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ginally compofed of true putty and oil; but atpreient it is nothing but wafhed chalk or whit-ing mixed up with linfeed oil; a fort of dudtilepafte is thus produced which becomes hard aftera few days expofure to the air.

PUZZOLANA.

Puzzolana appears to be nothing elfe thanvolcanic afhes more or lefs -compacted together.It occurs either white, red, or black, has adull earthy fratfure, and is eafily frangible. Itabounds in the vicinity of Vefuvius. The red-variety is the proper puzzolana, the black andthe white are called in Italy lapillo or rapillo.The allies which overwhelmed Pompeia nowform a vaft bed of white puzzolana. It is em-ployed in calcareous cements to which it givesthe property of hardening under water.PYRITES.

A name firft given by the ancient naturaliftsto' the native fulphuret of iron, becaufe when{truck againft a piece of fteel or other hardbody, it gives out fparks of fire, and becaufewhen heated red hot the fulphur that it containsburns with a lambent blue flame. Modernmineralogifts diftinguifh three fpecies of pyrites,namely, the ferruginous, the cupreous, and thearfenical, of which the firft is fulphuret of iron,the fecond fulphuret of copper, and the third isa compound of iron pyrites and arfenic. Allthefe fubftances have a general fimiiarity intheir external appearance: their colour variesfrom yellowifh white to gold yellow, and theyhave all a high metallic luftre. For furtherparticulars fee the ores of Arsenic, Copper ,and Iron .

PYROLIGNEOUS ACID]PYROMUCOUS ACIDPYROTARTAREOUS\ ACID J

PYROMETER. {Wedge-wood's.)

The range of the common mercurial ther-mometer extends no higher than the boilingpoint of this metal (that is about 6oo°) andeven this is a very inaccurate meafure of heatin its higher part on account of the very un-equal expanfion of mercury as it approachesboiling. But this point falls fhort of that ofthe loweft vifible red-heat, and from this to thehigheft temperature that furnaces will produce,a vaft range of heat is included, for whichno tolerably accurate method of meafurementexifted till the year 1782, 3 when the late Mr.Wedgewood invented his pyrometer. Previous■la this time chemifts were in the habit of ufing

See

Acetous acid,Empyrcumatic .

very vague terms to exprefs the heat employedin their operations, fuch as red heat , full-redheat', white beat, See. and the means ufed formeafuring it were too inaccurate for the expe-rimental chemift, though fufficient for the ma-nufafturer when aided by daily experience.Thefe were generally the degree of fufibility{hewn by refraftory earthy mixtures, the lengthof time which the crucible could endure theheat employed, the reduftion of metallic oxydsand the like. Another method which is aftu-ally employed in the potteries, and anfwers fuf-ficiently well, is the fhade of colour producedby the heat on a ring-of clay worked up with alittle oxyd of iron. This is a light brown, orflefli colour in the lower red heats, but graduallydarkens to a deep chocolate, or almoft to black-nefs as the fire reaches its utmoft intenfity.This mode however can hardly be applicable asa general ftandard of companion, as the {hadesof colour are not very definable, and not readilydeferibed, befides other objections which mightbe raifed.

Mr. Wedgewood, therefore, on examiningthis fubjedf, which his bufinefs as a potter ren-dered peculiarly interefting, found a new andmuch more accurate meafure of temperature inthe gradual, and on the whole pretty uni-form contraction, which clay undergoes in allheats from the fainteft red to the moft intenfewhich can be produced. From examiningwith minute attention the habitudes of claywith water and fire, and from the event ofmany experiments Mr. W. gives the followingas the general refult of his obfervations.

When a folid lj|mp of pure natural clay,(fuch as the fineft Devonftiire kindj is wellkneaded with water, and expofed very graduallyto an increafing heat, the firft change that takesplace is the fhrir.king produced by the . fimpleevaporation of water. The purer the clay is*the more water it takes to foften it, and there-fore the more it diminifhes in bulk by the lofsof moifture. The clay dries at a heat belowrednefs, and preferves its bulk unvaried up tothe approach of ignition. At this period it en-larges a little, owing probably to the efcape cfair with which it has been combined.

When this air has made its efcape, the claybegins to fhrink and returns to precifely thefame dimenfions which it had before, whenthoroughly dry. When thus prepared andbrought to this point, it is fit to be ufed for thepyrometer, and forms the beginning or loweftpoint of the fcale, which as above-mentioned is

• Phil. T rani, for 1782 , 4, and 6.