Another element occurring in allotropic forms is sulphur, of which many forms have been described.
Properties.-Sulphur exists in several allotropic modifications, but before considering these systematically we will deal with the properties of ordinary (or rhombic) sulphur.
Allotropic Modifications.-Sulphur assumes crystalline, amorphous and (possibly) colloidal forms. Historically the most important are the rhombic (Sa) and monoclinic (So) forms, discussed by E.
N"Ecv, to smell) observed in 1785 by Martin van Marum to be formed by the action of a silent electric discharge on the oxygen of the air; he showed it to be an allotropic modification of oxygen, a view subsequently confirmed by Marignac, Andrews and Soret.
The preparation of crystalline boron in 1856 by Wohler and Sainte Claire Deville showed that this element also existed in allotropic forms, amorphous boron having been obtained simultaneously and independently in 1809 by Gay Lussac and Davy.