_Glass_ is made by mixing sand, or prepared flint, with a certain proportion of soda (200) or potash (205); and exposing these substances, in a furnace, to a violent heat. When they are in a perfectly fluid state, part of the melted matter is taken out at the end of a long hollow tube. This is done by dipping the tube into it, and turning it about until a sufficient quantity is taken up; the workman, at each turn, rolling it gently upon a piece of iron, to unite it more intimately. He then blows through the tube till the melted mass, at the extremity, swells like a bubble; after which he rolls it again on a smooth surface to polish it, and repeats the blowing until the glass is brought as nearly to the size and form of the vessel required, as he thinks necessary. If he be forming a common bottle, the melted matter at the end of the tube is put into a mould of the exact size and shape of the body of a bottle; and the neck is formed by drawing out the ductile glass at the upper extremity. If he be making a vessel with a large or wide orifice, the glass, in its melted state, is opened and widened with an iron tool; after which, being again heated, it is whirled about with a circular motion, and, by the centrifugal force thus produced, is extended to the size required. Should a handle, foot, or any thing of similar kind be required, that is made separately, and stuck on in its melted state. _Window glass_ is made in a similar manner, except that the mass at the end of a tube is formed into a cylindrical shape. This being cut longitudinally by scissars or shears, is gradually bent back until it becomes a flat plate. Large _plate glass_ for looking─glasses is made by suffering the mass, in a state of complete fusion, to flow upon a casting table, with iron ledges. These confine the melted matter, and, as it cools, a metallic roller is passed over it, to reduce it to an uniform thickness. Glass utensils, unless very small and thin, require to be gradually cooled in an oven. This operation is called _annealing_, and is necessary in order to prevent them from cracking by change of temperature, wiping, or slight accidental scratches. It appears that the manufacture of glass was known very early; but glass perfectly transparent was esteemed of extremely high value. It is stated that the Emperor Nero purchased two glass cups with handles for a sum which was equivalent to 50,000_l._ of our money. The windows of some of the houses of the ancient city of Pompeii, which was buried by an eruption of Mount Vesuvius, in the year 79, were glazed, but the glass was thick, and not transparent. By many persons flint is used as a test for ascertaining the purity of silver coins. This is done by rubbing them upon the flint; and if the mark which they leave be not perfectly white, they are rejected as counterfeit.