made with scientific caution.
Though the observation seemed to prove the actual transformation of one
element into another, Professor Ramsay himself was by no means ready to
declare the absolute certainty of this. Yet the presumption in favor
of this interpretation of the observed phenomena is very strong; and so
cautious a reasoner as Professor Rutherford has declared recently that
"there can be no doubt that helium is derived from the emanations of
radium in consequence of changes of some kind occurring in it."*6*

"In order to explain the presence of helium in radium on ordinary
chemical lines," says Professor Rutherford, "it has been suggested that
radium is not a true element, but a molecular compound of helium with
some substance known or unknown. The helium compound gradually breaks
down, giving rise to the helium observed. It is at once obvious that
this postulated helium compound is of an entirely different character to
any other compound previously observed in chemistry. Weight for weight,
it emits during its change an amount of energy at least one million
times greater than any molecular compound known. In addition, it must
be supposed that the rate of breaking up of the helium compound is
independent of great ranges of temperature--a result never before
observed in any molecular change. The helium compound in its breaking
up must give rise to the peculiar radiations and also pass through the
successive radio-active change observed in radium.... On the other
hand, radium, as far as it has been examined, has fulfilled every
test required of an element. It has a well-marked and characteristic
spectrum, and there is no reason to suppose that it is not an element in
the ordinarily accepted sense of the term."*7*

THE SOURCE OF ENERGY OF RADIO-ACTIVITY

In 1903 Messrs. Curie and Laborde*8* made the remarkable announcement
that a crystal of radium is persistently warmer than its surrounding
medium; in other words, that it is perpetually giving out heat without
apparently becoming cooler. At first blush this seemed to contradict the
great physical law of the conservation of energy, but physicists were
soon agreed that a less revolutionary explanation of the phenomenon is
perfectly tenable. The giving off of heat is indeed only an additional
evidence of the dissipation of energy to which the radio-active atom
is subjected. And no one now believes that radio-activity can persist
indefinitely without actually exhau