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CEM

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it is very proper as a general lute for a greatvariety of purpofes, and- will never harden fo asto break the veffel.

Often a fire lute is required to join the coversto crucibles, or for fimilar purpofes, fo as tokeep them air-tight when hot. A very valuablecompofition of the kind is made of glafs of borax,brick-duft, and clay, finely powdered togetherand mixed with a little water when ufed. Novery great nicety is required in the proportions,but about a tenth of borax is quite fufficient tobring the earths to that ftate of femi-vitrificationwhich is defired. Litharge may alfo be ufedinftead of borax, but the latter is by far thebelt, as it promotes that thin fpreading fufionwhich is beft calculated to be equally appliedover an uneven furface, and befides, if a portionof the litharge-lute were to drop into the cru-cible it might poffibly be reduced, and lead in-troduced into the refult6 of the experiment.

A cement faid to be ufeful to ftop cracks ofiron veffels intended to be ftrongly heated, ismade of 6 parts of clay, i of iron filings, andlinfeed oil enough for mixture.

Another fpecies of cement is what is termedby the French Maftich chaw )j and confilts of dif-ferent kinds of oily and refinous fubftances, li-quid when hot, and which become more or lefsfolid by cooling. They are ufeful for a varietyof mifcellaneous purpofes, for experiments withgaffes over water or mercury, and others whereonly a very moderate warmth is ufed, and whereit is of importance to keep out air and water.Thel'e will alfo confine acid vapors, but not thevapors of alcohol, turpentine, or eflential oils,which diffolve moll refinous fubftances. Moltof them will Hick very well to glafs. Commonfealing-wax is one of the molt ufeful of thefecements. A cheaper and lefs brittle cement ismade fimply by melting bees wax with aboutone-eighth of common turpentine. This may bemade up into fticks to be ufed when wanted,being firlt melted or fpread evenly with a hotiron. A greater portion of turpentine rendersthis lute fofter, and more fufible, but fomewhatpliable.

A very firm cement is made by 4 parts ofrofin, 1 of bees wax, and when melted, 1 partof fine brick duff ftirred in. This adheres withextreme firmnefs. Table knives are cementedto their handles by this mixture, and turnerslife a fimilar compofition in fome fine works tofix them to the lathe.

Chaptal found after many trials that the pe-netrating vapours of fulphureous acid in the

manufacture of alum were completely confinedin a wooden chamber, lined very carefully witha mixture of equal parts of pitch, turpentine,and wax, boiled till all the eflential oil was dif-fipated (which was known by the cefl'ation ofthe bubbles) applied melted to the wood, andfpread with a hot trowel over the joints. Vint-ners ftop leaks in their calks with melted fuet crubbed over, when cooling, with lifted wood-afhes, or previoully mixed with the allies inmelting.

The ufe of gum arabic diffolved in water fo?cementing paper labels to bottles, and a greatvariety of mifcellaneous purpofes is known toevery one. A ftill better cement for the fameufes is ifinglafs diffolved in vinegar to a prettythick confiftence when warm. This congealson cooling, and before it is ufed it lhould begently warmed.

Many of the varnilhes and oil paints are em-ployed in rendering veffels air and water tight.Thus when canvas bags are faftened to a ftop-cock tube for air-holders, the joining is madeperfectly tight by tying over it flips of cloth orbladder foaked in fpirit varnilh.

The following cement is faid to be very ufe-ful in joining together glafs or fteel. Take ofmaftich five or fix bits as big as peas, diffolve inas much alcohol as will render them liquid. Inanother veffel diffolve as much ifinglafs (previ-oufly foaked in water) in brandy or rum as willmake two ounces by meafure of a ftrong glue ;warm it and incorporate with it, by rubbing,two or three fmall bits of galbanum or ammoni-acum, and then the maftich folution. Keep thecement in a bottle well flopped, and gently warmit before ufe.

Thofe fufible metallic compounds ufed tounite pieces of metal, form another totally difi-tin£l fpecies of cements. Thefe are termedSolders, under which they will be defcribed.

Some inconveniences occafionally attend theufe of thefe lutes in chemical operations. Theapplication of them takes up time and requiresa little manual dexterity, which can only belearned by practice. In fome nice experimentsof refearch, where fubftances are decompofedby diftillation, and all the produfts are to beanalyzed, the lute itfelf is often flightly corrodedor otherwife added on, and introduces lome con-' fufion in the refult. It is partly on this ac-count, and chiefly to fave trouble to the chemift,that the practice of clofely fitting glafs veffelsby grinding is fo much more ufed at prefentthan formerly; but this is always expenfive,

' Encycl. Arts & Metiers.