though oxygen occurs in the free state in the atmosphere,
its separation from the nitrogen and other gases with which it is mixed
is such a difficult matter that in the laboratory it has been found more
convenient to prepare it from its compounds. The most important of the
laboratory methods are the following:

1. _Preparation from water._ Water is a compound, consisting of 11.18%
hydrogen and 88.82% oxygen. It is easily separated into these
constituents by passing an electric current through it under suitable
conditions. The process will be described in the chapter on water. While
this method of preparation is a simple one, it is not economical.

2. _Preparation from mercuric oxide._ This method is of interest, since
it is the one which led to the discovery of oxygen. The oxide, which
consists of 7.4% oxygen and 92.6% mercury, is placed in a small, glass
test tube and heated. The compound is in this way decomposed into
mercury which collects on the sides of the glass tube, forming a silvery
mirror, and oxygen which, being a gas, escapes from the tube. The
presence of the oxygen is shown by lighting the end of a splint,
extinguishing the flame and bringing the glowing coal into the mouth of
the tube. The oxygen causes the glowing coal to burst into a flame.

In a similar way oxygen may be obtained from its compounds with
some of the other elements. Thus manganese dioxide, a black
compound of manganese and oxygen, when heated to about 700 deg.,
loses one third of its oxygen, while barium dioxide, when
heated, loses one half of its oxygen.

3. _Preparation from potassium chlorate (usual laboratory method)._
Potassium chlorate is a white solid which consists of 31.9% potassium,
28.9% chlorine, and 39.2% oxygen. When heated it undergoes a series of
changes in which all the oxygen is finally set free, leaving a compound
of potassium and chlorine called potassium chloride. The change may be
represented as follows:

/potassium\
| | (potassium / potassium \ (potassium
{ chlorine } = { } + oxygen
| | chlorate) \ chlorine / chloride)
\oxygen /

[Illustration: JOSEPH PRIESTLEY (English) (1733-1804)

School-teacher, theologian, philosopher, scientist; friend of Benjamin
Franklin; discoverer of oxygen; defender of the phlogiston theory; the
first to use mercury in a pneumatic trough, by which means he first
isolated in