Production of Helium by Radium. 587 



From the beginning it seemed probable that, if the a particles 

 consisted of any known kind of matter, they must either be 

 charged molecules of hydrogen or charged atoms of helium ; 

 and Rutherford * had pointed out, immediately after the 

 production of helium from radium had been experimentally 

 demonstrated, that the source of the helium was in all 

 probability the accumulated a particles expelled from radium 

 and its products. On this hypothesis, making use of the 

 imperfect data available at that time, an estimate was made 

 of the rate of production of helium by one gram of radium 

 in equilibrium, which indicated that the amount of helium 

 produced per year should be between 20 and 200 cubic 

 millimetres. 



The importance of determining the true nature of the 

 a. particles led to a more accurate measurement of the 

 velocities and the values of e/m of the a [(articles expelled 

 from the different types of active matter. The value of e\m 

 was found to be 5070 electromagnetic units f. 



The results indicated that the a particle was either a 

 hydrogen molecule carrying a single ionic charge or a helium 

 atom carrying two ionic charges (the latter possibility being- 

 considered the more probable), and suggested that helium 

 was a product of transformation of all types of radioactive 

 matter emitting a-rays. 



The question was further investigated by Rutherford and 

 Geiger |, who developed a method of directly counting the 

 a particles emitted by a radioactive substance. It was 

 found § that one gram of radium itself emitted 3*4 x 10 10 

 a particles per second and that radium in equilibrium with 

 its three a-ray products emitted four times this number. At 

 the same time the charge carried by each a particle was found 

 to be 9'3 x 10 ~ 10 electrostatic unit. From the various 

 evidence, it was concluded that the a particles did carry two 

 unit charges, and that the value of the unit charge, i. e. the 

 charge carried by the hydrogen atom, was 4*65 x 10 ~ 10 elec- 

 trostatic unit. The final proof of the connexion of the 

 a particles with the helium atom was supplied by Rutherford 

 and Royds ||, who showed that helium always accumulated 

 when a particles were allowed to penetrate into vacuous 

 space or into solid matter. It was thus definitely proved 



* Nature, vol. lxviii. p. 366 (1903). 

 t Rutherford, Phil. Majj. vol. xii. p. 358 (1906). 

 % Proc. Roy. Soc. vol. lxxxi. p. 151 (1908). 

 § Proc. Roy. Soc. vol. lxxxi. p. 162 (1908). 

 || Phil. Mag. vol. xvii. p. 281 (1909). 



