We know nothing of the elements of bodies, but we know some of their properties or qualities; and we distinguish their various matter by the effect or change produced on our senses; that is to say, by the variety of motion their presence excites in us. In consequence, we discover in them, extent, mobility, divisibility, solidity, gravity, and inert force. From these general and primitive properties flow a number of others, such as density, figure, colour, ponderosity, &c. Thus, relatively to us, matter is all that affects our senses in any manner whatever; the various properties we attribute to matter, by which we discriminate its diversity, are founded on the different impressions we receive on the changes they produce in us. A satisfactory definition of matter has not yet been given. Man, deceived and led astray by his prejudices, formed but vague, superficial, and imperfect notions concerning it. He looked upon it as an unique being, gross and passive, incapable of either moving by itself, of forming combinations, or of producing any thing by its own energies. Instead of this unintelligible jargon, he ought to have contemplated it as a _genus_ of beings, of which the individuals, although they might possess some common properties, such as extent, divisibility, figure, &c. should not, however, be all ranked in the same class, nor comprised under the same general denomination. An example will serve more fully to explain what we have asserted, throw its correctness into light, and facilitate the application. The properties common to all matter, are extent, divisibility, impenetrability, figure, mobility, or the property of being moved in mass. FIRE, beside these general properties, common to all matter, enjoys also the peculiar property of being put into activity by a motion that produces on our organs of feeling the sensation of heat; and by another, that communicates to our visual organs the sensation of light. Iron, in common with matter in general, has extent and figure; is divisible, and moveable in mass: if fire be combined with it in a certain proportion, the iron acquires two new properties; namely, those of exciting in us similar sensations of heat and light, which were excited by the element of fire, but which the iron had not, before its combination with the igneous matter. These distinguishing properties are inseparable from matter, and the phenomena that result, may, in the strictest sense of the word, be said to result necessarily.