ical separations, in isolating compounds of a
new and intensely radio-active substance which they named radium.

Radium resembles barium in its chemical properties, and is precipitated
with barium in the ordinary course of chemical analysis. It is separated
by a prolonged course of successive crystallisation, the chloride of
radium being less soluble than that of barium, and therefore sooner
separated from an evaporating solution. When isolated, radium chloride
has a composition, which, on the assumption that one atom of metal
combines with two of chlorine as in barium chloride, indicates that the
relative weight of the atom of radium is about 225. As thus prepared,
radium is a well-marked chemical element, forming a series of compounds
analogous to those of barium and showing a characteristic line spectrum.
But, unlike most other chemical elements, it is intensely radio-active,
and produces effects some two million times greater than those of
uranium.

In 1899 E. Rutherford, then of Montreal, discovered that the
radiation from uranium, thorium and radium was complex. (Rutherford,
"Radio-activity" (2nd edition), Cambridge, 1905.) Three types of rays
were soon distinguished. The first, named by Rutherford alpha-rays, are
absorbed by thin metal foil or a few centimetres of air. When examined
by measurements of the deflections caused by magnetic and electric
fields, the alpha-rays are found to behave as would positively
electrified particles of the magnitude of helium atoms possessing a
double ionic charge and travelling with a velocity about one-tenth that
of light. The second or beta type of radiation is much more penetrating.
It will pass through a considerable thickness of metallic foil, or many
centimetres of air, and still affect photographic plates or discharge
electroscopes. Magnetic and electric forces deflect beta-rays much
more than alpha-rays, indicating that, although the speed is greater,
approaching in some cases within five per cent. that of light, the mass
is very much less. The beta-rays must be streams of particles, identical
with those of cathode rays, possessing the minute mass of J.J. Thomson's
corpuscle, some eight-hundredth part of that of a hydrogen atom. A third
or gamma type of radiation was also detected. More penetrating even than
beta-rays, the gamma-rays have never been deflected by any magnetic
or electric force yet applied. Like Rontgen rays, it is probable that
gamma-rays are wave-pulses