342 BOLTWOOD— RADIOACTIVITY. [April 22, 



and Q is the number of atoms of B and A. the constant of change of 

 the product B. Under these conditions the substances A and B are 

 said to be in equilibritim with one another. The general mathemat- 

 ical theory of successive changes of this kind has been developed by 

 Rutherford. 



In the discussion of the characteristics of the alpha rays it has 

 been pointed out that the evidence supplied by the determination of 

 the ratio of the charge to the mass of these particles indicates that 

 their nature is the same in all cases. Let us consider briefly the 

 additional facts which are in support of this conclusion. 



The presence of considerable proportions of helium in crystalline 

 minerals containing m-anium and thorium has been very frequently 

 noticed. It was found by Ramsay and Soddy in 1903 that helium 

 could be detected in the residual gas set free when a specimen of 

 crystalline radium bromide was dissolved in water, and shortly after 

 this they showed that the spectrum of helium appeared with time in 

 a tube which initially contained only radium emanation. Debierne 

 found that helium was produced by a strong preparation of actinium, 

 and conclusive proof has also been obtained by Strutt and by Soddy 

 that helium results from the disintegration of both thorium and 

 uranium. 



During the past year I was able to experimentally demonstrate 

 the production of helium by ionium, and some earlier experiments 

 carried out by Professor Rutherford and myself showed that helium 

 appeared during the disintegration of polonium also. The latter 

 conclusion has since been confirmed b_\- the work of Mme. Curie and 

 Debierne. 



The data supphed by the counting experiments of Rutherford 

 and Geiger afford a basis for the calculation of a number of impor- 

 tant physical (luantitics, such as the mass of the hydrogen atom, the 

 number of atoms in one gram of h_\dr()gen and the number of mole- 

 cules per cubic centimeter of any gas at standard pressure and tem- 

 perature. In a similar matter Rutherford and Geiger have calcu- 

 lated the amount of helium produced per }ear by one gram of radium 

 containing equilibrium amounts of its three alpha ray products, the 

 emanation, radium A and radium C. The number obtained in this 

 way was 158 cubic millimeters of helium per year per gram of 



