t by M. Curie and Sir James Dewar seems to show
convincingly that this cannot be so. The spectrum of helium never
appears at first in the gas proceeding from pure bromide of radium;
but it shows itself, on the other hand, very distinctly, after the
radioactive transformations undergone by the salt.

All these strange phenomena suggest bold hypotheses, but to construct
them with any solidity they must be supported by the greatest possible
number of facts. Before admitting a definite explanation of the
phenomena which have their seat in the curious substances discovered
by them, M. and Madame Curie considered, with a great deal of reason,
that they ought first to enrich our knowledge with the exact and
precise facts relating to these bodies and to the effects produced by
the radiations they emit.

Thus M. Curie particularly set himself to study the manner in which
the radioactivity of the emanation is dissipated, and the
radioactivity that this emanation can induce on all bodies. The
radioactivity of the emanation diminishes in accordance with an
exponential law. The constant of time which characterises this
decrease is easily and exactly determined, and has a fixed value,
independent of the conditions of the experiment as well as of the
nature of the gas which is in contact with the radium and becomes
charged with the emanation. The regularity of the phenomenon is so
great that it can be used to measure time: in 3985 seconds[36] the
activity is always reduced one-half.

[Footnote 36: According to Professor Rutherford, in 3.77 days.--ED]

Radioactivity induced on any body which has been for a long time in
presence of a salt of radium disappears more rapidly. The phenomenon
appears, moreover, more complex, and the formula which expresses the
manner in which the activity diminishes must contain two exponentials.
To find it theoretically we have to imagine that the emanation first
deposits on the body in question a substance which is destroyed in
giving birth to a second, this latter disappearing in its turn by
generating a third. The initial and final substances would be
radioactive, but the intermediary one, not. If, moreover, the bodies
acted on are brought to a temperature of over 700 deg., they appear to
lose by volatilisation certain substances condensed in them, and at
the same time their activity disappears.

The other radioactive bodies behave in a similar way. Bodies which
contain actinium are particularly ri