MATTER, in physics, mechanics and chem istry, the tangible substance of which the world external to our minds is composed and which is characterized by the resistance which it opposes to muscular exerted upon it. Since we can know it only through the mediation of our senses, its actual objective reality has often been questioned by philosophers and metaphysicians; and in order to avoid doubtful points of this sort it is frequently defined merely as a °phys ical concepts without making any hypothesis as to its objective existence. (Consult Pearson, (The Grammar of In physics and chemistry we think and speak of it as having a real existence, however, just as we do in common life. For scientific purposes it is highly important to be able to accurately com pare the quantities of matter in two bodies; but the phrase "quantity of matter" is not com monly used in science, the word "mass" (or inertia) being substituted for it. The term "mass") is synonymous with "quantity of matter" so long as the bodies compared are identical in composition and in physicalstate; but by i its use we avoid the uncertainty the meaning of the longer phrase, when it is applied to bodies of dissimilar composition. Two bodies are said to have "equal masses" under the fol lowing conditions: Let the two bodies be con ceived to be initially at rest, and free from. the action of any external forces whatsoever. Let them then be exposed to the action of equal forces for an equal length of time. If, under these circumstances, the velocities that are produced in the two bodies are equal, the masses of the bodies are also equal, by defini tion. But if the velocities that are produced are unequal, then the masses are also unequal, and are (by definition) proportional to these velocities; the body which is moving the faster being the one whose mass is the less. It would be to carry out an elaborate ex periment of this kind every time we wished to compare the masses of a pair of bodies, and it fortunately happens that masses can be com pared with great accuracy by merely weighing the bodies against each other in a balance. For equality of i weight means that the earth's attrac tive force is the same upon both; and if (as we know to he the case, by experiment,) the two bodies would fafl with the same speed in a vacuum, it follows that equal forces acting upon the two bodies for equal times communicate to them equal velocities; and hence, by the defini tion of ((mass," the mass of the two bodies are equal. It will be noted that mass and weight are two essentially different things, although they are very commonly confused, because they are strictly proportional to each other, accord ing to the most accurate experiments that have i yet been made. Yet it is not at all impossible to imagine a state of things in which mass and weight would not be strictly proportional. It is
only by experiment, for example, that we know that the earth attracts a body with precisely the same whether the body is hot or cold. The attraction between two permanent magnets varies with the temperature, and it may, yet be found that gravitative attraction varies in the same way, though to a much smaller extent. Numerous experimenters have in fact fancied that they could detect an effect of this kind, though its reality is not yet admitted. But we can hardly admit that the mass of a body is a function of the temperature; and so we see that the apparently strict proportionality between mass and weight is not at all a necessity of thought, but merely an experimental fact, winch may any day be shown to be nothing but a close approximation to the actual truth.
So far as we know, matter can neither be de stroyed nor created by any experimental means at our command, nor by any process now oper ative in nature. This great fact is often called the "law of the conservation of matter," and it appears to be rigorously true. As has bees pointed out above, future experiment may show that the weight of a body depends to a slight extent upon its temperature, and it may also be found that the weight of a chemical compound is not always precisely equal to the sum of the weights of its constituents; but it is believed that any irregularities of this sort that may be discovered will hold true to the weight only, and will not affect the mass; and that the weight will also return to its original value when the substance that is weighed is brought back to the same chemical and physical condi tion again. The gravitative action of matter is one of its most singular attributes, and one which we are still far from understanding. It is apparently true that every particle of matter attracts every other particle with a force which is directly proportional to the product of the masses of the two particles and inversely proportional to the square of the distance be tween them, and independent of every other circumstance. It is not certain that this law holds true, nor that the attraction exists at all, at distances comparable with the distance from the earth to the fixed stars, but it appears to hold rigorously at distances commensurate with the dimensions of the solar system. Electric and magnetic forces may be either attractive or repulsive; but gravitative action is probably always attractive, the only phenomena which would countenance the opposite view being the apparent repulsion that the sun exerts upon the tails of comets and upon the coronal streamers that are seen at the time of a total solar eclipse.