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..