FORMATION OF GLASS.

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manufactured in this capital from 120 parts of white siliceoussand, 40 parts of pearl-ash, 35 of red oxid of lead, 13 of nitrateof potash, and 25 of black oxid of manganese.

This is the most fusible glass. It is used for bottles and otherutensils intended to be cut and polished, and for various orna-mental purposes.

Crown-glass differs from the preceding, in containing no lead.It is manufactured of soda and fine sand. This kind i3 used forpanes of windows, &c.

Bottle-glass is the coarsest of all. It is made of soda andcommon sand. Its green colour is owing to iron. It is the leastfusible.

Glass is often coloured by mixing with it while in a fluid statevarious metallic oxids. It is coloured Hue by the oxid of cobalt;red by the oxid of gold; green by the oxid of copper or iron;yellow by the oxid of silver or antimony; and violet by the oxidof manganese.

The properties of glass are well known. Its hardness is veryconsiderable; its gravity varies from 2.3 to 4, according to thequantity of metallic oxid which entered into its composition.Though glass when cold is brittle, it is one of the most ductilebodies known. If a thread of melted glass be drawn out andfastened to a reel, the whole of the glass can be spun off on thereel, and by cutting the threads at a certain length, there is ob-tained a sort of silver feather of glass. A thread of glass whenred-hot, may be drawn or spun so fine as to be scarcely visible tothe naked eye. It is almost perfectly elastic, and of course isone of the most sonorous bodies. Fluoric acid dissolves it atcommon temperatures, and alcalies at high degrees of heat.These are the only agents known which act upon it.

Glass utensils, unless very small and thin require to be gradu-ally cooled in an oven. This operation is called annealing, andis necessary to prevent their cracking by change of temperature,wiping, or slight accidental scratches.