Production of Helium hi/ Radium. 599 



Substituting in this equation the values of Q and T from 

 the two determinations, it follows that for 



First determination x = 0*0209 cub. mm. : 

 Second determination x = 0*0203 cub. mm., 



or an average production of 0*0206 cubic millimetres of 

 helium per day by the radium (element) present in the salt 

 used for these experiments. 



The quantity of radium contained in the salt was 192 milli- 

 grams (average of 191 and 193). The production of helium 

 per gram of radium was therefore 



= 0*107 cub. mm. per day, 



and per gram of radium in equilibrium with its first disin- 

 tegration products (the emanation, radium A, and radium C) 



0*107 x4x 365 = 156 cub, mm. helium per year . 



The observed rate of production of helium is in very close 

 agreement with the calculated rate of production (158 cub. 

 mm. per year) referred to earlier, and confirms in a striking 

 way the conclusions on which the calculations were based. 

 There can be no doubt that the a particle during its flight 

 consists of a helium atom carrying two unit positive charges, 

 and that helium itself is monatomic. The agreement between 

 calculation and theory verifies in a remarkable way the 

 essential correctness of the atomic theory of matter. The 

 number of helium atoms expelled per second by one gram 

 of radium have been directly counted, and the corresponding 

 volume of helium produced has been experimentally measured. 

 From these two experimental observations, it is possible to 

 deduce with a minimum of assumption the number of atoms 

 of helium in one cubic centimetre of that gas at standard 

 pressure and temperature. This number is 2 69 x 10 19 . By 

 Avogadro's hypothesis this gives the number of molecules in 

 a cubic centimetre of any gas under standard conditions. 



It will be seen that this method does not involve the 

 determination of the fundamental unit charge e, but that the 

 value of this can be directly deduced. It is important to 

 note that the agreement of the observed and calculated rates 

 of the production of helium is independent of the correctness 

 of the radium standard employed, for the same standard has 

 been employed in both experiments. When an international 

 radium standard is instituted, it will not be difficult to correct 

 the rate of production of helium in terms of the new 

 standard. 



2 R2 



