If it be a gaseous body,
it must form part of the argon group, and, like its other members, be
perfectly inert.
By studying the spectrum of the gas disengaged by a solution of salt
of radium, Sir William Ramsay and Professor Soddy remarked that when
the gas is radioactive there are first obtained rays of gases
belonging to the argon family, then by degrees, as the activity
disappears, the spectrum slowly changes, and finally presents the
characteristic aspect of helium.
We know that the existence of this gas was first discovered by
spectrum analysis in the sun. Later its presence was noted in our
atmosphere, and in a few minerals which happen to be the very ones
from which radium has been obtained. It might therefore have been the
case that it pre-existed in the gases extracted from radium; but a
remarkable experiment by M. Curie and Sir James Dewar seems to show
convincingly that this cannot be so. The spectrum of helium never
appears at first in the gas proceeding from pure bromide of radium;
but it shows itself, on the other hand, very distinctly, after the
radioactive transformations undergone by the salt.
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