active
radiation.

[Footnote 46: In reality M. Sagnac operated in the converse manner. He
took two equal _weights_ of a salt of radium and a salt of barium,
which he made oscillate one after the other in a torsion balance. Had
the durations of oscillation been different, it might be concluded
that the mechanical mass is not the same for radium as for barium.]

But for this explanation to be admitted, it would evidently need to be
supported by very numerous facts. It might, no doubt, appear still
more probable that the energy borrowed from the external medium by
radium is one of those still unknown to us, but of which a vague
instinct causes us to suspect the existence around us. It is
indisputable, moreover, that the atmosphere in all directions is
furrowed with active radiations; those of radium may be secondary
radiations reflected by a kind of resonance phenomenon.

Certain experiments by Professors Elster and Geitel, however, are not
favourable to this point of view. If an active body be surrounded by a
radioactive envelope, a screen should prevent this body from receiving
any impression from outside, and yet there is no diminution apparent
in the activity presented by a certain quantity of radium when it is
lowered to a depth of 800 metres under ground, in a region containing
a notable quantity of pitchblende. These negative results are, on the
other hand, so many successes for the partisans of the explanation of
radioactivity by atomic energy.

CHAPTER X

THE ETHER AND MATTER

Sec. 1. THE RELATIONS BETWEEN THE ETHER AND MATTER

For some time past it has been the more or less avowed ambition of
physicists to construct with the particles of ether all possible forms
of corporeal existence; but our knowledge of the inmost nature of
things has hitherto seemed too limited for us to attempt such an
enterprise with any chance of success. The electronic hypothesis,
however, which has furnished a satisfactory image of the most curious
phenomena produced in the bosom of matter, has also led to a more
complete electromagnetic theory of the ether than that of Maxwell, and
this twofold result has given birth to the hope of arriving by means
of this hypothesis at a complete co-ordination of the physical world.

The phenomena whose study may bring us to the very threshold of the
problem, are those in which the connections between matter and the
ether appear clearly and in a relatively simple manner. Thus in the