PYR 



PYR 



presents a striated fracture, the striae be- 

 ing gvKerail) diverging. 



It is rattier more liable to tarnish than 

 the preceding, and decomposes more 

 readily in a humid atmosphere. Accord- 

 ing- to Mr. Hatchet's analysis, it consists 

 of 



Sulphur 54 



Iron 46 



100 



The capillary pyrites occurs in delicate 

 capillary crystals, grouped, parallel, di- 

 verging or interwoven, slightly flexible, 

 having a metallic lustre, and a colour 

 passing from yellow to steel-grey. There 

 is, lastly, the hepatic pyrites, so named 

 from the liver-brown colour which it as- 

 sumes from exposure to the air. In the 

 fresh fracture its colour is pale brass-yel- 

 low, inclining to steel-grey. It occurs 

 massive, of various imitative forms, and 

 crystallized in six-sided prisms, or six-sid- 

 ed pyramids : it has less lustre than the 

 others, and is more subject to decompo- 

 sition. What has been named magnetic 

 pyrites, distinguished, as the name im- 

 plies, by its magnetic quality, of which 

 the others are destitute, has been consi- 

 dered as forming a distinct species. Its 

 colour is deeper, being intermediate be- 

 tween brass-yellow and copper-reel, and 

 approaching even to brown, often tar- 

 nished : its lustre is also inferior, but is 

 still metallic. It occurs only massive or 

 disseminated. Its fracture is compact : it 

 is hard and brittle : its specific gravity is 

 4.5. It appears from Mr. Hatchet's ana- 

 lysis of it to differ from the other iron py- 

 rites, in containing a larger proportion of 

 metal, to which, no doubt, its quality of 

 being attracted by the magnet is owing. 



PYROLA, in botany, -winter-green, a ge- 

 nus of the Decandria Monogynia class and 

 order. Natural order of Bicornes. Eri- 

 C3e, Jussieu. Essential character: calyx 

 five parted ; petals five ; capsule superior, 

 five-celled, opening at the corners, ma- 

 ny-seeded ; anthers with two pores. 

 There are six species, natives of the 

 North of Europe. 



PYROLIGNOUS and PVHOTARTAROUS 

 acids. When wood is distilled in close 

 vessels, it always yields more or less of 

 an acid juice : the same remark applies to 

 the salt called tartar. These liquids were 

 distinguished by the name of pyrolignous 

 and pyrotartarous acids : but they are 

 now known to be only the acetic disguis- 

 ed by the presence of a peculiar oil. 



PYROMETER, an instrument for rnea*> 

 suring the expansion of bodies by heat. 

 The whole art in forming an instrument, 

 adapted to this purpose, is so as to ren- 

 der it capable of showing very small ex- 

 pansions of solid bodies. Different instru- 

 ments have been invented for this pur- 

 pose ; of the greater number of which it 

 is scarcely necessary to give a detailed 

 account. " The difficulty of contriving au 

 unexceptionable instrument of this kind 

 has arisen partly from the difficulty of 

 finding a substance not liable to be alter- 

 ed by a high temperature, and which shall 

 suffer a change of volume sufficiently 

 perceptible to be accurately measured ; 

 and partly from that of rinding a measure, 

 which shall not itself be affected by the 

 high temperature, and be, At the same 

 time, sufficiently delicate. 



The pyrometer, in which, . perhaps, 

 these difficulties have been most effectu- 

 ally surmounted, and which has come in- 

 to most general use, is that invented by 

 the late Mr. Wedgwood. The pure 

 earth, named alumina, and the different 

 earths, (the clays) in which it predomi- 

 nates, have the singular property of not 

 expanding, but of contracting by heat. 

 This contraction begins to become evi- 

 dent, when the clay is raised to a red 

 heat, it continues to proceed until it vi- 

 trifies, and the total contraction, in pure 

 clays, exceeds considerably one-fourth 

 part of the volume in every direction. It, 

 occurred to Mr. \Vedgwood, that from 

 this property it might be employed in 

 the construction of a pyrometer. The 

 contraction that the clay suffers is per- x 

 manent, or it does not return to its for- 

 mer dimensions when cold. The degree 

 of contraction it has suffered, therefore, 

 can be ascertained without any source of 

 fallacy, and will indicate the extreme of 

 temperature to which it has been expos- 

 ed. 



This pyrometer consists of a gauge, 

 composed of two straight pieces of brass, 

 twenty-four inches long, divided into 

 inches and tenths, and fixed in a brass 

 plate, so as to converge ; the space be- 

 tween them, at the one extremity, being 

 five-tenths of an inch, and at the other 

 three-tenths. The pyrometrical pieces 

 of clay are small cylinders, flattened on 

 one side, made in a mould, so as to be 

 adapted exactly to the wider end. It is 

 evident that, in exposing one of these 

 pieces to a high temperature, the can- 

 traction it has suffered may be measured, 

 by the length to which it can be slid into 

 the converging groove or gauge. 



