P Y It 
P Y It 
P Y R 
527 
i greyq. brown, or blackish colour; of a loose, 
granular, or dusty and rough, porous, or 
spongy, texture, resembling a clay hardened 
by lire, and then reduced to a gross powder. 
It contains various heterogeneous substances 
mixed with it. Its specific gravity is from 
2300 to 2800 ; and it is, in some degree, 
magnetic : it scarcely effervesces with acids, 
though partially soluble in them. It easily 
melts per se ; but its most distinguishing pro- 
perty is, that it hardens very suddenly when 
mixed with one-third of its weight of lime 
and water, and forms a cement which is 
more durable in water than any other. 
According to Bergman’s analysis, 100 
parts of it contain from 55 to 60 of silica, 20 
of alumina, live or six of lime, and from 15 
to 20 of iron. Its effects, however, in ce- 
ment, may perhaps depend only on the iron 
which has been reduced into a particular 
substance by means of subterraneous lires ; 
evident signs of which are observable in the 
places where it is obtained, if the slate in 
Hetmeberg or Kennekulle, in the province of 
Westergottland, should happen to get lire, 
the uppermost stratum, which now consists 
of a mixture of iron and different kinds of 
rocks, called graberg in the account given 
of them, might perhaps be changed part- 
ly into slag and partly into terra puzzolana. 
It is evidently a martial argillaceous marl, 
that has suffered a moderate heat. Its hard- 
ening power arises from the dry state of the 
half-baked argillaceous particles, which 
makes them imbibe the water very rapidly, 
and thus accelerates the desiccation of the 
calcareous part. It is found not only in 
Italy but in France, and the provinces of 
Auvergne and Limoges ; and also in England 
and elsewhere. 
PYLORUS. See Anatomy. 
PYRAMID, in geometry, a solid stand- 
ing on a triangular, square, or polygonal 
basis, and terminating in a point at the top ; or 
according to Euclid, it is a solid figure, con- 
sisting of several triangles, whose bases are 
all in the same plane, and which have one 
common vertex. 
Hence the superficies of a given pyramid 
js easily found by measuring these triangles 
separately ; for their sum added to the area of 
the base, is the surface of the pyramid re- 
quired. 
It is no less easy to find the solid content 
of a given pyramid ; far the area of the base 
being found, let it be multiplied by the third 
part of the height of the pyramid, or the 
third part of the base by the height, and the 
^product will give the solid content, as is de- 
monstrated by Euclid, lib. 12. prop. 7. 
If the solid content of a frustum of a py- 
ramid is required, first let the solid content 
of the whole pyramid be found ; from which 
subtract the solid content of the part that 
is wanting, and the solid content of the frus- 
tum or broken pyramid will remain. 
Every pyramid is equal to one-third of its 
subscribing prism, or one that has the same 
base and height. All pyramids are in a ratio 
compounded of their bases and altitudes ; so 
that, if their bases are equal, they are in pro- 
portion to their altitudes ; and vice versa. 
Equal pyramids reciprocate their bases 
and altitudes ; that is, the altitude of one is to 
that of the other, as the base of the one is to 
that of the other. 
Pyramid, in architecture, a solid massive 
building, which from a square, triangular, or 
other base, rises diminishing to a vertex or 
point. 
Pyramids are sometimes used to preserve 
the memory of singular events; and some- 
times to transmit to posterity the glory and 
magnificence of princes. But as they are 
esteemed a symbol of immortality, they are 
most commonly used as funeral monuments. 
Such are that of Gestius of Rome ; and those 
very celebrated pyramids of Egypt, as fa- 
mous for the enormity of their size as their 
antiquity. These are situated on the west 
side of the Nile, almost opposite to Grand 
Cairo; the base of the largest covers more 
than ten acres of ground ; and it is, according j 
to some, near seven hundred feet high, j 
though others make it six hundred, and some j 
but little more than five hundred. The py- j 
ramid is said to have been, among the Egyp- j 
tians, a symbol of human life ; the beginning j 
of which is represented by the base, and the j 
end by the apex ; on which account it was, j 
that they used to erect them over sepulchres. j 
PYRAMIDALIA corpora. See Ana- j 
t omy. 
