by crystallizing
sulphur from its solution in carbon disulphide. The needle-shaped form
is called _monoclinic sulphur_. The two varieties differ also in density
and in melting point, the monoclinic sulphur melting at 120 deg..

Monoclinic and rhombic sulphur remain unchanged in contact with each
other at 96 deg.. Above this temperature the rhombic changes into
monoclinic; at lower temperatures the monoclinic changes into rhombic.
The temperature 96 deg. is therefore called the transition point of sulphur.
Heat is set free when monoclinic sulphur changes into rhombic.

~Amorphous sulphur.~ Two varieties of amorphous sulphur can be readily
obtained. These are white sulphur and plastic sulphur.

1. _White sulphur._ Flowers of sulphur, the preparation of which has
been described, consists of a mixture of rhombic crystals and amorphous
particles. When treated with carbon disulphide, the crystals dissolve,
leaving the amorphous particles as a white residue.

2. _Plastic sulphur._ When boiling sulphur is poured into cold water it
assumes a gummy, doughlike form, which is quite elastic. This can be
seen in a very striking manner by distilling sulphur from a small,
short-necked retort, such as is represented in Fig. 40, and allowing the
liquid to run directly into water. In a few days it becomes quite
brittle and passes over into ordinary rhombic sulphur.

[Illustration Fig. 40]

~Chemical properties of sulphur.~ When sulphur is heated to its kindling
temperature in oxygen or in the air it burns with a pale blue flame,
forming sulphur dioxide (SO_{2}). Small quantities of sulphur trioxide
(SO_{3}) may also be formed in the combustion of sulphur. Most metals
when heated with sulphur combine directly with it, forming metallic
sulphides. In some cases the action is so energetic that the mass
becomes incandescent, as has been seen in the case of iron uniting with
sulphur. This property recalls the action of oxygen upon metals, and in
general the metals which combine readily with oxygen are apt to combine
quite readily with sulphur.

~Uses of sulphur.~ Large quantities of sulphur are used as a germicide in
vineyards, also in the manufacture of gunpowder, matches, vulcanized
rubber, and sulphuric acid.

COMPOUNDS OF SULPHUR WITH HYDROGEN

~Hydrosulphuric acid~ (H_{2}S). This substance is a gas having the
composition expressed by the formula H_{2}S and is commonly called
hydrogen sulphide. It is found in the vapors issuing