ascription of adjectives to the class of concrete terms, upheld by J. S.
Mill, has been disputed on the ground that adjectives are applied both
to concrete and to abstract terms. Hence some logicians make a separate
class for adjectives, as being the names neither of things nor of
qualities, and describe them as Attributive terms.

CONCRETE, the name given to a building material consisting generally of
a mixture of broken stone, sand and some kind of cement. To these is
added water, which combining chemically with the cement conglomerates
the whole mixture into a solid mass, and forms a rough but strong
artificial stone. It has thus the immense advantage over natural stone
that it can be easily moulded while wet to any desired shape or size.
Moreover, its constituents can be obtained in almost any part of the
world, and its manufacture is extremely simple. On account of these
properties, builders have come to give it a distinct preference over
stone, brick, timber and other building materials. So popular has it
become that besides being used for massive constructions like
breakwaters, dock walls, culverts, and for foundations of buildings,
lighthouses and bridges, it is also proving its usefulness to the
architect and engineer in many other ways. A remarkable extension of the
use of concrete has been made possible by the introduction of scientific
methods of combining it with steel or iron. The floors and even the
walls of important buildings are made of this combination, and long span
bridges, tall factory chimneys, and large water-tanks are among the many
novel uses to which it has been put. Piles made of steel concrete are
driven into the ground with blows that would shatter the best of timber.
A fuller description of the combination of steel and concrete will be
given later.

Constituents.

The constituents of concrete are sometimes spoken of as the _matrix_ and
the _aggregate_, and these terms, though somewhat old-fashioned, are
convenient. The matrix is the lime or cement, whose chemical action
with the added water causes the concrete to solidify; and the aggregate
is the broken stone or hard material which is embedded in the matrix.
The matrix most commonly used is Portland cement, by far the best and
strongest of them all. The subject of its manufacture and examination is
a most important and interesting one, and the special article dealing
with it should be studied (see CEMENT), Here it will only b