Sensible heat

“ Again, let the required nature be heat or cold, of such a degree as not to be sensible from its weakness. They are rendered sensible by the thermometer, as we described it above; [139] for the cold and heat are not actually perceived by the touch, but heat expands and cold contracts the air. Nor, again, is that expansion or contraction of the air in [222] itself visible, but the air when expanded depresses the water, and when contracted raises it, which is the first reduction to sight. ”

“ Arrived in the cloud-region, this vapour, being condensed, liberates the latent heat which it borrowed from the air and water below; which heat, being now set free and made sensible, raises the temperature of the surrounding air, causing it to expand and ascend still higher ”

“ Aqueous vapor in the atmosphere, whether its presence is rendered sensible to the sight or not, implies the action of heat. Other things being equal, the greater the heat the greater the quantity of watery vapor in the air. In the summer, for instance—though many imagine the contrary—there is much more of such vapor in the air than there is in winter, the greater heat of the air enabling it to keep a greater quantity of the vapor in the invisible form. ”

“ Fifthly, The continued impression of heat makes us more sensible of cold. For when heat has rarefied the parts, and adapted them to a peculiar strength and frequency of vibrations, differing from the usual standard, the cold, whose difference from the usual standard lies on the other side, must raise a greater conflict, and produce a greater change, than if the parts had remained at the usual standard. The continued impression of cold must for the same reasons make us more sensible of heat. ”

“ Going out into the open air, in the temperate, and warm 324 months of the year, we often meet with bodies of warm air, which passing by us in two or three seconds, do not afford time to the most sensible thermometer to seize their temperature. Judging from my feelings only, I think they approach the ordinary heat of the human body. ”

“ Black the capacity of bodies for heat, is summed up in the facts, that different bodies contain different quantities of heat in the same bulk or weight; and different quantities of heat are required to raise different bodies to the same sensible temperature. ”

“ The first truth to be comprehended is, that all matter contains within its substance the power of creating heat—or as it may be expressed more plainly, solids, fluids, and gases contain what is termed latent or insensible heat, in contradistinction to the heat which is apparent when we touch a vessel containing warm water or approach a cheerful fire ”

“ In our demonstrations we tacitly assume that after a body has experienced a certain number of transformations, if it be brought identically to its primitive physical state as to density, temperature, and molecular constitution, it must contain the same quantity of heat as that which it initially possessed; or, in other words, we suppose that the quantities of heat lost by the body under one set of operations are precisely compensated by those which are absorbed in the others. ”

“ That this heat is furnished by the air, he proves, by finding, that that which we inspire contains more heat than that which we expire, or than the aqueous humor which we expire along with it, in a very considerable proportion; so that if the heat contained in the pure air did not become latent in the blood, it would raise its temperature higher than that of red-hot iron. ”

“ On the one hand it was held by the best-known physicists that heat is a substance which pervades all bodies in greater or less amount, and that heating and cooling are simply the absorption and the rejection of this “imponderable substance” by the body affected; while, on the other hand, it was asserted by a small but increasing number that heat is a “mode of motion,” a form of energy, not only imponderable, but actually immaterial; a quality of bodies, not a substance, and that it is identical, in its nature, with other forms of recognizable energy, as, for example, mechanical energy. ”

“ Under favorable conditions—that is, when the differences of temperature and moisture and the supply of warm, moist air at the surface are great—this action becomes very intense, and this intensity is still further increased by the fact that as the air rises its moisture is condensed, the latent heat thus liberated adding to the energy of the rising column of air. ”

“ If you take a red-hot ball out of a furnace and lay it down upon a frosty moor, two processes will go on—the ball will lose heat and the surrounding atmosphere will gain it. There are two ways by which you equalise the temperature of a hotter and a colder body: the one is by the hot one getting cold, and the other is by the cold one getting hot. If you are not heating the world, the world is freezing you. ”

“ Since solid bodies are susceptible of little change of temperature through changes of volume, and since the condition of the most effective employment of heat for the development of motive power is precisely that all change of temperature should be due to a change of volume, solid bodies appear but ill fitted to realize this power. ”

“ Vast ridges and crests of incandescent vapour are upheaved by the action of internal heat, which exceeds in intensity the temperature at which the most refractory of terrestrial substances can be volatilised ”

“ The approach toward a hot body increases heat in proportion to the approximation; a similar effect to that of light, for the nearer any object is placed toward the light, the more visible it becomes. 34. The union of different heats increases heat, unless the substances be mixed; for a large and small fire in the same spot tend mutually to increase each other's heat, but lukewarm water poured into boiling water cools it. 35. The continued neighborhood of a warm body increases heat. ”

“ We have already remarked upon this self-evident fact, or fact which at least appears evident as soon as we reflect on the changes of volume occasioned by heat: wherever there exists a difference of temperature, motive-power can be produced. Reciprocally, wherever we can consume this power, it is possible to produce a difference of temperature, it is possible to occasion destruction of equilibrium in the caloric. ”

“ Straw, sawdust, wool, feathers, confined air, are insulating substances which retain heat. We might surround with a double envelope of this kind the reservoir holding the sun-heated stones, and in this way we might have our store of solar heat, as now we have our store of ice. It is one problem whether we have to retain cold or to retain heat. ”

