P Y R

P Y R

( 2 G 5 )

the I owe ft heat of ignition, or that in which abody is juft perceptibly red-hot in the day time.From this point it ftirinks gradually and prettyuniformly in proportion as the heat is increafed,■till it reaches its utmoft degree of contraction,(which varies according to the nature of theclay,) thus forming a range which may be ufedfor thermometric purpofes. In the pureft na-tural clays it reaches nearly to the higheft arti-ficial heat procurable in furnaces, but in otherclays, though to the eye equally pure, the con-traction (tops very fhort of this, and then aJecond expanfion takes place.

At this time it is intenfely hard, and has atexture approaching to the vitreous, fo that thefecond enlargement appears to depend in partperhaps, on a further difengagement of gas,but chiefly probably on an alteration in its tex-ture. During the firft part of the pyrometricfcaie, (that is from low rednefs upwards) theclay continues bibulous and flicks to thetongue. This ceafes however fome time beforethe fecond expanfion comes on, whence Mr.Wedgewood conje£lures, that the porcellanomftate (which is intermediate between the earthyand the vitreous) comes on when the clayceafes to be bibulous, and remains till the vi-treous expanfion begins. It mnv be obferved,as before mentioned under the article Alumine,that the fhrinking which takes place after igni-tion cannot be attributed to the evaporation ofwater or any other fubftance, for the lofs ofweight is much too fmall to allow of this ex-planation. The entire contraction, from igni-tion to the higheft procurable furnace-heat, isfull one-fourth of the bulk of the clay in everydimenfion, when it remains unvitrified at fohigh a temperature; hut the lofs of weightfrom ignition to the heat that melts copper,which is about a fixth of the entire contraction,is only 2 per cent, of the weight of the clay,and above this latter point no lofs is perceptibleby the balance.

It has been mentioned above that famples ofclay though apparently equally pure and white,and even taken from the fame bed, though fur-ficiently fimilar for moft purpofes of manufac-ture, do not undergo the fame degree of fhrink-mg in the fire; for fome continue to contrailat every procurable intenfity, whereas others,above a certain point, again expand and afl'umea vitreous texture. Neither do the firjl ex-panfions, which take place juft at ignition, fuf-ficientiy correipond. This circumftance caufeda good deal of embarraffinent to the inventor, asafter the full fample of pyrometer clay was

VOL. 11.

exhaufted, (which happened to be of the kindthe moft refractory in the fire) he could not becertain of obtaining another mafs which wouldcorreipond with the former in contraftion.This led him to examine the contraction ofpure alumine precipitated from alum by an al-kali (that is to fay, as pure as it is ufually ob-tained without extreme precaution and greatexpence, and therefore {till retaining both ful-phuric acid and potafb) and he found that itcontinued to contrail, and even remained bi-bulous fo as hardly to become porcellanous, bythe higheft heat of a furnace. This prepared'alumine when mixed in certain proportionswith the natural clays alfo rendered them fufh-ciently unalterable in texture in the fire, andbrought their contractions to correfpond withthole of the pyrometer-pieces already made.The alumine is prepared by precipitating a fo-lution of alum with good pearlafh, and walhingit repeatedly with large quantities of boilingwater. The precipitated earth when it is fome-what condenfed by fettling, is to be ufed in thatgelatinous ftate undried, as if then mixes muchmore eafily with the natural clay. Fir. W.found that a mafs of ten hundred weight cf thefecond clay which he employed required all theearth of five hundred weight of alum to bringit to the fame ftate with the pyrometer-piecesalready made. This prepared alumine is muchmore tenacious of water than common clay, fothat the mixture requires a more careful dryingand at a higher temperature, before the fize ofthe pieces is adjufted, and this fhould be doneimmediately after giving them a boiling-waterheat, as they again abforb fome moifture fromthe air (owing doubtlefs to the potafh con-tained in the alumine) which would make thedifference of two or three degrees of the fcaie,if the adjuftment was delayed till that time.

The pyrometer is a&ually made in the fol-lowing manner. It con fills of two parts, theclay-pieces and the brafs gage. Each, clay-pieceis a feftioti of a cylinder of clay about thethicknefs of the little finger, and the lengthabout equal to the diameter. One fide is alittle flattened, that it may Aide more accuratelyalong the gage. This latter confifts of a longcanal formed by two perfectly ftraight bars ofbrafs fixed on a plate of the fame metal in aconverging direction, and graduated into 240equal parts, beginning at the wideft extremity.The width of the canal at one end is to that atthe ether in the proportion of r to 3. Thegage to be more portable, is cut in halves,which are placed fide by fide on the fame plate.

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