588 Profs. B. B. Boltwood and E. Rutherford on 



that the a particle is a projected atom of helium which carries 

 two unit charges. 



Rutherford and Geiger * calculated the rate of production 

 of helium by radium to be expected on the basis of the above 

 data. One gram of radium in equilibrium expels 13*6 x 10 10 

 « particles, i.e. atoms of helium, per second. Taking the 

 charge carried by the hydrogen atom as 4*65 x 10 -10 , it can 

 at once be calculated from the electrochemical equivalent 

 of hydrogen that one cubic centimetre of any gas at standard 

 pressure and temperature contains 2*72 x 10 19 molecules. 

 As helium is monatomic, the rate of production of helium 



13*6 X 10 10 

 per gram of radium is — -^ — Ynl9 c,c * P er secon ^, or 158 



cubic millimetres per year. 



The first direct determination of the rate of production of 

 helium by radium was made by Sir James Dewar |, using 

 70 milligrams of anhydrous chloride — part of the material 

 prepared by Dr. Thorpe for the determination of the atomic 

 weight of radium. The radium salt in the crystalline state 

 was contained in an exhausted vessel connected with a 

 MacLeod gauge. The salt was occasionally heated to drive 

 off the accumulated helium, and other gases which might be 

 present were removed by exposure to a small fragment of 

 coconut charcoal cooled by liquid air. An estimate of the 

 amount of hydrogen present in the gaseous mixture was 

 obtained by further cooling the charcoal to the temperature of 

 liquid hydrogen. 



In the first paper published it was concluded that one gram 

 of radium in equilibrium produced 0*37 cubic millimetre of 

 helium per day. In a later experiment J the radium was 

 sealed up for a period of nine months, and the average rate 

 of production of helium was estimated to correspond to 0*463 

 cubic millimetres per day. It was pointed out in the second 

 paper that when the results of the first experiment were 

 corrected for an error in the original calculation, the rate of 

 the production of helium in the first experiment corre- 

 sponded to 0*499 instead of 0*37 cubic millimetre per day 

 per gram of radium. The rate of the production of helium 

 per gram of radium per year indicated by these experiments 

 is therefore 182 cubic millimetres in the first and 169 cubic 

 millimetres in the second. These values are both somewhat 

 higher than the calculated quantity, viz. 158 cubic milli- 

 metres. 



* Proc, Roy. Soc. vol. lxxxi. p. 162 (1908). 

 t Proc. Roy. Soc. vol. lxxxi. p. 280 (1908). 

 j Proc. Roy. Soc. vol. lxxxiii. p. 404 (1910). 



