a paste consisting of (1)
phosphorus, (2) some oxidizing substance, such as manganese
dioxide or potassium chlorate, and (3) a binding material,
usually some kind of glue. On friction the phosphorus is
ignited, the combustion being sustained by the oxidizing agent
and communicated to the wood by the burning paraffin. In
sulphur matches the paraffin is replaced by sulphur.

In safety matches _red_ phosphorus, an oxidizing agent, and
some gritty material such as emery is placed on the side of the
box, while the match tip is provided as before with an
oxidizing agent and an easily oxidized substance, usually
antimony sulphide. The match cannot be ignited easily by
friction, save on the prepared surface.

~Compounds of phosphorus with hydrogen.~ Phosphorus forms several
compounds with hydrogen, the best known of which is phosphine (PH_{3})
analogous to ammonia (NH_{3}).

~Preparation of phosphine.~ Phosphine is usually made by heating
phosphorus with a strong solution of potassium hydroxide, the reaction
being a complicated one.

[Illustration: Fig. 71]

The experiment can be conveniently made in the apparatus shown
in Fig. 71. A strong solution of potassium hydroxide together
with several small bits of phosphorus are placed in the flask
A, and a current of coal gas is passed into the flask through
the tube B until all the air has been displaced. The gas is
then turned off and the flask is heated. Phosphine is formed in
small quantities and escapes through the delivery tube, the
exit of which is just covered by the water in the vessel C.
Each bubble of the gas as it escapes into the air takes fire,
and the product of combustion (P_{2}O_{5}) forms beautiful
small rings, which float unbroken for a considerable time in
quiet air. The pure phosphine does not take fire spontaneously.
When prepared as directed above, impurities are present which
impart this property.

~Properties.~ Phosphine is a gas of unpleasant odor and is exceedingly
poisonous. Like ammonia it forms salts with the halogen acids. Thus we
have phosphonium chloride (PH_{4}Cl) analogous to ammonium chloride
(NH_{4}Cl). The phosphonium salts are of but little importance.

~Oxides of phosphorus.~ Phosphorus forms two well-known oxides,--the
trioxide (P_{2}O_{3}) and the pentoxide (P_{2}O_{5}), sometimes called
phosphoric anhyd