P Y R 



moved to other holes made on purpofe in the fide pieces or 

 the frame c ece: tttl is the cyhndrical bottle, in which 

 hano- the rods in order to be heated at different degrees by 

 water of various temperatures ; :;, k, are the fiip.porters of the 

 bottle; X the thermometer fufpended in the water ; _yjr a 

 rod, to the lower end of which is fixed a fmall plate, to ilir 

 the water, by moving it up and down ; a z a fyphon, one 

 branch of which is within, and the other without, the bottle, 

 the latter being furnlfhed with a cock, which ferves to drav* 

 off the quantity of water that is neceflTary for changing the 

 temperature in the bottle. For a farther account of this 

 inllrument, together with the principle of its conilruftion 

 and ufe, both in the comparative meafure of the expanfions 

 of bodies by heat, and the mealure of their abfolute ex- 

 panfion, and the experiments made with it, we mull refer 

 to an elaborate efTay of M. De Luc on Pyrometry, &c. in 

 the Phil. Tranf. vol. Ixviii. part i. art. 20. p. 419—546. 



All the contrivances above defcribed mud be confidered 

 as inferior to that conilruftcd by Mr. Ramfden, of which 

 general Roy has given an accurate and minute account in 

 the Phil. Tranf. vol. Ixxv. p. 462. We (hall here give fuch 

 an account as our limits will allow of this inftrument, the 

 conllruftion of which is fo accurate, fays major-general 

 Roy, that it feems not eafy to improvp it ; and refer for a 

 fuller account illuftrated by appropriate drawings. This 

 pyrometer, named the microfcopir, becaufe by means of two 

 microfcopes attached to it, the expanfion is meafured, con- 

 fifts of a ftrong deal frame, 5 feet long, nearly 28 inches 

 broad, and about 42 inches in height. The metallic bar, 

 whofe expanfion is to be meafured, and which may be even 

 five feet long, is placed in a copper trough little longer than 

 five feet ; and this is placed over 1 2 fpint lamps, the flames of 

 which heat the water of that trough fully to the boiling 

 point, and of coiirfe heat the bar which is plunged into it. 

 Two other wooden troughs, alfo full of water, are placed 

 parallel to, and at a httle dillance from, the copper one. 

 Each of thefe contains a caft-iron prifmatic bar. To the 

 ends of one of tkefe bars two microfcopes are fattened in 

 an horizontal iituation, perpendicularly to the bars. One 

 of thefe microfcopes is furnifhed with a micrometer, or 

 mechanifm, to meafure the magnified image of an objeft ; 

 \}\e other microfcope has a fimple mark. The parts of the 

 microfcopes, as well as the proper marks for meafurement, 

 are fo feparated and dii'pofed, partly upon the ends of the 

 caft-iron rods, and partly upon the ends of the rod under 

 examination, that if any of them be lengthened or (hortened, 

 that alteration is clearly perceived through the microfcopes, 

 and may be meafured by means of the micrometer. It fol- 

 lows, that if the temperature of the caft-iron prifmatic 

 bars be kept unaltered, whilft that of the bar under exa- 

 mination is increafed, then the increafe of length, which 

 is meafured by the micrometer, mull be attributed to that 

 bar only, and by thefe means the expanfions of feven fub- 

 ftances were afcertained. For further particulars relating 

 to the conitruftion and ufe of this inftnunent, we refer to 

 the Tranfaftions (ubi fupra), and to the article ExPANSIOX 

 in this Cyclopaedia. For an account of Wedgwood's pyro- 

 meter, fee THirKMOMETER. Alfo, for a defcription of Mr, 

 Troughton's pyrometer, we are under the neceffity of re- 

 ferring to the fame article. 



