~Physical properties.~ Roll sulphur is a pale yellow, crystalline solid, without marked taste and with but a faint odor. It is insoluble in water, but is freely soluble in a few liquids, notably in carbon disulphide. Roll sulphur melts at 114.8 deg.. Just above the melting point it forms a rather thin, straw-colored liquid. As the temperature is raised, this liquid turns darker in color and becomes thicker, until at about 235 deg. it is almost black and is so thick that the vessel containing it can be inverted without danger of the liquid running out. At higher temperatures it becomes thin once more, and boils at 448 deg., forming a yellowish vapor. On cooling the same changes take place in reverse order. ~Varieties of sulphur.~ Sulphur is known in two general forms, crystalline and amorphous. Each of these forms exists in definite modifications. ~Crystalline sulphur.~ Sulphur occurs in two crystalline forms, namely, rhombic sulphur and monoclinic sulphur. 1. _Rhombic sulphur._ When sulphur crystallizes from its solution in carbon disulphide it separates in crystals which have the same color and melting point as roll sulphur, and are rhombic in shape. Roll sulphur is made up of minute rhombic crystals. 2. _Monoclinic sulphur._ When melted sulphur is allowed to cool until a part of the liquid has solidified, and the remaining liquid is then poured off, it is found that the solid sulphur remaining in the vessel has assumed the form of fine needle-shaped crystals. These differ much in appearance from the rhombic crystals obtained 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]