) were four times more active (activity being
measured by the phenomenon of the ionization of the air) than metallic
uranium. Now, no compound containing any other active metal than
uranium or thorium ought to show itself more active than those metals
themselves, since the property belongs to their atoms. It seemed,
therefore, probable that there existed in pitchblendes some substance
yet unknown, in small quantities and more radioactive than uranium.
M. and Madame Curie then commenced those celebrated experiments which
brought them to the discovery of radium. Their method of research has
been justly compared in originality and importance to the process of
spectrum analysis. To isolate a radioactive substance, the first thing
is to measure the activity of a certain compound suspected of
containing this substance, and this compound is chemically separated.
We then again take in hand all the products obtained, and by measuring
their activity anew, it is ascertained whether the substance sought
for has remained in one of these products, or is divided among them,
and if so, in what proportion.
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