RADIUM AND THE ELECTRON^ — RUTHERFORD. 199 
The proof by the Curies of the rapid and continuous emission of 
heat from radium showed clearly the vast amount of energy that 
must be stored up in radioactive matter and released by its transfor- 
mation. This heat emission has been shown to be a secondary effect 
of radioactivity, for it is a measure of the energy of the expelled 
radiations, the greater part being due to the energy of the expelled 
a-particles. 
The transformation of an atom is the result of an explosion of 
intense violence in which a part of the atom, whether a helium atom 
or an electron, is shot out with great speed. In order to produce 
a-, ^-, or Y-rays of equal energy to those emitted by radioactive sub- 
stances, potential differences of about 2,000,000 volts applied to a 
vacuum tube would be necessary. These spontaneous radiations have 
been of great utility in studying the ionization, scattering, and other 
properties of particles moving at high speed, while in the very pene- 
trating Y-rays we have a type of X-rays of much shorter wave-length 
than can be produced at present or is likely to be produced by 
laboratory methods. 
The properties of the a-rays have been very closely studied and 
their speed and mass have been determined accurately. The definite- 
ness of the range of a-particles, to which Bragg first directed atten- 
tion, is a matter of remark, and so far the apparent disappearance of 
the a-particle while still moving with a high velocity has not been 
adequately explained. The analysis of the ^-rays has disclosed the 
presence of groups of electrons emitted at a definite velocity, so that 
the pencil of ^-raj^s deflected in a magnetic field shows a veritable 
magnetic spectrum. The presence of these groups of ^-rays appears to 
be connected with the emission of characteristic X-radiation from the 
atom, and the evidence as a whole strongly supports the view that the 
Y-rays from radioactive substances, like the X-rays from a vacuum 
tube, contain rays of a wide range of frequency in which the char- 
acteristic rays from the atom predominate. 
Space does not allow me to do more than mention the extraordinary 
delicacy and definiteness of the electrical methods devised for measur- 
ing minute quantities of radioactive matter. By their aid the chemi- 
cal properties of the numerous radioactive elements have been studied 
and their position in the periodic table established. The orderly 
sequence of changes in the chemical properties of successive elements 
in the radioactive series has been shown to be intimately connected 
with the type of radiation, whether a- or ^-ray, emitted by the pre- 
ceding element. One of the most important fruits of these chemical 
investigations has been the proof of the existence of nonseparable 
elements, named isotopes by Soddy, which are identical in ordinary 
physical and chemical properties, but have different atomic weights. 
