oth the analysis and
synthesis of matter have been revealed through the discoveries of
radio-activity.

Discovery of Helium

It is of interest at this point to learn something of the history of
helium and its occurrence. In 1868 there was discovered by Janssen and
Lockyer a bright yellow line in the spectrum of the sun's
chromosphere. Because of its origin the name helium was given to the
supposed new element causing it. Later it was found in the spectra of
many of the stars, and because of its predominance in some of these
they were called helium stars. Its existence on our planet was not
detected for nearly thirty years.

In 1895, in connection with the discovery of argon in the atmosphere,
a search was made to see if the latter element could be obtained from
mineral sources. In analyzing certain uranium minerals Hillebrand had
found considerable quantities of a gas which he took to be a peculiar
form of nitrogen. Ramsay made a further examination of the gas coming
from these minerals and the spectroscope revealed the yellow line of
helium, thus at last proving the presence of this element on the
earth. It is known now to be present in thorium minerals, in the
waters of radio-active wells, and in minute amounts in the atmosphere.
Its occurrence in every case, in the light of the experiment described
above, would seem to be due to the presence of radio-active changes.

Characteristics of Helium

Helium, on account of its chemical inactivity and physical properties,
is classed along with argon, neon, krypton, and xenon in the zero
group of the Periodic System, and forms with them the monatomic, inert
gases. In this class are now placed also the three radio-active gases,
emanating respectively from radium, thorium, and actinium. These are
generally known as radium emanation, thorium emanation, and actinium
emanation. The first mentioned was once called niton. Emanium was the
name originally proposed by Giesel for the body now known as actinium.

The calculated rate of production of helium in the series in
equilibrium with one gram of radium is 158 cubic millimeters per year.
This corresponds quite well with the experimental results.

Table of Constants

Some of the more important atomic and radio-active constants are given
in the following table. They are recorded here to show how helpful the
study of radio-activity has been in working out the composition of
matter, and to give some idea of the magnitude