SULPHUR AND HYDROGEN SULPHIDE 251 



is very viscous, so that, as its quantity increases, the whole liquid 

 becomes thick. At 120 molten sulphur is a limpid fluid, at 260 

 a vessel containing it can be inverted without loss of material. 



Amorphous sulphur is a super-cooled liquid, and not a true solid, 

 for true solids are all crystalline (see p. 94). At room tempera- 

 ture it changes into rhombic sulphur, but so slowly that the 

 transformation even of a small part of it can be detected (by 

 treating with carbon disulphide) only after the lapse of many 

 months. At 100 the change is complete in less than an 

 hour (compare p. 242). 



Elastic sulphur. When melted sulphur is chilled, the 

 amorphous sulphur does not at once become hard. Sulphur 

 which has been heated to a high temperature, therefore, and then 

 suddenly cooled, consists at first of a sticky, transparent, elastic 

 material, called elastic or plastic sulphur. In the course of forty- 

 eight hours, however, this becomes opaque and hard, because of 

 the separation of the crystalline and the hardening of the amor- 

 phous varieties. 



*. 



Melting and Freezing -Points. Amorphous sulphur, like 

 glass and other amorphous substances, softens when heated, but 

 has no sharp melting temperature. The two crystalline forms 

 have different melting-points, rhombic melting to form S\ at 

 112.8, and monoclinic at 119.25. But these are difficult to ob- 

 serve, as the rhombic begins to turn into monoclinic above 96.5, 

 and gradual transformation of S\ to S M , to produce an equilibrium 

 mixture of the two, occurs in both cases in the liquid state. Hence, 

 the only temperature which is easy to observe is that at which 

 both the solid forms melt when heated very slowly, and that at 

 which the liquid freezes if cooled very slowly, namely 114.5. 

 This is the so-called natural freezing-point of sulphur. 



Chemical Properties. The vapor density of sulphur indi- 

 cates that the vapor is a mixture of the molecules S 8 , S 6 and 82, 



