Porbeagle

ted among the effective ;me groups, and with a
relatively small proportion of infants and de-
pendents represented. An increasing popula-
tion resulting from a high birth rate is actually
less desirable than a low death rate and a grea-
er average duration of life. At the present time,
the increase of population is of less concern
than its character, and sociologists and econo-
mists are investigating carefully the great
problems arising from the rapid development
of densely populated centers. There is no abso-
lute standard of population or over-popula-
tion. The density of population—the average
number of persons per square mile—may in-
crease without causing serious suffering, pro-
vided the opportunities to secure subsistencc
increase correspondingly. A certain density of
population is necessary to really effective social
and political life; while an excessive density is
detrimental to the health, and destroys the
comfort of the inhabitants.

Porbeagle (Lamma cornubica}, a shark
which occurs in the North Atlantic Ocean, and
which is known among American fishermen as
a mackerel shark. It reaches a length of ten
ft., and feeds chiefly on fish.

3790________________________Porphyry

Porch, a covered space immediately in
front of the entrance to a building, open in
front, and more or less enclosed at the sides.
A porch is only a subordinate part of a build-
ing, whereas the1 portico may be the whole of
a front.

Porcupine, a family of rodents. In Eu-
rope, Asia, and Africa occur the members of
the genu< Hystrix. The most familiar species
is //. cri\tatut found in S. Europe and N.
Africa. The American porcupines differ in
several respects from the Old World forms;
they are all arboreal in their habits, and with
the exception of the northern forms (Ercthi-

Porbeagle.

Porcelain, the material of the highest class
of ceramic ware, is composed of a kind of clay
(kaolin) mixed with feldspar and covered with
a glaze. It is frequently decorated cither under
or over the glaze. By porcelain is now gen-
erally meant hard porcelain, the secret of
which was discovered in China.

The process that distinguishes porcelain
from pottery is the hard firing. In Chinese
porcelain the body of kaolin and the glaze of
feldspar are fired together at a temperature of
about 1,400° Centigrade, or over 3,000° Fah-
renheit. In European porcelain the body, of
mixed kaolin and feldspar, is fired at a low
temperature; then, having been dipped in the
glaze, it is fired at a very high temperature.

The materials of porcelain were discovered
in Europe in 1709 by Bottler, chemist to the
Elector of Saxony, who established the Dres-
den manufactory. The Sevres factory, estab-
lished by royal decree, began to make porce-
lain in 1768, while factories were established
in the towns of Plymouth and Bristol in Eng-
tad.

\orth

on it pine.

zlon dvrstitits and K. <'/?Aw/////,s), all possess
prehensile tails. In the common porcupine of
Eastern North America the spines are short,
and are concealed by the long hair. They are
also easily detached, and are slightly barbed
at the points. The tuil is broad and massive,
and is the chief active means of defence, the
animal delivering powerful lateral blows with
it.

Porgy. See Scup.

Pork, the flesh of the pig, exclusive of bacon
and ham. Bacon is the sides and back of the
animal and ham the thigh, both being salted
and smoked.

Porosity, a term indicating the presence of
minute holes or pores throughout an otherwise
solid body. Pores may vary much in size: thus
in sponge and pumice stone the pores are ob-
vious; but in charcoal and ungluzerl earthen-
ware they require to be demonstrated by the
way in which such substances can suck up
liquids or gases, or allow the passage of liquids
or gases, through them. Porosity is made use
of to absorb liquids and gases, as in the use
of blotting-paper, or charcoal for filtration of
liquids, and in separating gases by diffusion.

Porphyry, u beautiful igneous rock show-
ing bright-mi spots on a dark-red ground.
The parfido rasao (tntwo was much admired
by the ancients, who used it for interior dec-
orations and objects of art. It? color is due to
the presence of a red nr pink variety of cpi*
dote,. The rock itself would at the present day
be classed rather among the porphy rites than
the porphyries, as its feldspar is mainly phurio-
clase; fc contains also dark-brown hornblende,