692 Dr. Geiger and Prof. Rutherford on the JS J umber of 



uranium emits one a particle for one from the radium itself 

 in equilibrium with it. This suggests that the uranium 

 atom in its transformation emits at least two a particles. In 

 the present state of our knowledge it is not certain whether 

 this can, be ascribed to the existence of an additional a ray 

 product which is always separated with the uranium, or to 

 the expulsion of two or more a particles in the transforma- 

 tion of the uranium atom. 



Supposing, for the purpose of calculation, that the uranium 

 in a mineral emits two a particles for one from each of the 

 subsequent six a ray products, viz. ionium, radium emana- 

 tion, radium A, radium C, radium F (polonium), the number 

 of a particles emitted per second per gram of uranium in a 

 mineral is 8 N, or four times the number emitted by ordinary 

 purified uranium. In this calculation no account has been 

 taken of the actinium which occurs in all uranium minerals, 

 and which Boltwood has shown stands in a genetic relation 

 with uranium. However, Boltwood (loc. cit.) has found 

 that the actinium and its four a ray products contributes an 

 activity to the mineral equal to only '21 of that of the 

 uranium. The relative number of a particles is still smaller, 

 for the a particles from actinium have an average range of 

 about 5*7 cms. of air, while the a, rays of uranium, according 

 to Bragg, have a range of 3" 5 cms. Taking as a first 

 approximation that the ionization due to an a particle is 

 proportional to its range, the number of a particles emitted 

 by the actinium in a mineral should be about *17 of that 

 from uranium. The total number of ex. particles emitted by 

 a mineral containing one gram of uranium should con- 

 sequently be 2*34 N + 6 N = 8*34 N. Since N by calculation 

 is 11,600, the total number of a particles emitted per second 

 from a mineral containing one gram of uranium should be 

 9*67 x 10 4 , and the number per second from one gram of 

 ordinary purified uranium should be 2*32 x 10 1 . 



It was the object of the present experiments * to deter- 

 mine the number of a particles experimentally, and to test 

 the agreement with the calculated number. 



* The experiments described later were, for the most part, completed 

 more than a year ago. Recently, J. N. Brown (Proc. Roy. Soc. vol. A. 

 lxxxiv. p. 151, 1910) has counted the scintillations from a uranium 

 mineral and found a value per gram of uranium of 7*36 X 10 4 , which 

 is somewhat smaller than our experimental value given later, viz. 

 96 Xl0 4 



