free from impurities. Glass-makers differ in their recipes for colour, of which there exists a great variety. The above will be sufficient to give a general idea of the principle of colouring.

Suitable cullet, or broken glass, may be mixed with the batch of the above glasses, in quantities at discretion.

The base of all Glasses is sand; and suitable alkali is the chief solvent. In the Compound Glasses, oxide of lead, whether in the form of litharge or red lead, as well as the colouring metals, are active fluxes. These Glasses, therefore, require less alkali, in proportion to the sand, than the Simple. Flint Glass, which ought rather to be termed metallic Glass, needs, for its fusion, less caloric than plate Glass, or any of the Simple Glasses. As the Simple Glasses require intense heat, they are melted in open pots, so that the flame comes at once in contact with the materials. Flint and the Compound Glasses must be melted by the heat going through the pot, which requires to be covered, or hooded; or the fumes and smoke of the coal would carbonize or deoxydize the lead, and precipitate it to the bottom, in its original metallic state. Dried beech or oak wood is used for fuel in some parts of France for Flint Glass, with open pots; and so little carbon is produced by the smoke, as not to affect materially the metal, although the flames play upon its surface. Formerly, flints were calcined and ground, for Glass-making; but for many years past, Isle of Wight, Lynn, or Reigate, sand have been substituted; these sands are not only more free from iron, but less expensive in the preparation, than flints when washed and calcined.

Some manufacturers cleanse sand by hand-washing, and others have machines: to render the Isle of Wight sand fit for