MOR

MUF

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cr metallic veftel, and may therefore be ufedfor numerous diftillations, particularly in thofecafes where the reft of the apparatus is requiredto be kept cool. To increafe the heat of thislamp fome ingenious artifts have inventeddouble concentric wicks, which renders it equalto a fand-heat, but the fupports of the wicksare apt to be out of order, owing to the fufionof the folder. Smaller lamps are alfo required,and for thefe, alcohol is a very valuable fuel, asits flame is perfectly clean.

MATRASS. This' is a thin glafs vefiel ofthe lhape of a flalk, which will bear a lamp-heat at bottom without breaking. It is eitherround at .the bottom as in Fig. 57. a. or flatteneda little. Fig. 48. Fig. 18. PI. I. is a very ufefulfmall matrafs of about 2 ounces capacity.

MORTAR. This is a molt effential articlein a laboratory, to reduce to powder a variety offubftances, and the material of which it is madeis of confiderable confequence. Very hardbodies, luch as the gems, quartz-eryftal, &c.can only be ground to powder in a mortar ofagate, or flint, or fome other hard filiceous ftone.They muft previoufly however be crufhed intofragments, either in a fteel mortar, or wrappedup in many folds of ftrong paper, and bruifedon an anvil with a heavy hammer. The frag-ments are then collected, and put into the flintmortar with a little water, and a very longcontinued fri&ion is required to bring it to animpalpable powder. It fhould always be re-membered that when a very hard body is groundto powder, the friftion wears the mortar aswell as the fubftance pulverized, confequentlyfor delicate experiments it is neeeffary to weighthe mortar before and after the procefs, andallow for the lofs of weight by an equivalentaddition to the powder obtained. Hence toothe compofition of the mortar itfelf fhould pre-vioufly be known, and by the analyfis of Klap-roth it appears that black flint and agate are fonearly pure filex, that the minute quantity oftheir other conftituent parts may be entirely ne-glefted.

For grinding fubftances of moderate hard-nefs, fteel mortars with very fmooth peftles ofthe fame materials, are kept in fome of thefliops. Very ufeful mortars are alfo made ofcall iron, which are very hard, but are notfinifhed quite fo fmooth as thofe of fteel. Largemortars of this kind are ufed bv the druggiftsfor pounding barks, woods, and to leflenthe manual labour, the peftle is fattened to theend of a flexible wooden pole, which hangsover the mortar, by the elafticity of which the

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peftle is lifted up again to the proper heightafter the ftroke is made. For lighter purpdfesvery good mortars are made of bell-metal, withpeftles either of the fame or of iron. T hefe arecall very fmooth fo that the material to bepounded does not flick to the fides. Moftneutral falts may be conveniently pulverized inthem, but their chemical adlion on the copperought to be kept in mind, as (for example) iffal-ammoniac is left for fome hours in a bell-metal mortar it becomes green by the oxyd ofcopper which it diflblves. Thefe mortars areoften fitted with wooden covers, perforated toadmit the peftle, fo as to prevent fragmentsfrom flying out by the ftroke.

Mortars are alfo made of marble, glafs, andWedgwood ware. The latter are highly ufe-ful, being very hard, fmooth, and not adted onby any chemical reagent. They will readilybreak, however, by a lmart blow. ■

MUFFLE. FI. I. Fig.'30. A muffle is avaulted flat-bottomed earthen veflel, intended tobe put in the midft of a furnace, of properconftrudtion, fo as to afford a fpace ftronglyheated, but protected from the actual contactof the fuel, in which fmall veffels of any kind ■may be fet. The muffle is entirely open at oneend, and clofed at the other, and it has fome-times fmall fide flits, to allow the freer accefsof the hot air. It is'always fet in a furnacewith the open end directly oppofite a fide door,of the fame arched fhape, and which it touches, .fo as to prevent any of the fuel from getting in.

The door for this p.urpofe is feen in PI.Via. Fig. 77. & 78. d. and Fig;. 83. e. The'muffle will juft Aide in and out of thefe open-ings with eafe, and iron bars or fome otherkind of fupport are placed in the furnace to 1keep it at the proper level. When the fufc--ftance in the muffle is' to be heated withoutaccefs of air the ftopper of the furnace may bethruft in upon it; but when a current of heatedair from without is wanted to pais through themuffle, which is the cafe in allaying, the ftop-per of the furnace muft be removed, and infteadof it a fmall iron plate (like the flat part of afhovel with the edges a little turned up) isfixed on a level with the bottom of the opening,on which are heaped a number of long piecesof charcoal. to block it up. Thefe prefentlykindle by the heat of the furnace, fo thatthe air which paffes freely between theminto the muffle is heated in its pafiage, and themuffle may be cooled at pleafure by taking'away fome of the pieces of charcoal. The fen all3 Y