250 SMITH'S INTERMEDIATE CHEMISTRY 



slowly to cool. As the temperature is now above 96, the crystals 

 which grow in the liquid are of the monoclinic variety. They are 

 long, transparent, pale-yellow needles (Fig. 67), almost rectangular 

 in section, and bevelled at the points. The specific gravity is 1.96. 

 This form can be kept indefinitely above 96, but, when allowed 

 to cool below that temperature, it slowly becomes opaque, chang- 

 ing into particles of rhombic sulphur. 



The temperature at which a substance 

 changes its crystalline form is called a transition 

 point. It is analogous to the fusion point in the 

 case of a solid and a liquid; only at this one point 

 can both forms exist together in equilibrium. 



_ The Two Liquid Forms: Amorphous Sul~ 



phur. When sulphur is melted, and the liquid 

 is heated, two fluid, mutually soluble forms of sulphur are pro- 

 duced. These are known as S\ and S M or amorphous sulphur. 

 As the temperature rises, the second variety increases in quantity 

 at the expense of the first variety. When the temperature is 

 lowered, the reverse change occurs: 



S x + S M (amorphous). 



If the temperature is lowered gradually, therefore, only mono- 

 clinic sulphur (by crystallization of the S\) is obtained, the reac- 

 tion proceeding to completion in the reverse direction owing 

 to the removal of S\ (compare p. 239). But the change from S M 

 to S\ takes place only very slowly, except at temperatures near 

 the boiling-point. Consequently, if the liquid is quickly chilled, 

 by pouring into a cold vessel or into cold water, the S M is found 

 as a non-crystalline substance mixed with the crystalline form. 

 The crystalline form can be dissolved out with carbon disulphide, 

 leaving the amorphous sulphur which is not soluble. The propor- 

 tion of S M varies from 3.6 per cent at 120, to 11 per cent at 160 

 and about 34 per cent at 445 (the boiling-point of sulphur). S M 



