The experimental evidence collected during the last few years
has strongly supported the view that the α particle is a charged
helium atom, but it has been found exceedingly difficult to give
a decisive proof of the relation. In recent papers, Rutherford
and Geiger have supplied still further evidence of the
correctness of this point of view. The number of α particles
from one gram of radium have been counted, and the charge
carried by each determined. The values of several radioactive
quantities, calculated on the assumption that the α particle is
a helium atom carrying two unit charges, have been shown to be
in good agreement with the experimental numbers. In particular,
the good agreement between the calculated rate of production of
helium by radium and the rate experimentally determined by Sir
James Dewar, is strong evidence in favour of the identity of the
α particle with the helium atom. The methods of attack on this
problem have been largely indirect, involving considerations of
the charge carried by t