Matter. It is impossible to give a really satisfactory definition of this term. We may employ as equivalents such words as Stuff, Substance,
Body, &c., but all are inadequate. The reason is simply that we do not yet know what Matter is, and it is probable that we shall never be able to obtain an exact and complete conception of its true nature. Metaphysicians differ among themselves more perhaps on this subject than on almost any other. Some of them deny altogether the possibility of objective existence. Many, however, tell us that 'Matter is whatever can be perceived by the senses.' Others vary the phrase slightly, and call matter a 'Permanent possibility of sensation.' Hegel defines it as 'Nature self-externality in its most universal form, with a tendency to self-internality or individuation shown in the nius of gravitation'! Scientific men can, as yet, define matter only by some of its properties. One of their favourite definitions is Matter is what can occupy space. Another is that which possesses inertia. Again, it may be regarded as the vehicle of Energy (q.v.), inasmuch as energy is never found except in association with matter. But the scientific man, though confessing that none of his definitions can be adequate, knows that each of them expresses some part of the truth; and he also knows that the metaphysical definitions cited above (so far at least as they are intelligible) are erroneous. For it is by the senses alone that we know of the existence of energy, and energy is certainly not matter. Again, the notion of Force (q.v.) is entirely sense-suggested, and force is not matter—has not even objective existence.
But, though the ultimate nature of matter is unknown, we already know much about its structure and properties, as well as about what (for the present at least) we must call its various kinds. To a brief sketch of these the present article is devoted. Beside that unique and all-pervading species of matter which we call the Ether (q.v.), which certainly satisfies the scientific definitions quoted above, but about which we know little more, we have what is called gross or ordinary matter. This we find in two forms, solid or fluid. These are sharply distinguished from one another by their elastic properties. For, while solids (as a rule) possess, in a more or less imperfect degree, elasticity alike of bulk and of form, fluids possess the first in perfection and are absolutely devoid of the second. Fluids again are divided into liquids, vapours, and gases. These distinctions, in the case of any one substance, as well as that between solids and fluids, are found to depend mainly upon temperature.
The existence of the gaseous state, with its very special features, has enabled us to obtain great insight into the structure of matter. For experiment has assured us that a gas is not a continuous substance, but an assemblage of an enormous number of perfectly distinct and independent particles, each of which moves freely till it collides with another, and thus some eight thousand million times per second has its motion completely changed. The number of such separate particles in a single cubic inch of air contains twenty-one figures—i.e. is expressed in hundreds of trillions. Yet they are very far from filling that space. Their total bulk probably amounts to less than the five-hundredth part of it. We are unable to discover any differences among the particles of the oxygen group, or among those of the nitrogen; though, by delicate processes not involving chemistry, we can detect a difference between the properties of an oxygen and of a nitrogen particle.
As all known simple substances can be brought into the gaseous form, we have a proof that every simple solid or fluid must be built up of particles absolutely equal to one another. We figure to ourselves what we call Molecular Forces (q.v.) as the cause of the agglomeration, and ascribe the various states of solid, liquid, vapour, and gas in any one substance to the greater or less relative activity of the molecular forces (attractive) and of the thermal motions (disjunctive or dispersive). The modern kinetic theory of Gases (q.v.) has thus enabled us to account for at least the more simple of their physical properties, such as the experimental relations among pressure, volume, and temperature, known as the laws of Boyle and Charles; and the equality of numbers of particles in a cubic inch of each of two gases at the same temperature and pressure, known as Avogadro's Law; as well as to study the mechanism of gaseous viscosity, diffusion, and heat-conduction. Much has already been done towards the explanation of the 'critical temperature' and the vapour state with its relation to that of liquid, and further progress may soon be expected. On the other hand, a great deal of information as to the liquid aggregate has been obtained from experiments on Capillarity (q.v.) and Compressibility (q.v.); and as to the solid aggregate from its elastic properties, &c., but specially from the forms of crystals (see CRYSTALLOGRAPHY). Besides the molecular forces-mentioned above, which are generally understood as those exerted between particles of the same kind, we have those of Chemical Affinity, which are exerted between any two particles of different kinds. Physical experiments, following those of Andrews on the compressibility of gaseous mixtures, promise to give us much information on this subject; but, in the main, it is at present more immediately in the domain of Chemistry (q.v.), to which we must refer for the discussion of Atomic Weights, Combining Volumes, Valency, &c.
