Could
the trail be due to a molecule? Two atoms of hydrogen
bound to a single neon atom would measure 22 on this
scale, but to produce the weak trail at 11 the molecule
would need to have gained a double charge by losing
a second electron. Thomson knew it was very unusual
for a molecule to lose a second electron.
Another
theory, which also relied on a molecule carrying a double
charge, was that the 22 trace could be due to doubly
ionised carbon dioxide. Thomson removed all the carbon
dioxide from the gas, but the trace was still present.
Thomson
correctly interpreted the traces as showing isotopes
of neon. The trace at 20 was ionised neon, as expected,
but the trace at 22 was also ionised neon. The two types
of neon were chemically identical but their masses were
different. The trace at 11 was this second type of neon
which had lost two electrons. Isotopes of radioactive
elements had already been discovered by Soddy in Canada,
but this was the first demonstration that a non-radioactive
element could have isotopes.
