Glass possesses the three first quali ties in an eminent degree, and may be heated red-hot without melting: Its greatest inconvenience is its disposition to crack or break in pieces when sudden ly heated or cooled. As this is occasion ed by its unequal expansion or contrac tion, it is best remedied by forming the vessels very thin, and giving them, in ge neral, a rounded shape. Glass vessels should also be well annealed, that is, cooled very slowly, after being blown, by placing them immediately in an oven while they are yet in a soft state. When ill annealed, or cooled suddenly, glass is apt to fly in pieces on the slightest change of temperature, or touch of a sharp point. We may sometimes take advantage of this imperfection ; for by means of a red-hot wire glass vessels may be cut into any shape. Where there is not a crack already in the glass, the point of the wire is applied near the edge, by which a crack is formed ; and this is afterwards easily led in any direction we wish.
Ileaumur's porcelain is also glass, which, by being surrounded with hot sand, is made to cool so slowly that it as sumes a crystalline texture that destroys its transparency, but imparts to it every other quality desirable in chemical ves sels. The coarser kinds of glass are commonly used in making it ; but as there is no manufacture of this valuable substance, its employment is still very limited.
Lutes also form a necessary part of chemical apparatus. They are composi tions of various substances, intended to close the joining of vessels, to coat glass vessels, and to fine furnaces. Lutes of the first description are commonly em ployed to confine elastic vapours. They should, therefore, possess the following properties : viscidity, plasticity, compact ness, the power of resisting acrid va pours, and certain degrees of heat. The viscidity of lutes depends on the pre sence either of unctuous or resinous substances, mucilaginous substances, or clay.
Lutes of the first kind possess viscidity, and resist acrid vapours in an eminent degree ; but they are in general so fusi ble, that they cannot be employed when they are exposed even to very low de grees of heat, and they will not adhere to any substance that is at all moist. The following are a few of this kind that have been most frequently employed : Eight parts of yellow wax melted with one of oil of turpentine, with or without the addition of resinous sub stances, according to the degree of plia bility and consistence required. Lavoi sier s lute.
Four parts of -wax melted with two of varnish and one of olive oil. Saussure's lute.
Three parts of powdered clay worked up into a paste, with one of drying oil, or, what is better, amber varnish. The
drying oil is prepared by boiling 22.5 parts of litharge in 16 of linseed oil, un til it be dissolved. Fat lute.
Chalk and oil, or glaziers' putty, is well fitted for luting tubes permanently into glass vessels, for it becomes so hard that it cannot be easily removed.
Equal parts of litharge, quick-lime, and powdered clay, worked into a paste with oil varnish, is sometimes used to daub over the cracks in glass vessels, so as to render them again fit for some purposes.
Melted pitch and brick dust.
Mucilaginous substances, such as flour, starch, gum, and glue mixed with water, with or without some powder, are suffi ciently adhesive, are dried by moderate degrees of heat, and are easily removed after the operation, by moistening them with water. But a high temperature de stroys them, and they do not resist cor rosive vapours. Of these take the fol lowing forms : Slips of bladder macerated in water, and applied with the inside next the ves sels. They are apt, however, from their great contraction on drying, to break weak vessels.
One part of gum arabic with six or eight of chalk, formed into a paste with water.
Flour worked into a paste with pow dered clay or chalk.' Almond or linseed meal formed into a paste with mucilage or water.
Quicklime in fine powder, hastily mix ed with white of egg, and instantly ap plied, sets very quickly, but becomes so hard that it can scarcely be removed.
Slaked lime in fine powder, with glue, does not set so quickly as the former.
The cracks of glass vessels are some times mended by daubing them and a suitable piece of linen over with white of egg, strewing both over with finely powdered quick lime, and instantly ap plying the linen closely and evenly.
Earthy lutes resist very high tempera tures, but they become so hard that they can scarcely be removed, and often hard en so quickly after they are mixed up, that they must be applied immediately. Examples : Quick-lime well incorporated with a sixth part of muriate of soda.
Burnt gypsum, made up with water.
One ounce of borax dissolved in a pound of boiling water, mixed with a suf ficient quantity of powdered clay. Mr. Watts's fire lute.
One part of clay with four of sand form ed into a paste with water. This is also used for coadng glass vessels, in order to render them stronger, and capable of re sisting violent degrees of heat. It is then made into a very thin mass, and applied in successive layers, taking care that each coat be perfectly dry before another be laid on.