The Radioactivity of Uranium. 441 



hydrogen at 0° C. and 760 mms. of mercury pressure is 



9 X 10~ 5 gram, the number of molecules (N) in one cubic 



9 x 10 — 5 

 centimetre of hydrogen is - — . or approximately 



N = 4xl0 19 . ^ xl ° 



The mean resultofthe present experiments, viz. £ = 3'1 x 10~ 10 

 of an electrostatic unit, cannot be very far from the truth. 

 I think that it may be considered established by these experi- 

 ments that e lies between 2 x 10~ 10 and 4x 10" 10 E.S. unit. 



The values of 1ST which have been obtained from the kinetic 

 theory of gases vary between rather wide limits. The value 

 obtained depending usually on the radius assigned to a mole- 

 cule of the gas under consideration. 0. E. Meyer ('Kinetic 

 Theory of Gases/ p. 333) gives the value N = 6'lxl0 19 , 

 which is based on the assumption that the average radius of 

 a molecule of air is 10~ 8 cm. If N =4 x 10 19 then the average 

 radius of a molecule of air must be 1*2 X 10~ 8 cm. 



A great many different lines of argument (see Meyer's 

 ' Kinetic Theory of Gases ') lead to values for the radius of a 

 molecule or sphere of molecular action near to 10 -8 cm., but 

 the magnitude of this quantity certainly cannot be considered 

 to be established except within limits not very near together. 

 The agreement between the value of N obtained from the 

 present experiments, and the values deduced from the kinetic 

 theory of gases, may consequently be considered as good as 

 could have been expected. 



In conclusion I wish to say that my best thanks are due 

 to Prof. J. J. Thomson for much valuable advice during 

 the carrying out of these experiments in the Cavendish 

 Laboratory. 



XLIII. The Radioactivity of Uranium. i?z/E. Rutherford, 

 M.A.. D.Sc, Macdonald Professor of Physics, McGill 

 University, and F. Soddy, M.A. (Oxon.)*. 



THE radioactivity of the element uranium has been ex- 

 amined in the light of the theory put forward by the 

 the authors to explain the radioactivity of thorium. The 

 constant radiating power of that element was shown to be 

 caused by an equilibrium process, in which the decay of 

 activity with time was balanced by the continuous production 

 of fresh active matter at a constant rate. This explanation 

 embraces equally well the radioactivity of uranium, although 

 the changes that occur differ widely in degree and complexity 

 from those that maintain the radioactivity of thorium. 



* Communicated by the Authors. 



