The above may be considered as constituting by far the most generally useful hues for connecting chemical apparatus together ; there arc, besides these, another class of lutes, which is of high value to the practical che mist, and of some of them we shall now give a short ac count. As glass vessels, when subjected to a consi derable temperature, are liable to melt, they are fre quently coated with lutes, which protect them, and obviate the inconveniences which would result from com plete exposure. Earthen vessels being naturally porous, arc apt to allow any volatile matters contained in them to escape ; this is also remedied by similar means. Tn secure glass vessels, sand is mix«I with as much clay as binds the whole mass together, and is beaten up with any fibrous matter, which gives an additional degree of mechanical security. Windsor loam answers well ; butt an equally useful cement may be formed from an admix ture of sand and coarsely ground potter's ware ; to which horse dung, chopped straw, chaff, horse or cow's hair, or tow, may be added, in such proportions as will ren der the whole mass perfectly ductile. An ounce of hail is said to he sufficient for five pounds of the earthy mix ture. The mass should be beaten together with great care, so as to diffuse the hair throughout the whole of it It may he then applied to the surface, to be coated either in the dry state, or by repeated clippings into a thick mix ture of the mass with water. If the coating be smeared with lintseed oil after it is thoroughly dry, it will be lesi, liable to crack.
As a coating for earthen ware vessels, various coin pounds may be formed of a variety of fluxes, mixed into a thin paste with clay and lime. One ounce of borax dissolved in a pint of boiling water, added to as much lime as will form a thin paste, will be an excellent coating of this kind. After this has been applied to the earthen vessel, and has become properly dry, a lute of lime and linseed oil should be laid over the whole. When it is completely di y, the vessel is fit for use. By renewing the last lute, the vessel may he used several times with safety. This preparation has been chiefly recommend ed as a coating for vessels in which phosphorus is pre pared.
It is frequently necessary to lute the covers of cruci bles, so as to exclude any communication with the ex ternal air ; and it is an object of importance, to have a lute that can bear a high temperature. An excellent
compound of that kind is formed by borax first calcined and then fusvd, brick dust and clay finely powdered, and moistened with a little water. About a tenth of the whole may be borax. Lithargc may be used, though it is in ferior to the borax.
Clay, iron filings, and water, make a durab:e cement for iron vessels exposed to a high temperature.
Very useful lutes arc made of oily substances united to resinous matters. They answer extremely well at a moderate temperature, and with those vapours that do not dissolve their component parts. They will adhere to glass, which enhances their value. Bees' wax, soft ened with one-eighth of the spirits of turpentine, forms a cement of this kind.
One part of bees' wax, four of rosin, melted and mix ed up with one part of brick dust finely powdered, forms the cement used by turners, with which the handles of knives are cemented to their blades.
Equal parts of pitch, turpentine, and wax, boiled till the essential oil is dissipated, makes a coating to a wooden chamber, which is perfectly impervious to the vapours of sulphureous acid. It is therefore useful in the manufacture of alum. Suet and wood ashes make a simple cement, which is well adapted to stop leaks in casks.
Gum arable, dissolved in water, is a cement well known for applying paper to glass. &c. Isinglass, dis solved in vinegar, is still better.
The following formula has been given for a cement to unite glass or steel : " Take of mastich five or six bits as big as peas, dis solve in as much alcohol as will render them liquid. In another vessel, dissolve as much isinglass "(previously soaked in water) in brandy or rum as will make two ounces, by measure, of a strong glue, warm it, and in corporate with it, by rubbing, two or three small bits of galhanum or ammoniacum, and then add the mastich solution; keep the cement in a bottle well stopped, and gently warm it before use." Of late, the use of cements in chemical experiments has been nearly superseded by the introduction of accu rately ground glasses, which, without the introduction of other substances, may be made perfectly air tight. Much inconvenience certainly arises from the use of them, which is thus obviated. They still, however, are necessary in many large manufactories, and arc retained by the practical chemist. See BRIDGE, p. 510 .4plzendix to CHEMISTRY; and CIVIL ARCHITECTURE.
(c. M.)