a that the stream of projectiles of which they
consisted was a flight of helium atoms. Ramsay and Soddy, confining a
minute bubble of radium emanation in a fine glass tube, were able
to watch the development of the helium spectrum as, day by day, the
emanation decayed. By isolating a very narrow pencil of alpha-rays,
and watching through a microscope their impact on a fluorescent screen,
Rutherford has lately counted the individual alpha-projectiles, and
confirmed his original conclusion that their mass corresponded to that
of helium atoms and their charge to double that on a univalent atom.
("Proc. Roy. Soc." A, page 141, 1908.) Still more recently, he has
collected the alpha-particles shot through an extremely thin wall of
glass, and demonstrated by direct spectroscopic evidence the presence of
helium. ("Phil. Mag." February 1909.)

But the most thorough investigation of a radio-active pedigree is found
in Rutherford's classical researches on the successive disintegration
products of radium, in order to follow the evidence on which his results
are founded, we must describe more fully the process of decay of the
activity of a simple radio-active substance. The decay of activity of
the body known as uranium-X is shown in a falling curve (Fig. 1.). It
will be seen that, in each successive 22 days, the activity falls to
half the value it possessed at the beginning.

This change in a geometrical progression is characteristic of simple
radio-active processes, and can be expressed mathematically by a simple
exponential formula.

As we have said above, solid bodies exposed to the emanations of radium
or thorium become coated with a radio-active deposit. The rate of decay
of this activity depends on the time of exposure to the emanation, and
does not always show the usual simple type of curve. Thus the activity
of a rod exposed to radium emanation for 1 minute decays in accordance
with a curve (Fig. 2) which represents the activity as measured by the
alpha-rays. If the electroscope be screened from the alpha-rays, it is
found that the activity of the rod in beta- an gamma-rays increases for
some 35 minutes and then diminishes (Fig. 3.).

These complicated relations have been explained satisfactorily and
completely by Rutherford on the hypothesis of successive changes of
the radio-active matter into one new body after another. (Rutherford,
"Radio-activity" (2nd edition), Cambridge, 1905, page 379.) The
experimental curve re