alt is added to a solution of a sulphate or
sulphuric acid:

BaCl_{2} + H_{2}SO_{4} = BaSO_{4} + 2HCl.

This precipitate is used, as are also the finely ground native sulphate
and carbonate, as a pigment in paints. On account of its low cost it is
sometimes used as an adulterant of white lead, which is also a heavy
white substance.

Barium compounds color the flame green, and the nitrate (Ba(NO_{3})_{2})
is used in the manufacture of green lights. Soluble barium compounds are
poisonous.

RADIUM

~Historical.~ In 1896 the French scientist Becquerel observed that the
mineral pitchblende possesses certain remarkable properties. It affects
photographic plates even in complete darkness, and discharges a
gold-leaf electroscope when brought close to it. In 1898 Madam Curie
made a careful study of pitchblende to see if these properties belong to
it or to some unknown substance contained in it. She succeeded in
extracting from it a very small quantity of a substance containing a new
element which she named radium.

In 1910 Madam Curie succeeded in obtaining radium itself by the
electrolysis of radium chloride. It is a silver-white metal melting at
about 700 deg.. It blackens in the air, forming a nitride, and decomposes
water. Its atomic weight is about 226.5.

~Properties.~ Compounds of radium affect a photographic plate or
electroscope even through layers of paper or sheets of metal. They also
bring about chemical changes in substances placed near them.
Investigation of these strange properties has suggested that the radium
atoms are unstable and undergo a decomposition. As a result of this
decomposition very minute bodies, to which the name corpuscles has been
given, are projected from the radium atom with exceedingly great
velocity. It is to these corpuscles that the strange properties of
radium are due. It seems probable that the gas helium is in some way
formed during the decomposition of radium.

Two or three other elements, particularly uranium and thorium, have been
found to possess many of the properties of radium in smaller degree.

~Radium and the atomic theory.~ If these views in regard to radium should
prove to be well founded, it will be necessary to modify in some
respects the conception of the atom as developed in a former chapter.
The atom would have to be regarded as a compound unit made up of several
parts. In a few cases, as in radium and uranium, it would appear that
this unit is unstable