Cements, in general, are substances employed to unite bodies in close adhesion, for which purpose they are applied in a semi-fluid or pasty state, so as to be brought into intimate contact with the opposite surfaces, and becoming solid as the moisture exhales, the whole form, as it were, one mass. Cements are variously composed, according to the nature of the surfaces to which they are applied, and their exposure to heat or moisture. We shall here annex the processes of preparing and mode of employing those which are in the most esteem.

Cement for Steam. The following methods are adopted by machinists to join the flanges of iron cylinders, and other parts of hydraulic and steam engines. Boiled linseed oil, litharge, red and white lead, mixed together to a proper consistence, applied on each side of a piece of flannel, previously shaped to fit the joint, and then interposed between the pieces before they are brought home, as the workmen term it, to their place, by the screws or other fastenings employed, make a close and durable joint. The quantities of the ingredients may be varied without inconvenience, taking care not to make the mass too thin. It is difficult in many cases instantly to make a good fitting of large pieces of iron work; this renders it necessary sometimes to join and separate the pieces repeatedly before a proper adjustment is obtained. When this is required, the white lead ought to predominate in the mixture, as it dries much slower than the red. A workman knowing this fact, can be at little loss in exercising his own discretion in regulating the quantities.

It is safest to err on the side of the white lead, as the durability of the cement is not thereby affected; a longer time only is required for it to dry and harden.

When the fittings will not easily admit of so thick a substance as flannel being interposed, linen, pasteboard, or even paper, may be substituted. Stones that are broken, however large, may be joined together very well with this cement, and cisterns put together with it will never leak. In this case, the stones need not be entirely bedded in it; an inch, or even less, of the edges that are to be next the water, need only be so treated; the rest of the joint may be filled up with good lime.

Iron Cement. Take two ounces of muriate of ammonia, one of flour of sulphur, and sixteen of cast-iron filings or borings. Mix them well in a mortar, and keep the powder dry. When the cement is wanted for use, take one part of this mixture, twenty parts of clear iron borings or filings, grind them together in a mortar, mix them with water to a proper consistence, and apply them between the joints. This cement is in extensive use by iron founders, and is found to be very excellent, as in time it unites with the iron as one mass.

Cement for Iron Boilers. Six parts of clay, one of iron filings, and linseed oil sufficient to form a thick paste, make a good cement for stopping cracks in iron boilers.

Cement for Copper Boilers. Powdered quick lime mixed with bullock's blood is often used by coppersmiths, to lay over the rivets and edges of the sheets of copper in large boilers, as a security to the junctures, and also to prevent cocks from leaking.

Cements for Jewellers, Lapidaries, etc. It sometimes happens, that jewellers, in setting precious stones, by accident break off pieces; in this case they join them, so that it cannot easily be seen, with gum mastic, the stone being previously made hot enough to melt it. By the same medium cameos of white enamel, or coloured glass, are often joined to a real stone as a ground, to produce the appearance of an onyx. Mastic is likewise used to cement false backs or doublets to stones, to alter their hue. The jewellers in Turkey, who are generally Armenians, ornament watch-cases and other trinkets with gems, by glueing them on. The stone is set in silver or gold, and the back of the setting made flat to correspond with the part to which it is to be applied. It is then fixed on with the following cement: isinglass, soaked in water till it swells up and becomes soft, is dissolved in French brandy or in rum, so as to form a strong glue. Two small bits of gum galbanum, or gum ammonicum, are dissolved in two ounces of this by trituration, and five or six bits of mastic, as big as peas, being dissolved in as much alcohol as will render them fluid, are to be mixed with this by means of a gentle heat.

This cement is to be kept in a phial closely stopped; and when used, it is to be liquefied by immersing the phial in hot water. This cement will resist moisture. Temporary cements are wanted in cutting, grinding, or polishing optical glasses, stones, and various small articles of jewellery, which it is necessary to fix on blocks, or handles, for the purpose. Four ounces of resin, a quarter of an ounce of wax, and four ounces of whiting, made previously red hot, form a good cement of this kind, as any of the above articles may be fastened to it by heating them, and removed at pleasure in the same manner, though they adhere very firmly to it when cold. Pitch, resin, and a small quantity of tallow, thickened with brick dust, is much used at Birmingham for these purposes. Four parts of resin, one of bees'-wax, and one of brick-dust, likewise make a good cement. This answers extremely well for fixing knives and forks in their hafts; but the manufacturers of cheap articles of this kind too commonly use resin and brick-dust alone.

On some occasions, in which a very tough cement is requisite, that will not crack though exposed to repeated blows, as in fastening to a block metallic articles that are to be cut with a hammer and punch, workmen usually mix some tow with the cement, the fibres of which hold its parts together. Seven or eight parts of resin, and one of wax, melted together, and mixed with a small quantity of plaster of Paris, is a very good cement to unite pieces of Derbyshire spar, or other stone. The stone should be made hot enough to melt the cement, and the pieces should be pressed together as closely as possible, so as to leave as little as may be of the cement between them. This is a general rule in cementing, as the thinner the stratum of cement interposed, the firmer it will hold.