XVTI 



SULPHUR AND ITS COMPOUND 



251 



EXPT. 258. Allow these crystals to remain in air for a few 

 days, and examine again. 



They will be now found to have become opaque, and have 

 in fact changed to the ordinary form of sulphur, each crystal 

 becoming converted to a number of minute rhombic crystals. 



Allotropy. We have now formed at least three modifications of 

 sulphur, viz., the rhombic, monoclinic, and plastic sulphur, all 

 consisting solely of sulphur. This property, which some elements 

 possess, of existing in different forms is known as ciUotropy, and 

 the several modifications are known as allotropic forms. A fourth 

 allotropic form of sulphur also exists : a white powder insoluble 

 in carbon disulphide, and which is present in flowers of sulphur. 



Oxides of Sulphur. We have already learnt that sulphur in 

 burning combines with oxygen to form an oxide which we call sul- 



Fio. 128. Preparation of Sulphur Dioxide. 



phur dioxide, and which we know to be a colourless gas with a 

 very pungent smell, soluble in water, forming an acid solution 

 (p. 222). As, however, when so prepared it is always mixed with 

 air or nitrogen, we must obtain it by some other method in order 

 to have it in a purer state. 



