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this way, also, see further the correlation between what may be called photo-therapeutics and radioactive therapeutics.

The ss-ray, whether we obtain it directly from the transforming radioactive atom 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 hydrogen.  In these cases, also, recombination can be partially effected by the rays.

We can be quite sure that within the complex structure of the living cell the ionising effects which everywhere accompany the ss-rays must exert a profound influence.  The sequence of chemical events which as yet seem