Allotropy. 29 



sesses a considerable degree of transparency, and a fair amount 

 of tenacity and freedom from brittleness. It can be readily cut 

 or trimmed with a knife, having very much the consistence of 

 a hard horny cheese. Advantage is taken of this by the 

 makers of plaster medallions and the copiers of old coins and 

 medals. They moisten a plaster medallion, or grease slightly 

 the surface of a medal, and then, securing a paper rim around 

 it, pour on melted sulphur. This solidifies into the transparent 

 prismatic variety, and may be cut and trimmed into shape, 

 serving as a mould in which new plaster copies may be cast. 

 After a few days, however, the sulphur resumes its octohedral 

 brittle form, and the attempt to use it as a mould when in this 

 condition generally results in its being defaced in consequence 

 of its extreme brittleness. 



Other peculiar aliotropic forms of sulphur are produced by 

 melting it at different temperatures. 



When heated to a degree not exceeding 120 Cent., sulphur 

 forms an exceedingly limpid mobile liquid, that possesses the 

 property of taking sharper casts than any other substance, 

 hence its employment as previously mentioned. 



If it is heated to a higher temperature, it becomes gradu- 

 ally darker and extremely thick and viscid, so that the flask in 

 which it is being melted may be inverted without its running 

 out. The greatest degree of thickness and viscidity is attained 

 at a heat of about 250° Cent. If it be heated to a higher degree 

 it becomes more liquid again, though never to the same extent as 

 when at a lower temperature. If in this highly heated state it 

 is poured in a thin stream into water, sulphur assumes the extra- 

 ordinary form of a rich amber brown transparent substance, 

 possessed of a very high degree of elasticity and capable of be- 

 ing drawn out into threads. In this extraordinary condition it 

 is quite insoluble in bisulphide of carbon and other menstrua 

 that so freely dissolve octohedral or common sulphur. 



In the course of a few hours, however, it returns to the 

 common brittle condition, the change being* accompanied by 

 the evolution of heat, and, what is very remarkable, this change 

 may be instantly brought about by placing the elastic sulphur 

 in boiling water. 



In the elastic state, sulphur is evidently in the vitreous or 

 glassy form. This form is dependent on the fact that the sul- 

 phur has united with a proportion of heat, which has become 

 latent in effecting this change. 



A very good illustration of the vitreous condition assumed 

 by some aliotropic substances exists in barley sugar. This is 

 formed by boiling sugar with the smallest possible quantity of 

 water capable of dissolving it when aided by heat; as thus 

 formed it is, whilst heated, a soft vitreous substance, capable of 



