gaseous form hydrochloric acid, sulphur dioxide, and
ammonia]

The evolution of the oxygen begins at about 400 deg.. It has been found,
however, that if the potassium chlorate is mixed with about one fourth
its weight of manganese dioxide, the oxygen is given off at a much lower
temperature. Just how the manganese dioxide brings about this result is
not definitely known. The amount of oxygen obtained from a given weight
of potassium chlorate is exactly the same whether the manganese dioxide
is present or not. So far as can be detected the manganese dioxide
undergoes no change.

[Illustration: Fig. 4]

~Directions for preparing oxygen.~ The manner of preparing oxygen from
potassium chlorate is illustrated in the accompanying diagram (Fig. 4).
A mixture consisting of one part of manganese dioxide and four parts of
potassium chlorate is placed in the flask A and gently heated. The
oxygen is evolved and escapes through the tube B. It is collected by
bringing over the end of the tube the mouth of a bottle completely
filled with water and inverted in a vessel of water, as shown in the
figure. The gas rises in the bottle and displaces the water. In the
preparation of large quantities of oxygen, a copper retort (Fig. 5) is
often substituted for the glass flask.

[Illustration: Fig. 5]

In the preparation of oxygen from potassium chlorate and manganese
dioxide, the materials used must be pure, otherwise a violent explosion
may occur. The purity of the materials is tested by heating a small
amount of the mixture in a test tube.

~The collection of gases.~ The method used for collecting oxygen
illustrates the general method used for collecting such gases as are
insoluble in water or nearly so. The vessel C (Fig. 4), containing the
water in which the bottles are inverted, is called a _pneumatic trough._

~Commercial methods of preparation.~ Oxygen can now be purchased stored
under great pressure in strong steel cylinders (Fig. 6). It is prepared
either by heating a mixture of potassium chlorate and manganese dioxide,
or by separating it from the nitrogen and other gases with which it is
mixed in the atmosphere. The methods employed for effecting this
separation will be described in subsequent chapters.

[Illustration: Fig. 6]

~Physical properties.~ Oxygen is a colorless, odorless, tasteless gas,
slightly heavier than air. One liter of it, measured at a temperature of
0 deg. and under a pressure of one atmosphere, weigh