Matter. In philosophy (Aristotle's hyle), that which is in itself not definite, but by receiving a form becomes a substance. In physics, ithas been urged that only two kinds of things are known to exist in the physical world; of these one is matter, and the other energy. Both are indestructible; by no conceivable processcan the quantity of matter in the universe be increased or diminished by a single ounce, and this. pioperty is known as the conservation of matter.
Matter is characterised by its inertia; it always tends to remain in the same state, either of rest or of uniform motion in a straight line, unless some force compels it to change that state. This was. enunciated by Newton, and is known as his first law of motion. The most popular idea of matter is that it is something which has size, which can occupy space; and regarding the ultimate structure of matter many hypotheses have been propounded. The Greek philosophers more than 2,000 years ago conceived that matter could be subdivided into particles smaller and smaller until an indivisible atom was reached. That these atoms were hard, with space between them, was accepted by Newton r while Boscovich eliminated the material property of the atom altogether, and considered it to be nomore than a mathematical point acted on by forces. This theory was not accepted for long, since it was unable to account for the property of inertia. Another theory assumed matter to be continuous not separated into distinct particles. The most modern theory, propounded by Sir W. Thomson and investigated by Helmholtz, imagines matter to be merely the rotating parts - vortex atoms - of some fluid which possesses inertia and entirely fills space; this vortex theory is, however, still in its infancy. Some properties of matter are familiar to all, such as its weight, divisibility, cohesion, plasticity, ductility, viscosity, rigidity, elasticity, transparency, colour, capillarity, etc.