“ This indefatigable investigator of the heavenly phenomena shows, by many substantial proofs, drawn from natural philosophy, that heat is produced by the sun’s rays only when they act on a calorific medium; and that they cause the production of heat by uniting with the matter of fire which is contained in the substances that are heated,—as the collision of flint and steel will inflame a magazine of gunpowder, by uniting with its latent fire, and bring the whole into action. ”

“ If, in place of compressing the air, we allow it to expand to two litres, four litres, eight litres, etc., it will be necessary to supply equal quantities of heat to maintain the temperature always at the same degree. ”

“ When we now come to assemble these partial inductions so as to raise from them a general conclusion, we consider, 1st, That all the conclusions we have come to have a reference to that first general fact—the cooling of the exposed surface of the body dewed below the temperature of the air. Those surfaces which part with their heat outwards most readily, and have it supplied from within most slowly, will, of course, become coldest if there be an opportunity for their heat to escape, and not be restored to them from without. Now, a clear sky affords such an opportunity. ”

“ An objection to the theory has likewise been urged, from the well-known property and tendency of heat to become equally diffused through all the particles of any body in which it exists. Thus, it is an established principle, that heat not only diffuses itself on all sides, but passes continually from bodies in which the temperature is greater to those in which it is less ”

“ The caloric fluid was considered intangible, since even the most careful experiments in adding or subtracting heat by nondestructive heating or cooling of a substance failed to produce any changes in its weight.
Heat is energy We know now that heat is not a fluid, nor a substance at all, but is really one of the several forms of energy. ”

“ But what can we do when the surrounding temperature is higher than blood heat? The outside heat may be kept from flowing in by surrounding ourselves with heat-insulating clothing, but the internal heat then has no means of radiating away from our bodies; it accumulates, and we become overheated. ”

“ Very often, too, aqueous vapour as well as air is carried up by means of the sun's heat to the upper and colder atmospheric regions, and there deposited in the shape of rain, or hail, or snow, which ultimately, finds its way back again to the earth, often displaying in its passage immense mechanical energy. ”

“ The resistance of the surfaces is so great that little elevation of temperature is experienced. Professor Plücker has stated, that by washing the arm with ether previously to plunging it into melted metal, the sensation produced while in the molten mass is that of freezing coldness.—R. Hunt, F.R.S.
PROTECTION FROM INTENSE HEAT.
The singular power which the body possesses of resisting great heats, and of breathing air of high temperatures, has at various times excited popular wonder. In the last century some curious experiments were made on this subject. ”

“ If we reflect that heat is essentially distributive in its nature, we shall soon perceive the reason for this peculiar law; for, in virtue of its nature, heat is always rushing from a body of high temperature to one of low, and if left to itself it would distribute itself equally amongst all bodies, so that they would ultimately become of the same temperature. ”

“ Rise of temperature, expansion, liquefaction, evaporation. The first indication of the presence of heat is discovered by an elevation in temperature. Though man is not a reliable thermometer, he would be able, ordinarily, even if blind, to chronicle the progress of the sun, from horizon to horizon, by the increasing and decreasing warmth. The little thermometer placed beneath the tongue of the invalid gives reliable report of the combustion going on within his system. We see a thousand illustrations of the expansive effects of heat, many of which are familiar to all. ”

“ The gentlest summer breezes and the fiercest blasts of winter are produced by the unequal heating of air. We have seen that the air nearest to a stove or hot object becomes hotter than the adjacent air, that it tends to expand and is replaced and pushed upward and outward by colder, heavier air falling downward. We have learned also that the moving liquid or gas carries with it heat which it gradually gives out to surrounding bodies.
When a liquid or a gas moves away from a hot object, carrying heat with it, the process is called convection. ”

“ However, we find that when a body is melting, vaporizing, or otherwise suffering an abrupt change in physical properties, a quantity of heat is absorbed which disappears without changing the temperature of the body. ”

“ When heat is applied to a substance, the molecular activity increases and the temperature rises. Conversely, if a substance loses heat, the molecular activity decreases and so does the temperature. We can think of heat, then, as the energy of molecules in motion.
Energy cannot be created or destroyed in any way that we know. But it can be changed from one form to another, and this transformation is constantly going on, both in nature and through the activities of man. ”

“ The generation of heat is a mystery, but enough of the theory of caloric has already been developed to induce us to acquiesce in the notion that it will hereafter, and perhaps at no very distant period, be possible to produce heat at will, and to warm the most ungenial climates as readily as we now raise the temperature of our apartments to whatever degree we may deem agreeable or salutary. ”

“ The warmer will part with its heat to the cooler, and the latter will in like manner reduce the temperature of the former. By covering, then, the surface of the body, it is prevented from giving its heat directly to the air, for the clothes intercept it by absorbing the heat themselves. ”

“ His assumption of the materiality of heat led, logically, to the conclusion that the same quantity of heat was finally stored in the refrigerator as had, initially, left the furnace, and that the effect produced was a consequence of a fall of temperature analogous to a fall of water ”

“ The atmosphere, then, is heated or cooled according as it lies upon a warm or cold part of the earth’s surface; and, by means of its aqueous vapor, it serves to store up and distribute this heat, keeping the earth from such extremes of climate as would otherwise prevail.
The air lying next to a hot surface is heated; the air touching a cold surface is cooled. And upon such differences of temperature in the air the formation of winds depends.
Hot or warm air is lighter than cold air. You have learned how heat expands bodies. ”