352 



SCIENCE 



[N. S. Vol. XXVII. No. 687 



Cooke found that the penetrating radiation 

 produced about 4.5 ions per c.c. per second and 

 McClennan has recently found 9. We thus 

 see that the radium content of the ground is 

 too small to account for all of the ionization 

 produced by the external penetrating radia- 

 tion. Eve {Phil. Mag., August, 1907) reaches 

 a similar conclusion. 



Taking A for air to be .00004 and assuming 

 Cooke's value for the ionization produced by 

 the penetrating radiation, one can calculate 

 the radium content of the air necessary to 

 furnish a sufficient amount of y radiation. 

 Supposing these radioactive products to be 

 contained by the lower strata of air — say for 

 a height of 300 m.- — one finds that this is 

 equivalent to the radium content of some 

 60 centimeters depth of ground, taking Q 

 equal to .9(10)"'". This depth is rather large, 

 but is not entirely impossible. The diurnal 

 temperature wave penetrates to a depth of 

 about one meter and barometric waves prob- 

 ably penetrate much deeper. 



It has been found by Campbell, Wood, the 

 writer and McKeon that the value of the ex- 

 ternal penetrating radiation which produces 

 part of the ionization in closed vessels varies 

 very considerably in amount, these variations 

 often amounting to more than one or two 

 hundred per cent, in the course of two or three 

 days. If this penetrating radiation is due to 

 the y I'ays from the ground it should be prac- 

 tically constant. Moreover, Dike has found 

 that the active deposit in the air also varies 

 very greatly in amount. The writer has 

 found in several instances that the penetrating 

 radiation falls almost to zero value during a 

 heavy rain or snow (Science, July 12, 1907). 

 Now T^ilson, Eutherford and Allen have 

 shovra. that rain and snow carry dovm. radio- 

 active pi'oducts. If this is true, then, accord- 

 ing to our theory, the penetrating radiation 

 would be considerably decreased. During a 

 very heavy rain the decrease should be very 

 noticeable, and might reach a very low value. 

 If the external penetrating radiation is due 

 to radium products in the air, then after a 

 heavy rain the penetrating radiation should 

 increase rapidly for one or two hours after 

 the stopping of the rain, since the products 



radium A, B and G will reach equilibrium 

 values in that length of time, and the y rays 

 come from radium G only. Results indicating 

 a recovery of the penetrating radiation similar 

 to this have been noted in one instance. 



The writer has tried glass electroscopes with 

 practically all metal parts covered with sul- 

 phur. Since glass and sulphur give off very 

 little intrinsic radiation, the ionization should 

 be produced chiefly by the external penetrating 

 radiation. The results found so far are so 

 irregular that they are as yet hard to interpret. 



During the summer of 1907 some observa- 

 tions were made in the Cumberland valley 

 (Trenton limestone similar to that analyzed by 

 Eve) seven miles from Harrisburg, and one 

 mile from any town. The electroscopes were 

 lined on the inside with aluminium. Read- 

 ings during the day were made every few 

 hours. During the night, as the leak was 

 very slow, few readings were made, so that 

 readings were not frequent enough to show 

 the double daily period. The curves do 

 show the enormous increase of the ioniza- 

 tion during the early hours of the day. 

 During the night the ionization was very 

 small. The electroscope was placed in a 

 cistern in the ground and was thus surrounded 

 on all sides by at least four feet of water. In 

 this case the penetrating radiation was entirely 

 cut off and it was found that there was no 

 difference in the ionization found during the 

 various parts of the day. Moreover, the 

 amount was roughly the same as that ob- 

 sen'ed during the night on the surface of the 

 ground. 



The conclusion is that during the early 

 hours of the day the penetrating radiation was 

 quite large, the walls of the electroscope being 

 thick enough to absorb any ;8-like rays. 



Curve 1 represents the values found Septem- 

 ber 4, 1907. 



Curve 2 represents averages found for sev- 

 eral days during August and September, 1907. 



During the early hours of the day, accord- 

 ing to Elster and Geitel's theory, the ground 

 would become warmer and its emanating 

 power would be greatly increased. Radio- 

 active products would thus accumulate in the 

 lower strata of the air, and as the coefficient 



