r properties and the secrecy which was maintained about
its preparation, it remained a very rare and costly substance until the
demand for it in the manufacture of matches brought about its production
on a large scale.

~Occurrence.~ Owing to its great chemical activity phosphorus never occurs
free in nature. In the form of phosphates it is very abundant and widely
distributed. _Phosphorite_ and _sombrerite_ are mineral forms of calcium
phosphate, while _apatite_ consists of calcium phosphate together with
calcium fluoride or chloride. These minerals form very large deposits
and are extensively mined for use as fertilizers. Calcium phosphate is a
constituent of all fertile soil, having been supplied to the soil by the
disintegration of rocks containing it. It is the chief mineral
constituent of bones of animals, and bone ash is therefore nearly pure
calcium phosphate.

~Preparation.~ Phosphorus is now manufactured from bone ash or a pure
mineral phosphate by heating the phosphate with sand and carbon in an
electric furnace. The materials are fed in at M (Fig. 70) by the feed
screw F. The phosphorus vapor escapes at P and is condensed under
water, while the calcium silicate is tapped off as a liquid at S. The
phosphorus obtained in this way is quite impure, and is purified by
distillation.

[Illustration: Fig. 70]

~Explanation of the reaction.~ To understand the reaction which
occurs, it must be remembered that a volatile acid anhydride is
expelled from its salts when heated with an anhydride which is
not volatile. Thus, when sodium carbonate and silicon dioxide
are heated together the following reaction takes place:

Na_{2}CO_{3} + SiO_{2} = Na_{2}SiO_{3} + CO_{2}.

Silicon dioxide is a less volatile anhydride than phosphoric
anhydride (P_{2}O_{5}), and when strongly heated with a
phosphate the phosphoric anhydride is driven out, thus:

Ca_{3}(PO_{4})_{2} + 3SiO_{2} = 3CaSiO_{3} + P_{2}O_{5}.

If carbon is added before the heat is applied, the P_{2}O_{5}
is reduced to phosphorus at the same time, according to the
equation

P_{2}O_{5} + 5C = 2P + 5CO.

~Physical properties.~ The purified phosphorus is a pale yellowish,
translucent, waxy solid which melts at 43.3 deg. and boils at 269 deg.. It can
therefore be cast into any convenient form under warm water, and is
usually sold in the market in the form of sticks. It is quite sof