ol Particles emitted by Uranium and Thorium. 697 



The agreement between theory and experiment confirms 

 in another way the correctness o£ Boltwood's conclusion that 

 uranium emits two a particles for one from each of its later 

 products. The experiments are not of sufficient accuracy to 

 confirm the data on the relative activity of actinium and 

 radium. There is no doubt, however, that the number of 

 a particles to be ascribed to actinium is very small compared 

 with that to be expected if actinium and its series of products 

 emitted one a particle for one from radium. The connexion 

 of actinium with the uranium-radium series is difficult to 

 determine, and remains one of the chief outstanding problems 

 in the analysis of radioactive changes. 



Production of Helium by Uranium, Uranium Minerals, 

 and 'lhorium. 



Since the a. particle is a charged atom of helium, it is a 

 simple matter to deduce the rate of production of helium 

 from the active materials considered. Calculation and 

 experiment show that one gram of radium in equilibrium 

 with its three «-ray products produces 158 cubic mm. of 

 helium per year. Since radium and each of its products 

 emits 3*4 x 10 10 a. particles per gram per second, uranium, 

 which emits 2*37 x 10 4 a particles per gram per second, 

 produces 2*75 x 10" 5 cubic mm. per year. The rate of 

 production of helium for the different materials is given 

 below. 



Production of Helium 

 per gram per year. 



Uranium 2*75 X 10~ 5 cubic mm. 



Thorium 3\L X lO" 5 



Uranium mineral in equi- ) -. -. #n in- 5 



librium j 



Radium in equilibrium ... 158 „ 



A simple calculation allows us to estimate the production 

 of helium for a mineral like thorianite containing both 

 uranium and thorium. 



Range of the a particles from Uranium. 



The range of the a particles from uranium has been 

 difficult to determine directly on account of the smallness of 

 the activity of the thin films of the substance. By observa- 

 tions of the decrease of the ionization due to a layer of 

 uranium when sheets of thin aluminium were placed over it, 

 Bragg * deduced that ihe range in air of the a particle from 

 uranium was about 3*5 cms. In the course of counting the 

 scintillations from a thin film of ionium, it was observed that 

 * Bragg, Phil. Mag. 1906, xi. p. 754. 



