s will suffice to recall
as much as is needed for the purpose of this chapter. In their study of
the emanations from uranium compounds the Curies were led to isolate
the various elements of the compounds until they discovered that the
discharge was predominantly due to one specific element, radium. Radium
is itself probably a product of the disintegration of uranium, the
heaviest of known metals, with an atomic weight some 240 times greater
than that of hydrogen. But this massive atom of uranium has a life that
is computed in thousands of millions of years. It is in radium and its
offspring that we see most clearly the constitution of matter.

A gramme (less than 15 1/2 grains) of radium contains--we will economise
our space--4x10 (superscript)21 atoms. This tiny mass is, by its
discharge, parting with its substance at the rate of one atom per second
for every 10,000,000,000 atoms; in other words, the "indestructible"
atom has, in this case, a term of life not exceeding 2500 years. In the
discharge from the radium three elements have been distinguished. The
first consists of atoms of the gas helium, which are hurled off at
between 10,000 and 20,000 miles a second. The third element (in the
order of classification) consists of waves analogous to the Rontgen
rays. But the second element is a stream of electrons, which are
expelled from the atom at the appalling speed of about 100,000 miles
a second. Professor Le Bon has calculated that it would take 340,000
barrels of powder to discharge a bullet at that speed. But we shall see
more presently of the enormous energy displayed within the little system
of the atom. We may add that after its first transformation the radium
passes, much more quickly, through a further series of changes. The
frontiers of the atomic systems were breaking down.

The next step was for students (notably Soddy and Rutherford) to find
that radio-activity, or spontaneous discharge out of the atomic systems,
was not confined to radium. Not only are other rare metals conspicuously
active, but it is found that such familiar surfaces as damp cellars,
rain, snow, etc., emit a lesser discharge. The value of the new
material thus provided for the student of physics may be shown by
one illustration. Sir J. J. Thomson observes that before these recent
discoveries the investigator could not detect a gas unless about a
billion molecules of it were present, and it must be remembered that the
spectroscope had alr