over a wide range on different occasions, and therefore 



(/■') the relative amounts of the radium and thorium 

 deposits were not constant, but depended on the height at 

 which the wire had been exposed. 



Theoretical Considerations. 

 (4) Considering the comparatively slow rate of transfor- 

 mation of the radium emanation, and the existence of some 

 convection current of air even on calm days, it will be 

 reasonable to suppose that the amount of the emanation does 

 not appreciably vary within a few metres above the earth's 

 surface. Let Ne denote the number of atoms of the emanation 

 in each c.c. of air. These uniformly distributed atoms of the 



* One of us (S. Ouo) having- left the Laboratory in July 1910, we were 

 assisted in the later part of the experiment by Mr. T. Sone, to whom 

 our thanks are due. 



3 I 2 



