PYR 
Bttd wliich differ principally in structure, 
«r in the form under which they occur. 
The striated or radiated pyrites presents 
a striated fracture., the striae being generally 
diverging. 
It is rather more liable to tarnish than 
tlie preceding, and decomposes more rea- 
dily in a humid atmosphere. According to 
Mr. Hatchet’s analysis, it consists of 
Sulphur... 54 
Iron.... 46 
too 
The capillary pyrites occurs in delicate 
capillary crystals, grouped, parallel, diverg- 
ing, 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 assumes from expo- 
sure to the air. In the fresh fracture its 
colour is pale brass-yellow, inclining to 
steel-grey. It occurs massive, of various 
imitative forms, and crystallized in six- 
sided prisms, or six-sided pyramids : it has 
less lustre than the others, and is more sub- 
ject to decomposition. What has been 
named magnetic pyrites, distinguished, as 
the name implies, by its magnetic quality, 
of which the others are destitute, has been 
considered as forming a distinct species. 
Its colour is deeper, being intermediate 
between brass-yellow and copper-red, 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 
genus of tire Decandria Monogynia class 
and order, Natural order of Bicornes. 
Ericae, Jussieu. Es.sential character: ca- 
lyx five-parted ; petals five ; capsule supe- 
rior, five-celled, opening at the corners, 
many-seeded ; anthers with two pores. 
There are six species, natives of the North 
of Europe, 
PYROLIGNOUSandPyROTAETARous 
flcids. When wood is distilled in close ves- 
sels, 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 
PYR 
and pyrotartarous acids : but they are now- 
known to be only the acetic disguised by 
the presence of a peculiar oil. 
PYROMETER, an instrument for mea- 
suring the expansion of bodies by heat. 
The whole art in forming an instrument, 
adapted to this purpose, is so as to render 
it capable of showing very small expan- 
sions of sohd bodies. Different instruments 
have been invented for this purpose ; of the 
greater number of which it is scarcely ne- 
cessary to give a detailed account. The 
difficulty of contriving an unexceptionable 
instrument of this kind has arisen partly 
from the difficulty of finding a substance 
not liable to be altered by a high tempera- 
ture, and which shall suffer a change of 
volume sufficiently perceptible fo be accu- 
rately measured; and partly from that of 
finding 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 effectually sur- 
mounted, and which has come into 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 predominates, have the singular 
property of not e.xpauding, but of contract- 
ing by heat. This contraction begins to 
become evident, when the clay is raised to 
a red heat, it continues to proceed until 
it vitrifies, and the total contraction, in 
pure clays, exceeds considerably one-fourth 
part of the volume in every direction. It 
occurred to Mr. Wedgwood, that from this 
property, it might be employed in the con- 
struction of a pyrometer. The contraction 
that the clay suffers is permanent, or it does 
not return to its former dimensions, when 
cold. The degree of contraction it has suf- 
fered, therefore, can be ascertained w'ithout 
any source of fallacy, and will indicate the 
extreme of temperature to which it has been 
exposed. 
This pyrometer consists of a guage, com- 
posed of two straight pieces of brass, twen- 
ty-four inches long, divided into inches and 
tenths, and fixed in a brass plate, so as to con- 
verge; the space between them, at the one 
extremity, being five-tenths of an inch, and 
at the other three-tenths. The pyrometri- 
cal pieces of clay are small cylinders, flat- 
tened 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 tliese 
pieces to a high temperature, the contrac- 
tion it has suffered may be measured, by 