PYRITES, a genus of inflammable sub- j 
stances, composed of sulphur, which has ; 
dissolved or saturated itself with metals. 1 
Thus there are many kinds of pyrites ; as of 1 
gold, arsenic, iron, &c. It is also the prim- 1 
cipal ore of sulphur ; particularly that called j 
martial pyrites, copperas-stone, or marcasite. j 
This is verv common, containing a quan- 
tity of sulphur in proportion to the iron ; ! 
and, when thoroughly inflamed, burns by it- j 
self. It is either of a compact texture, steel- 
grained, coarse-grained, or crystallised. In 
this last form, it shoots mostly into cube 
and octahedral figures, though it is met with 
also in innumerable other forms. The liver- 
coloured marcasite has an appearance be- 
tween that of the preceding and the blue 
copper-ore. The iron predominates in this 
kind, so that it is less fit than the other for 
extracting sulphur for it, or for the smelting 
of copper ores. It is formed of a compact 
texture, coarse-grained, and steel-grained. 
See Sulphurets, Iron, &c. 
PYROLA, winter-green, a genus of the 
monogynia order, in the decandria class of 
plants ; and in the natural method ranking 
under the 18th order, bicornes. The calyx 
is quinquepartite ; there are five petals ; the 
capsule is quinquelocular, opening at the j 
angles. There are six species, natives of j 
Britain. 
PYROMETER, an instrument for mea- 
suring the expansion of bodies by heat, j 
Muschenbroeck, who was the original in- 
ventor of this machine, has given a table of 
the expansion of the different metals in the 
same degree of heat. Having prepared cv- 
lindric rods of iron, steel, copper, brass, tin, 
and lead, he exposed them first to a pyro- 
meter with one flame in the middle ; then 
with two flames ; and successively to one , 
with three, four, and five flames. But pre- | 
vious to this trial, he took care to cool them j 
equally, by exposing them some time upon 
the same stone, when it began to freeze, ' 
an ! Fahrenheit’s thermometer was at thirty- 
two degrees. The effects of this experi- 
ment are digested in the following table, j 
where the degrees of expansion are marked 
in parts equal to 1-1250011) part of an inch. 
Expansion of 
Iron 
Steel 
Copp. ! Brass 
Tin 
Lead. 
By' 1 flame. 
80 
85 
89 
110 
153 
155 
By 2 flames 
placed close 
together. 
117 
123 
115 
220 
274 
By 2 flames 
2^ inches 
distant. 
109 
94 
92 
141 
219 
263 
By 3 flames 
placed close 
together. 
142 
168 
193 
275 
By 4 flames 
placed close 
together. 
211 
270 
273 
361 
By 5 flames. 
230 
310 
310 
377 
It is to be observed of tin, that it v. ill 
easily melt when heated by two flames placed 
together. Lead commonly melts with three 
flames placed together, especially if they 
burn long. 
From these experiments, it appears at first 
view that iron is the least rarefied of any of 
these metals, whether it is heated by one or 
more flames; and therefore is most proper for 
making machines or instruments which we 
would have free from any alterations by heat 
or cold, as the rods of pendulums for clocks,, 
&c. So likewise the measures of yards or 
feet should be made of iron, that their length 
may be as nearly as possible the same' 
in summer and in winter. The expansion of 
lead and that of tin are nearly the same ; that 
is, almost double of the expansion of iron. 
It is likewise observable, that the tlames pla- 
ced together, cause a greater rarefaction than 
when they have a sensible interval between 
them ; iron in the former case being ex- 
panded 117 degrees, and only 109 in the 
latter ; the reason of which difference is ob- 
vious. 
By: comparing the expansions of (he same 
metal produced by one, two, three, or more 
flames, it appears that two flames do not 
cause double the expansion of one, nor three 
flames three times that expansion, but always 
less ; and these expansions differ so much 
the more from the ratio of the number of 
flames, as there are more flames acting at the 
same time. 
It is also observable, that metals are not 
expanded equally at the time of then- melting, 
but some more and some less. Thus tin began 
to run when rarefied 219 degrees; whereas 
brass was expanded 377 degrees, and yet 
was far from melting. 
Mr. Ellicot found, upon a medium, that 
the expansion oi bars of different metals, as 
nearly of the same dimensions as possible, by 
the same degree of heat, were as follow : 
Gold, .Silver, Brass, Copper, Iron, Steel, 
73 103 95 89 60 56 
Lead 
149 
The great difference between the expan- 
sions of iron and brass has been applied with 
good success to remedy the irregularities in 
pendulums .rising from heat. 
Mr. Graham used to measure the minute 
