138 PRACTICAL LESSONS IN SCIENCE. . 



so does sulphur. Nearly all the compounds of sulphur as 

 well as most other substances of the mineral world have defi- 

 nite crystalline forms, and so constant and definite is this 

 form that it is one of the marks for the identification of min- 

 erals. The natural crystallization of sulphur is the octahedra, 

 and it takes this form when crystallizing from solutions, but 

 when crystallizing from the melted state it takes the form of 

 oblique prisms. The difference between these two crystalline 

 forms of sulphur extends to their specific gravities and the tem- 

 peratures of their melting points. Ordinary rolled sulphur con- 

 tains many prismatic crystals when first made which gradually 

 change to octahedra. This change in structure of the solid gives 

 rise to a state of tension which accounts for the extreme brittle- 

 ness of sulphur. 



Sulphur is one of the most interesting of the non-metallic ele- 

 ments, and it may serve as the basis of many interesting and 

 instructive experiments . A large part of the sulphur of commerce 

 comes from Sicily. As the melting point and boiling point of 

 sulphur are low it is comparatively easy to separate it from im- 

 purities by the aid of heat. Large quantities of sulphur are also 

 obtained by decomposing iron pyrites or bisulphide of iron. 

 Sulphur is used in the manufacture of gunpowder and matches. 



Sulphur unites with oxygen, forming many interesting com- 

 pounds. When sulphur is burned in the air, sulphur dioxide, 

 formula S0 2 , is formed with the evolution of light and heat. It 

 occurs among the products of volcanic action, is a transparent 

 colorless gas, having a pungent suffocating odor, such as arises 

 from a burning match. It may be prepared for examination 

 by treating copper with sulphuric acid. It is readily absorbed 

 by water but may be collected by displacement, as in the case of 

 chlorine or carbon dioxide, as it is heavier than air. It may 

 be condensed to a liquid by cold or pressure, and is a solid at 

 105 F. Sulphur dioxide S0 2 -hwater, H 2 forms sulphurous 

 acid, H 2 S0 3 . When neutralized by a base this acid forms a class 

 of salts called sulphites, as the sulphite of soda. 



Sulphur dioxide is extensively used as a bleaching agent. Al- 



