or whether we obtain it as a result of the
effects of the y- or x-rays upon the atom, is an ionising agent
of wonderful power. What is meant by this? In its physical aspect
this means that the atoms through which it passes acquire free
electric charges; some becoming positive, some negative. This can
only be due to the loss of an electron by the affected atom. The
loss of the small negative charge carried in the electron leaves
the atom positively electrified or creates a positive ion. The
fixing of the wandering electron to a neutral atom creates a
negative ion. Before further consideration of the importance of
the phenomenon of ionisation we must fix in our minds that the
agent, which brings this about, is the ss-ray. There is little
evidence that the y-ray can directly create ions to any large
extent. But the action of liberating high-speed ss-rays results in
the creation of many thousands of ions by each ss-ray liberated.
As an agent in the hands of the medical man we must regard the
y-ray as a light wave of extremely penetrating character, which
creates high-speed ss-rays in the tissues which it penetrates,
these ss-rays being most potent ionising agents. The ss-rays
directly obtained from radioactive atoms assist in the work of
ionisation. ss-rays do not

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penetrate far from their source. The fastest of them would not
probably penetrate one centimetre in soft tissues.

We must now return to the phenomenon of ionisation. Ionisation is
revealed to observation most conspicuously when it takes place in
a gas. The + and - electric charges on the gas particles endow it
with the properties of a conductor of electricity, the + ions
moving freely in one direction and the - ions in the opposite
direction under an electric potential. But there are effects
brought about by ionisation of more importance to the medical man
than this. The chemist has long come to recognise that in the ion
he is concerned with the inner mechanism of a large number of
chemical phenomena. For with the electrification of the atom
attractive and repulsive forces arise. We can directly show the
chemical effects of the ionising ss-rays. Water exposed to their
bombardment splits up into hydrogen and oxygen. And, again, the
separated atoms may be in part recombined under the influence of
the radiation. Ammonia splits up into hydrogen and nitrogen.
Carbon dioxide forms carbon, carbon monoxide, and oxygen;
hydrochloric acid forms chlorine and hyd