HEAT, the unknown cause of the sensation ofwarmth, and of a multitude of common phenomena in nature and art. In considering this subject scientifically, it is necessary, at the outset, to discard the ideas conveyed by the popular use of such words as heat and cold. A number of bodies, however different, left for a long enough time in the same room, must, as we shall see further on, acquire the same temperature, or become in reality equally warm. Yet in popular language, some, as metals, stones, etc., are pronounced to be cold, and others, as flannel and fur, warm. The touch, then, is not a means by which we can acquire any definite idea of the temperature of a body.
Nature of heated body is no heavier than it was before it was heated; if, therefore, heat be a material substance, as it was long considered, it must be imponder able. And, in fact, under the name of caloric or phlogiston, it is classed, in almost all but modern treatises, as one of the family of imponderables. But if it were matter, in any sense of the word, its quantity would be unchangeable by human agency. Now we find that there are cases in which heat is produced in any quantity without flame, combustion, etc., as in melting two pieces of ice by rubbing them together, and also cases in which a quantity of heat totally disappears. This is utterly, inconsistent with the idea of the materiality of heat. The only hypothesis that at all accords with the phenomena is, that heat is a form of motion, and with this idea we shall start.
Measure of it bee vibration, such as light and sound (in some cases, it certainly is), or consist in a succession of impacts of the particles of bodies on each other (as in some cases it has been considered to be), it is none the less certain that the amount of heat in a body is to be measured by the vis-viva (see FoncE) of moving par ticles. But as we cannot observe those particles so as to ascertain their vis-viva, we must have some means of measuring the temperature of a body, depending upon an of heat. Whatever that effect maybe, it is obvious that, as the laws of nature are uniform, it will afford us a reproducible standard, by which we can estimate its amount' at any time and in any place, and compare that amount with another observed some.
where else; just as the French metre (q.v.) is reproducible at any time, being the ten millionth part of a quadrant of the meridian.
Dilatation or the most general and notable effect which heat pro duces on matter W. to exfiand it. The length of a metallic bar varies with every change of temperature, and is ever the same at the same temperature. The fixing of the tire of a cart-wheel is a very good instance. No hammering could fit an iron hoop so tightly on the woodwork of the wheel, as the simple enlarging of the tire by heat, and its subse quent contraction by cold. It is thus possible to slip it on, and an enormous force is secured to bind the pieces together. In almost every kind of structure, the expansion and contraction from changes of temperature require to be guarded against. In the huge iron tubes of the Britaunia bridge, the mere change of the seasons would have produced sufficient changes of length to tear the piers asunder, had each end of the tube been fixed to masonry. Watches and clocks, when not compensated (see PENDULUM), go faster in cold weather, and slower in hot, an immediate consequence of the expansion or contrac tion of their balance-wheels and pendulums.
If a flask full of water or alcohol be dipped into hot water or held over a lamp, a por tion of the liquid runs over; a glass shell which floats in a vessel of water, sinks to the bottom when the water is heated; and as water is heated, the hotter water continually rises to the surface. Indeed, if the latter were not the case, it would be impossible to prevent explosions every time we attempted to boil water or any other fluid. If a blad der, partly tilled with air, and tightly tied at the neck, be heated before a fire, the con tained air will expand, and the bladder will be distended. As it cools, it becomes flaccid again by degrees.