ts to pass through, but sufficiently
thick to withstand atmospheric pressure. The thickness of the glass
wall was in most cases less than .01 mm. On introducing the emanation
into the tube, the escape of the alpha particles from the emanation
was clearly seen by the scintillations produced at some distance on a
zinc sulphide screen. After this test the glass tube _A_ was
surrounded by a glass tube _T_ and a small spectrum tube _V_ attached
to it. The tube _T_ was exhausted to a charcoal vacuum. By means of
the mercury column _H_, the gases in the tube _T_ could at any time be
compressed into the spectrum tube _V_ and the nature of the gases
which had been produced determined spectroscopically. It was found
that two days after the introduction of the emanation into _A_ the
spectrum showed the yellow line of helium, and after six days the
whole helium spectrum was observed. In order to be certain that the
helium, coming possibly from some other source, had not diffused
through the thin walls of the tube _A_, the emanation was pumped out
and helium substituted. No trace of helium could be observed in the
vacuum tube after several days, showing that the helium observed in
the first experiment must have originated from the alpha particles
which had been propelled through the thin glass tube into the outer
tube.

Most of the alpha particles are propelled with such force that they
penetrate some distance into the walls of the outer tube and some of
these gradually diffuse out into the exhausted space. The presence of
helium in the spectrum tube can be detected after a shorter interval
if a thin cylinder of lead is placed over the emanation tube, since
the particles fired into the lead diffuse out more rapidly than from
glass.

A still more definite proof of the identity of the alpha particle with
the helium atom was obtained by removing the outer glass tube _T_ and
placing a cylinder of lead over the emanation tube in the open air.
Helium was always detected in the lead after it had remained several
hours over the thin tube containing a large quantity of the emanation.
In order to test for the presence of helium in the lead, the gases
present were released by melting the lead in a closed vessel. There
can thus be no doubt that the alpha particle becomes a helium atom
when its positive charge is neutralized.

Thus the chemist was afforded the experience of the building up of at
least one element under his observation, and b