in the Earth and the Atmoqjliere. 37£> 



is necessary to account for the penetrating 

 radiation observed in the atmosphere, and calls attention to 

 the fact that such a quantity is about four times greater than 

 the average amount which Strutt has found to be contained 

 in rocks. 



Now I wish to call attention to a fact which was pointed 

 out by me at the meeting of the " Societa Italiana di Fisica " 

 that took place in Rome during the month of April 1906. 

 I then communicated the results of an investigation made by 

 me on the excited activity obtained by exposing a negatively 

 charged wire to the open air, showing that a great part of 

 the active matter contained in the atmosphere (at least in 

 Rome and its surroundings) consists of transformation 

 products of radiothorium. 



A simple calculation shows that, owing to the respective 

 values of the constants of decay of radium B and thorium A, 

 while an exposure lasting for 3*1 hours is sufficient to obtain 

 99 per cent, of the maximum activity in the case of radium, 

 an exposure lasting for 73*5 hours is necessary in order to 

 obtain the corresponding result in the case of radiothorium. 



It is interesting to note that already three years ago Sella * 

 had seen that the activity excited on a wire by exposure to 

 electric effluvium in the open air could be considerably in- 

 creased by prolonging the exposure for many hours ; a fact 

 which was not consistent with the hypothesis of radium 

 emanation being the only cause of the observed eifects. 



Bumstead f later, found that by prolonged exposure of a 

 wire to the open air, in Newhaven, the excited activity of 

 rapid decay was followed by a residual activity which was 

 similar to that obtained by exposure to thorium emanation ; 

 and Dadourian J also noticed, in the same locality, a residual 

 activity of the thorium type on a wire exposed to air extracted 

 from the soil. 



Burbank § finally, has observed a similar effect for wires 

 made active in the open air, first in Grottingen and afterwards 

 in Washington. 



My object was therefore to establish as exactly as possible 

 the relative amount of radium and radiothorium excited 

 activity to be obtained from atmospheric air in Rome and its 

 surroundings. 



An insulated brass wire, about twelve metres long, kept 



* Sella, Rendic. Ace. Lincei, Sem. 1, 1902. 

 t Bumstead, Amer. Joarn. Science, xviii. July, 1904. 

 X Dadourian, Amer. Journ. Science, xix. January. 190o. 

 § Burbank. Phys. Zeit. vi. p. 436, 1905 ; < Atmosph. Electr. and Terr. 

 Magn.' 1906. 