Beyond the state of equal independent particles, as in a simple gas, we know as yet nothing. Many of the properties of the individual particles can be obtained from the properties of the aggregate, others by the help of Spectrum Analysis (q.v.). But in answer to the question, Do the particles of different simple substances consist of one and the same ultimate material, or no? intensely attractive as it is, we have absolutely nothing to say. So it is with the question, Are these particles themselves further divisible, or are they atoms? Atoms (q.v.), whether Lucretian or Vortical, are not even proved to exist. We must, therefore, in further discussing the subject, content ourselves with a few brief statements as to the Properties of Matter.
One of the most remarkable of these, what has been called Conservation of Matter, is the experimentally ascertained fact that no process at the command of man can destroy even a single particle of matter. Still less can it create a new one. It is on this definite basis that the great science of chemistry has been securely built. The Balance (q.v.), used to determine quantity of matter with the utmost precision, is its chief instrument. And this attribute of unchangeable quantity furnishes the most powerful of the arguments for the objective reality of matter.
Quantity of matter, or mass, as it is technically called, is measured by Inertia, which (as expressed in Newton's First Law of Motion) may be looked on as the fundamental property of matter. For it is a property possessed by every body, even a particle, in itself, and independently of the vicinity or even the existence of any other body. It is in virtue of its inertia that a body can possess energy of motion, and that work is required in order to set in motion even the smallest particle of matter. Similarly, until it can transfer its energy to some other body, a moving mass must continue to move.
Next in order of simplicity to inertia, which, as we have seen, is a property of every single particle, come the properties in virtue of which any system of two bodies, even if they be mere particles, possesses energy depending directly on the mass of each, and also upon their distance from one another. This part of the energy of a system gives rise to the phenomena of Gravitation (q.v.), Molecular Action, and Chemical Affinity. It has been shown by Sir W. Thomson that the first of these might suffice to account for the second if not the third (at all events in aggregates of particles), provided the structure of the aggregate were sufficiently heterogeneous. Be this as it may, we know much more about gravitation than about the other phenomena referred to, and will therefore confine our further remarks to it. And yet all that we know about gravitation can be summed up in the following statement: The potential energy of a system of two particles of matter is less when they are at a finite distance apart than when they are infinitely distant from one another, by an amount which is directly as the product of their masses and inversely as their distance apart. This statement, it is to be particularly observed, contains no allusion to attraction or (so-called) force of any kind; yet it suffices for the complete formation of the equations of motion of any system of gravitating masses, be it as complex as the solar system itself. The rest of the calculation is a matter of mathematics and of numerical data alone. Many attempts, often extremely ingenious, but all alike fruitless, have been made to explain gravitation. Such failure, however, in the eyes of a genuine scientific man is only an encouragement to perseverance; and the very remarkable success which has attended Clerk-Maxwell's attempt to explain electric and magnetic phenomena by means of the luminiferous medium renders it at least probable that the properties of the ether will, some day, explain gravitation—possibly inertia also. Mere speculation, of course, is of use in science only in so far as it originates or directs inquiry, so that we must be content simply to express the idea that the ether may be the one material substance in the universe (urstoff), gross matter being simply differentiated portions of it—denser or less dense than the rest, perhaps mere cavities or bubbles. If so, the words of Fresnel may in time be verified: 'La Nature ne s'est pas embarrassée des difficultés d'analyse; elle n'a évité que la complication des moyens.'
The various properties of matter, discussed for the most part under their several heads, may be roughly divided into two classes—those which belong more particularly to matter in itself, and those which are specially related to various forms of energy. Among the former class may be mentioned, but only as examples, Capillarity, Cohesion, Compressibility, Density, Elasticity, Friction, Gravitation, Hardness, Inertia, Impenetrability, Malleability, Plasticity, Rigidity, Tenacity, Viscosity. Among the latter we have Colour, Absorptive Power, Transparency, Refractive and Reflective Power, Melting and Boiling Points, Specific Heat, Latent Heat, Conductivity (Thermal and Electric), Thermo-electric Power, Expansibility, Specific Inductive Capacity, Magnetic Permeability, &c. Even to name all the more important would greatly exceed our limits. See also MOLECULE.