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P t It
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the uiLiculty in adjufting a fufficiently accurateftandard of comparifon.
r Ihe inventor obferves that a correfpondingpyrometer may always be conftrudted by mak-ing tor a gage a converging canal with thewidths at the two ends in the proportion of 5to 3, with the fides perfeftly ftraight, and di-vided into 230 equal parts—and by procuringnatural or artificial clay, pieces of which willjuft enter the gage at a low red-heat, will goto 27 0 by the heat at which copper melts, toabout 90° by the welding heat of iron, andabout 160 by the greateft heat of an air fur-nace, the clay pieces all the while remainingperfectly refraftory.
But the great difficulty which the inventorhimfelf feerns to have had in preparing a fe-cond parcel of clay fimilar in thefe refpefts tothat already employed is of itfelf fufficientlydifeouraging, ncr are the points which he givesfor the conftrudtion of the fcale at ail compar-able in accuracy with thofe by which the com-mon thermometers are graduated. The heatof boiling water and of melting ice are fixedpoints in which [with a few fimple precautions)every one would agre?, but it is not fo with ared heat barely vifible in day-light, and ftilllefs with the utmoft power of a wind-furnaceof given dimenfions. Even the fufing point ofcopper is liable to fome variations according tothe purity of the metal, and that of caft-iron isftill more variable on this account, fo thatthough we may confider Mr. Wedgewood’sfcale as affording eery ufeful approximations tothe expreffions of comparative heats, it does notapproach the accuracy of the mercurial ther-mometer. Neither nave we any means ofafeertainiug in the higher heats whether thecontraftion of clay is at all equable, that is,whether equal numbers on the pyrometricalfcale expreis equal increments of heat, as theydo pretty accurately with the mercurial ther-mometer.
The analyfis of Mr. Wedgewood’s pyrometerpieces as condufted by Scherer, a fhews them tobe made of a very pure natural clay compofedof about 54 per cent, of alumine, and 44 offxlex, with hardly a trace of any other ingredient.As the Devonfhire clays contain a much lefsproportion of alumine than is here given, theabove pieces were doubtlefs made, as indicatedby Mr. Wedgewood, by adding to the naturalclay a confiderable portion of precipitatedalumine. r
But on the other hand, Gazeran b in his ex-
• An, Chun. tom. kxx i. p. 17*. b u; tt0) to;
periments on this fubjeet round the ben imita-tion of the Wedgewood pieces to he producedby a clay in which the alumine was to the filexas 34 to 43, and pieces made of this materialcorrefponded very clofely with the originalWedgewood pieces at the heat of 160°, andfhewed no fign of vitrification at this hightemperature.
This fubjedl therefore, which is highly in-terefting to the chemift and manufacturer,requires further experiments.
PYROPHORUS, Homberg’s.
This preparation was lirlt difeovered by thechemift whofe name it bears, as the refidue ofthe diftillation of fecal matter with alum andvitriol. Improvements with refpeCt to thecleanlinefs of the procefs were foon introduced,and the following is now the common mode ofpreparing this fubftance. 3 Take three parts ofRoman alum in powder, and one part of coarfebrown fugar, mix them together and heat themafs over a common fire in an iron ladle or afhovel. By the firft impreffion of the heat boththe alum and fugar melt and mix intimatelywith each other; after a little time when moftof the water is evaporated, the refidue begins tofoam and fwell confiderably, and to give out apungent empyreumatic acid: it mult now bellirred conftantly with an iron fpatula, takingcare to detach it from the ladle in proportion asit dries, and to heat every part as uniformly aspoflible: when the laft remains of clamminefsare deftroyed, and the fpungy black matter thatis left is quite friable, it muff: be transferredwhile hot into a mortar, and haftily reduced toa fomewhat coarfe powder. As foon as it ispulverized, a green glafs vial of the capacity ofabout fix ounces, (previoufly luted on die infideby rinfing it out with a weak folution of boraxthickened with pipe day to the confidence ofcream) is to be nearly filled with it and thenfet up to the neck in a crucible filled with fand,its mouth being 'loofely flopped with a piece ofcharcokl. The crucible is now to be placed ina furnace, or even in an open fire, and raifedgradually to a moderate red-heat. At firft adenfe white vapour will iflue from the vial fornearly a quarter of an hour, and will then befucceeded by an inflammable gas, burning witha light blue flame: as foon as this makes itsappearance, the heat is to be kept up at anuniform temperature for about 20 minuteslonger, after which, the crucible with its con-tents being removed from the fire, and the ’
. xxxvi. a Beaurac Chim. exper. F. p. 338,
2 L 2