t and
can be easily cut with a knife, but this must always be done while the
element is covered with water, since it is extremely inflammable, and
the friction of the knife blade is almost sure to set it on fire if cut
in the air. It is not soluble in water, but is freely soluble in some
other liquids, notably in carbon disulphide. Its density is 1.8.

~Chemical properties.~ Exposed to the air phosphorus slowly combines with
oxygen, and in so doing emits a pale light, or phosphorescence, which
can be seen only in a dark place. The heat of the room may easily raise
the temperature to the kindling point of phosphorus, when it burns with
a sputtering flame, giving off dense fumes of oxide of phosphorus. It
burns with dazzling brilliancy in oxygen, and combines directly with
many other elements, especially with sulphur and the halogens. On
account of its great affinity for oxygen it is always preserved under
water.

Phosphorus is very poisonous, from 0.2 to 0.3 gram being a fatal dose.
Ground up with flour and water or similar substances, it is often used
as a poison for rats and other vermin.

~Precaution.~ The heat of the body is sufficient to raise
phosphorus above its kindling temperature, and for this reason
it should always be handled with forceps and never with the
bare fingers. Burns occasioned by it are very painful and slow
in healing.

~Red phosphorus.~ On standing, yellow phosphorus gradually undergoes a
remarkable change, being converted into a dark red powder which has a
density of 2.1. It no longer takes fire easily, neither does it dissolve
in carbon disulphide. It is not poisonous and, in fact, seems to be an
entirely different substance. The velocity of this change increases with
rise in temperature, and the red phosphorus is therefore prepared by
heating the yellow just below the boiling point (250 deg.-300 deg.). When
distilled and quickly condensed the red form changes back to the yellow.
This is in accordance with the general rule that when a substance
capable of existing in several allotropic forms is condensed from a gas
or crystallized from the liquid state, the more unstable variety forms
first, and this then passes into the more stable forms.

~Matches.~ The chief use of phosphorus is in the manufacture of
matches. Common matches are made by first dipping the match
sticks into some inflammable substance, such as melted
paraffin, and afterward into