PYROMUCOUS Acid, or Empyreumatic acid of fugar, 

 or Syrupous acid, is different from the acid of fugar, or 

 Oxalic ylc'id, (which fee,) and denotes that pungent acid 

 vapour of fugar, or any faccharine matter ftrongly heated, 

 which is familiar to every perfon who has ever entered a 

 fugar bakint;houfe. It is prepared by chemifts in the 

 following manner : Put into a very large glafs retort any 



P Y 11 



qaantity of pulverized fugar of any kind, fo as to fill oidy 

 ^th of it, as the m.atter very much fwells in the procefs : 

 adapt to it a large receiver, not clofely luted, and heat it 

 gradually on a fand-bath. A great quantity of gas arifes 

 when the fugar begins to be fcorched, which is moftly car- 

 bonic acid mixed with an inflammable gas, probably the 

 gafeous oxyd of carbon. In the receiver is condenfed a 

 weak acidulous liquor, coloured by an oily matter, and 

 alfo apparently fouled by a portion of fuliginous matter 

 volatilized during the diftillafion. The quantity of acid, 

 obtainable in this procefs, varies according to the regulation 

 of the fire, which ftiould at leaft be pufticd fo as to make 

 the retort red-hot, and to reduce the fugar to a perfeft 

 charcoal ; but in general about ixXn of the weight of fugar 

 may be obtained of the diftilled acid. Gum mucilage, 

 manna, honey, ftarch, and other mucous or mucofo-faccha- 

 rine fubftances, yield, by diftillation, the fame acid as fugar. 

 Aikin, Dia. Min. ' 



PYRONOMIA, a term ufed by the chemical writers to 

 expreis tiie art of regulating fire, fo as to make it fubfer- 

 vient to all their proceftcs in a detetjninate degree. 



PYROPE, in Mineralogy, a precious ftone, formerly 

 called tlie Bohemian garnet. (See Garnet.) The colour 

 is a dark blood red: when held between the eye and the 

 light, the red colour inclines to a yellow. The pyrope is 

 never found cryftaUized, but occurs in JVnall round or an- 

 gular fragments imbedded in ferpentiiie rock, or fcattered in 

 the fands on the fea-fhore, and in alluvial ground. It Icratches 

 quartz. The fpeciiic gravity is ilated by Klaproth 3.718. 

 On account of its lullre, tranfparency, colour, and hardnefs, 

 it is much prized by jewellers. The fmall grains, when 

 powdered, are applied to cut fofter ftones. According to 

 Klaproth, the conftituent parts of pyrope are 



13- 



PYROPHAGI. See YmiL-Ealers. 



PYROPHANES, in Mineralogy, a variety of the femi- 

 opal ; fo called becaufe, being heated in a fpoon, it be- 

 comes tranfparent, but returns to its opaque ftate when 

 cold, as Mr. Landriani has difcovered. M. SauflTurc, junior, 

 renders cormnon hydrophanes tranfparent, and of a topaz 

 colour, when heated by digefting them in melted wax . It 

 is faid that fome pyrophanes are found in Armenia, which 

 are tranfparent while expofed to the fun, and opaque at 

 night. See Gem. 



PYROPHORUS, formed of ^i/f, /re, and ^;:., /fe^r, 

 in Chemijlry, the name ufually given to that fubftance called 

 hy [ome blach phofphorus ; a chemical preparation pofTefling 

 the Angular property of kindling fpontaneoufly when ex- 

 pofed to the air. 



This fubftance was accidentally difcovered by M. Rom- 

 berg, who prepared it of alum and human faeces. (See 

 Phosphorus Fizcalis. ) See alfo M. Le Fevre's prepara- 

 tion, defcribed under Phosphorus of Sulphur. 



It was apprehended for a confiderable time after the dif- 

 covery, that human faeces were effential to the operation, 

 till the youngeft fon of the great Lemery found, that honey, 

 fugar, flour, and, indeed, any animal or vegetable matter, 

 might be fubftituted inftead of the human fjeces ; and fince 

 that time, M. de Suvigny has (hewn that moift vitriolic falts 

 may be fubftituted for the alum ; having added to the alu- 



12 mingus 



