§ 89. The Radioactivity of Radium.
Radium salts give off three distinct sorts of rays, referred to by
the Greek letters α, β, γ. The α-rays have been shown to
consist of of electrically charged (positive) particles, with a mass
approximately equal to that of four hydrogen atoms; they are slightly
deviated by a magnetic field, and do not possess great penetrative
power. The β-rays are similar to the kathode rays, and consist of
(negative) electrons; they are strongly deviated by a magnetic field, in
a direction opposite to that in which the α-particles are deviated,
and possess medium penetrative power, passing for the most part through
a thin sheet of metal. The γ-rays resemble X-rays; they possess
great penetrative power, and are not deviated by a magnetic field. The
difference in the effect of the magnetic field on these rays, and the
difference in their penetrative power, led to their detection and allows
of their separate examination. Radium salts emit also an emanation,
which tends to become occluded in the solid salt, but can be
conveniently liberated by dissolving the salt in water, or by heating
it. The emanation exhibits the characteristic properties of a gas, it
obeys Boyle's Law (
i.e., its volume varies inversely with its
pressure), and it can be condensed to a liquid at low temperatures; its
density as determined by the diffusion method is about 100. Attempts to
prepare chemical compounds of the emanation have failed, and in this
respect it resembles the rare gases of the atmosphere—helium, neon,
argon, krypton, and xenon—whence it is probable that its molecules are
monatomic, so that a density of 100 would give its atomic weight as
200.
5 As can be seen from the table on
pp. 106, 107, an atomic weight of about 220 corresponds to a position in
the column containing the rare gases in the periodic system. That the
emanation actually has an atomic weight of these dimensions was
confirmed by further experiments carried out by the late Sir William
Ramsay and Dr. R. W. Gray.
6 These
chemists determined the density of the emanation by actually weighing
minute quantities of known volume of the substance, sealed up in small
capillary tubes, a specially sensitive
balance being employed. Values for the density varying from 108 to
113½, corresponding to values for the atomic weight varying from
216 to 227, were thereby: obtained. Sir William Ramsay, therefore,
considered that there could no longer be any doubt that the emanation
was one of the elements of the group of chemically inert gases. He
proposed to call it
Niton, and, for reasons which we shall note
later, considered that in all probability it had an atomic weight of
about 222½.
