It is uncertain whether glass ought to be considered a chemical combination, or a mere mechanical mixture of the different silicates. A circumstance highly favourable to the view of its being a definite chemical compound, is, that the nearer the materials for the glass are compounded in atomic proportions, the better, in general, is the quality of the glass, other circumstances of the manufacture being equal. If an excess of the alkali is introduced, a portion of the excess volatilizes when the mixture is fused, and the resulting glass affords to boiling water a small quantity of the alkaline silicate, much more readily than it would had the materials been compounded in proportions rigorously atomic. The power of glass to resist the continued action of boiling water and acid and alkaline liquids may be ascertained by boiling concentrated sulphuric acid upon the glass: if good, the surface remains smooth and transparent; but if bad, it becomes rough. The only acid which acts upon glass with energy is the hydrofluoric.

If a moderately thick glass vessel is cooled to the ordinary temperature soon after it is formed, the brittleness of the glass is increased to such an extent that a small change of temperature or the merest scratch is apt to cause the vessel to fly to pieces. The two philosophical toys known as the Bologna phial and Prince Rupert's drops, and the "tests" or "proofs" which may be obtained at the flint-glass works, are well adapted for illustrating the properties of glass in this peculiar condition. The first of these is a common-shaped phial with a thick bottom and thin sides, which has been cooled to the ordinary

temperature immediately after being blown. If a small piece of any hard and angular mineral capable of abrading glass, such as a grain of sand, or what is better, a fragment of a gun-flint, is dropped into the phial, the latter immediately falls to pieces, the fracture commencing from the part abraded. The phial, however, will sustain a heavy blow with a wooden mallet, or the concussion caused by the fall of a leaden bullet, without injury. Prince Rupert's drops are small, solid, pear-shaped pieces of quickly cooled glass with an elongated stem, made by allowing the red-hot glass to drop into cold water. The spherical part of the drop will bear without injury a heavy blow from an instrument the surface of which is not sufficiently hard to scratch the glass; but if the minutest portion of the stem or tail is broken off, the whole drop instantly falls into a multitude of small fragments; and if the drop is held by the fingers, a slight stinging sensation is perceived from the sudden rupture.

These remarkable properties of quickly cooled glass are not difficult of explanation; they are produced merely by the unequal contraction of different parts of the same vessel through an unequal rate of cooling. Owing to the imperfect conducting power of glass, the surface of the vessel in contact with the atmosphere becomes cooled and solidified while the interior substance remains in the soft and expanded state. When quite cold, the glass is therefore in a state of tension, the relative positions of the exterior and interior portions not being exactly the same as if the whole vessel had been allowed to contract equally by a very gradual cooling. In this state of tension,

which has been compared to that of a piece of cloth stretched unequally in different parts, the slightest scratch on the surface is sufficient to derange the equilibrium; a fracture consequently ensues, which, from the elastic quality of the glass, proceeds through the entire thickness of the vessel.

The glass may obviously be prevented from acquiring these properties by subjecting it to a slow refrigeration, commonly called" annealing," so that the contraction of the interior and exterior portions may be as equable as possible. The disposition of badly annealed glass vessels to fly to pieces may even be completely corrected long after the vessel is become quite cold, by heating it slowly to a temperature about that of boiling water and then allowing it to cool very gradually. If heated to redness and slowly cooled, the Bologna phial and Prince Rupert's drops may be entirely deprived of their extreme susceptibility of breaking. The glass experiences an increase in density by this treatment, which shews that an approximation of the particles takes place.

If glass is maintained for a long time, however, at a high degree of heat, but lower than that requisite for its fusion, it is apt to suffer another change in its molecular structure; becoming opaque or translucent, fibrous, considerably harder, more difficult of fusion, a conductor of electricity, and a better conductor of heat. After being thus treated, the glass is so hard as to cut common glass with facility, and to give sparks when struck with steel; it is incapable of becoming electrically excited by friction, unless insulated; and if the heating has been con

tinued a long time, the glass is enabled to sustain without breaking a sudden cooling from a red heat to the ordinary temperature, by being plunged into cold water. Several interesting observations on the properties and uses of glass in this devitrified state were made by the French naturalist, M. Reaumur, and communicated to the Academy of Sciences in 1739. The opaque glass is commonly known as

"Reaumur's porcelain."

All the varieties of true glass are probably subject to this change in their molecular structure, but some kinds are much more susceptible of it than others. The glass which acquires it with the greatest facility is the common green bottle-glass, which differs from the other varieties in containing a greater number of bases. It was formerly believed that the change produced in the glass is owing to the chemical action of the powdered gypsum or sand with which it is necessary to surround the vessel in order to preserve its shape, the powder being supposed to act by a process like that distinguished by the name of cementation. But the devitrification of the glass proceeds as regularly forward in the absence of gypsum, sand, or any other cementing substance; it must be effected, therefore, by the action of heat alone. The glass sometimes experiences, however, a slight change in its composition, during the prolonged heating, through the volatilization of a trace of its alkali; but the loss of alkali is by no means an essential part of the process of devitrification, as is believed by some. The bases in the glass as

sume a new molecular arrangement; and the silicates, instead of continuing in a vitreous state, become crys

tallized and opaque. The higher the temperature to which the glass is exposed, provided it is below the point at which fusion takes place, the more rapid is the change. At a full red heat, a piece of a common wine-bottle has been completely devitrified in two hours. The toughness of the altered glass, its difficult fusibility, and its power of withstanding the action of acid liquids and of resisting sudden alternations of temperature, would probably render vessels of this material of great service in the laboratory as a substitute for those of ordinary glass, porcelain, and even in some cases, for those of platinum.

If the devitrified glass is exposed to a very intense heat, it fuses and reassumes its original vitreous characters, which it retains permanently if cooled in the ordinary manner. The farther the process of devitrification had been carried, the higher is the temperature necessary for fusion.

III. ON THE MATERIALS USED IN THE MANUFACTURE OF GLASS.

The quality of glass depends in a great measure on the judgment with which the ingredients from which it is prepared are selected, and on the proper application of certain correctives for the impurities that may be present. The only essential constituents of all the common kinds of glass are silicic acid, potash, soda, lime, oxide of lead, alumina, and oxide of iron; but some of these may be introduced in several different forms, in his choice of which, the manufacturer is guided in a great measure by the relative