THE TECHNOLOGIST. [June 1, 1864. 



502 ON GRANITE AND ITS USES. 



In like manner, sulphur may be procured in large, brilliant, beauti- 

 fully transparent crystals ; and also by beating to a certain point and 

 suddenly cooling, as a glass which long retains plasticity and pliancy ; 

 and, further, by protracted heating and subsequent irregular cooling, 

 as an uncrystalline, unvitreous mass. 



The gums, resins, inspissated balsams, and other exudations from 

 trees, along with amber, oscillate as it were between the glassy and 

 amorphous conditions, being generally glassy, sometimes amorphous, 

 and, most rarely of all, crystalline. No substance, however, exhibits the 

 contrast between at least the vitreous and the crystalline condition, and 

 the ready transition from the one to the other, better than sugar. Sugar, 

 as it occurs in brown sugar, or in a sugar-loaf, or, still better, in sugar- 

 candy, is one of the most perfectly crystallized bodies. Keep that 

 sugar for some time melted, and it changes into a- glass, and hardens as 

 such. If you give a piece of it to a child as a plaything, and tell it that 

 it is glass, sugar-glass, or vitreous sugar, it will smile at you, and tell 

 you it is not glass, but barley-sugar ; and so it is, but none the less 

 glass ! It has all the essential properties of glass. 



Lastly, the great ingredient of household glass, silica, can easily 

 assume the crystalline, the vitreous, or the amorphous condition ; and 

 as it transfers this property to all the kinds of glass containing it, the 

 glass-maker is often hard put to it, to keep the happy medium between 

 amorphous shapelessness and crystalline symmetry. Thus, the most 

 beautiful pyramid of colourless quartz, of purple amethyst, or yellow 

 cairngorm, may be uncrystallised and changed into glass simply by being 

 melted. It is true that no ordinary fire, or even seven times heated 

 furnace, will melt such crystals, nor any artificial heat easily procurable, 

 except that produced by the burning together of the elements of water, 

 hydrogen, and oxygen; but under this heat the hardest crystals of quartz 

 melt into glass. 



Electricity also can furnish a heat sufficient to effect this result, and 

 it is seen on the grandest scale when thunder-storms send their dis- 

 charges into beds of pure sand, and the white hot lightning melts its 

 grains into glass. Tubes thus made in a moment out of lightning- 

 melted sand, may be seen in the British Museum and Jermyn street 

 Museum, and are justly reckoned objects of great interest and value. 



And if it be possible to uncrystallise rock crystal into glass, it is 

 still more easy to change both the rock crystal and its glass into 

 amorphous, structureless silica. To do this it is only necessary to expose 

 either to the vapour of the corroding hydrofluoric acid which fluorspar 

 gives out when wetted with strong sulphuric acid. The crystal or the 

 glass equally changes into a pulpy, gelatinous, starchy mass, which dries 

 up like gum and hardens like glue. Chalcedony, common and precious 

 opal, perhaps flint, jasper, and agate, are examples of this. 



The glass-maker has thus a difficult task. Phosphorus, or sulphur, 

 or barley-sugar, will scarcely do as the materials of window-panes and 



