twentieth-century scientist is impelled to call 'the country that is
not ours'.

The realm thrown open to science by Crookes's observations, which human
knowledge now entered as if taking it by storm, was that of the
radioactive processes of the mineral stratum of the earth. Many new and
surprising properties of electricity were discovered there - yet the
riddle of electricity itself, instead of coming nearer, withdrew into
ever deeper obscurity.

The very first step into this newly discovered territory made the
riddle still more bewildering. As we have said, Maxwell's use of a
material analogy as a means of formulating mathematically the
properties of electro-magnetic fields of force had led to results which
brought electricity into close conjunction with light. In his own way
Crookes focused, to begin with, his attention entirely on the
light-like character of electric effects in a vacuum. It was precisely
these observations, however, as continued by Lenard and others, which
presently made it necessary to see in electricity nothing else than a
special manifestation of inert mass.

The developments leading up to this stage are recent and familiar
enough to be briefly summarized. The first step was once more an
accident, when Röntgen (or rather one of his assistants) noticed that a
bunch of keys, laid down by chance on top of an unopened box of
photographic plates near a cathode tube, had produced an inexplicable
shadow-image of itself on one of the plates. The cathode tube was
apparently giving off some hitherto unknown type of radiation, capable
of penetrating opaque substances. Röntgen was an experimentalist, not a
theorist; his pupils used to say privately that in publishing this