106 JUNIOR GRADE SCIENCE 



Varieties of sulphur. Sulphur is one of the few simple substances 

 which exist in several forms. When such substances have more than 

 one modification, all of them with the same chemical composition 

 but possessed of different physical properties, such as density, colour, 

 crystalline form, and so on, it constitutes an instance of what is called 

 allotropy, and the different varieties of the substance are called allotropic 

 forms. Sulphur, oxygen, carbon and phosphorus all have allotropic 

 forms. Sulphur has four allotropic forms, though it is only necessary 

 here to mention three of them. These are octahedral, prismatic and 

 plastic sulphur. Though the properties of these varieties of sulphur 



are so different, yet all the varieties are 

 composed entirely of sulphur. 



Octahedral sulphur. Ordinary roll- 

 sulphur, or brimstone, is composed of 

 tiny crystals of this variety of sulphur 

 compactly massed together. This can be 

 seen by breaking a roll of sulphur in two 

 and examining the broken ends, when 

 FIG. 82. Crystals of sulphur. crystals will be distinctly visible in the 

 centre of the roll. But much larger 



crystals are obtained by dissolving powdered roll-sulphur in carbon 

 bisulphide and allowing the solution to evaporate slowly into the air, 

 when fairly large, perfectly formed octahedra of sulphur will be 

 obtained (Fig. 82). This kind of sulphur is the most stable form; 

 the other varieties gradually change into octahedral sulphur if left to 

 themselves exposed to the air. 



Prismatic sulphur. The second crystalline variety of sulphur is 

 called prismatic sulphur. It is obtained in the form of clear, needle- 

 shaped crystals by carefully melting powdered roll-sulphur in and 

 evaporating basin and allowing the liquid obtained to cool slowly. 

 As soon as a film of a solid sulphur has formed on the liquid sulphur, 

 two holes are pierced, and the remaining liquid rapidly poured through 

 one of them. If the film be removed and the inside of the basin 

 examined, a number of clear, needle-shaped crystals of prismatic 

 sulphur are seen (Fig. 83). But when the basin is examined again 

 after an interval of a few days the crystals are no longer clear ; they 

 have become opaque owing to the transformation of each needle into 

 a number of minute crystals of octahedral sulphur, which, as has been 

 remarked, is the stable form of sulphur. 



Plastic sulphur. When boiling sulphur, which may be obtained by 

 melting powdered roll-sulphur in a large test-tube, is cooled suddenly 

 by pouring it into cold water, it undergoes a remarkable change. If a 



