Latent heat

“ How unphilosophical, too, to talk of latent heat in connection with evaporation, at the lowest temperature known. Meteorologists must revise their opinions on the subject of condensation. This latent heat has never been actually met with; on the contrary, the most sudden and complete condensations of the vapor of the atmosphere are attended by as sudden and extraordinary productions [Pg 318] of cold, and consequent hail, and the connection between condensation and electricity is shown by too many facts to permit the old theory to stand.
Fog never forms with the thermometer below 32°. ”

“ But the calorimeter affords us no more idea of the absolute quantity of heat contained in a body, than the thermometer; for though by means of it we extricate both the free and combined caloric, yet we extricate them only to a certain degree, which is the freezing point; and we know not how much they contain of either below that point.
EMILY.
According to the theory of latent heat, it appears to me that the weather should be warm when it freezes, and cold in a thaw: for latent heat is liberated from every substance that it freezes, and such a large supply of heat must warm the atmosphere ”

“ When energetic motions raise the ascending air to such a height that the temperature, cooled dynamically in ascending, goes below the dew-point, then the great store of latent heat thereby set free becomes, it is assumed, the main source of energy in maintaining the upward convectional movement. The subsidiary causes are attributed to the diminution of pressure on the collapse of the vapor, and also to the direct absorption of the sun's heat at the upper cloud surface. ”

“ Unconscious, perhaps, of the remote tendency of his own labours, he undermined that doctrine of material heat, which he seemed to support. For, by his advocacy of latent heat, he taught that its movements constantly baffle, not only some of our senses, but all of them; and that, while our feelings make us believe that heat is lost, our intellect makes us believe that it is not lost. Here, we have apparent destructibility, and real indestructibility. To assert that a body received heat without its temperature rising, was to make the understanding correct the touch, and defy its dictates. ”

“ The reason of this is, that we cannot feel warm till we have been cold, and vice versâ."
"Our bodies," resumed Fritz, "however much the thermometer descends, never mark less than thirty-five degrees above zero. In winter the skin shrinks, and becomes a bad conductor of heat from without; but, at the same time, does not allow so much gas and vapor to escape from within. In summer, on the contrary, the skin dilates and allows perspiration to form, a process that consumes a considerable amount of latent heat. ”

“ The varying occurrences thus produced, tend to multiply and extend their own variety. The ascending streams of vapour carry with them that latent heat belonging to their gaseous state, which, when they are condensed, they give out as sensible heat. They thus raise the temperature of the upper regions of air, and occasion changes in the pressure and motion of its currents. The clouds, again, by shading the surface of the earth from the sun, diminish the evaporation by which their own substance is supplied, and the heating effects by which currents are caused. ”

“ This heat, which thus ceases to exhibit itself as temperature, is otherwise occupied. Its energy is partly devoted to the work of increasing the bulk of the water to the above-named extent, and partly in conferring on the steam its gaseous specialty—that is, in overcoming liquid cohesion, and substituting for it the opposite property of internal repulsive energy which is characteristic of gases. My reasons for thus defining and separating these two functions of the so-called “latent” heat will be seen when we come to the philosophy of the interesting researches of Dr. ”

“ But heat can also be produced by the friction of liquids, in which there could be no question of changes in structure, or of the liberation of latent heat. ”

“ The impact of a falling stone generates heat; a waterfall is hotter at the bottom than at the top--the falling particles of water, on striking the ground, generate heat; and most chemical changes are attended by heat changes. Energy may remain latent indefinitely in a lump of wood, but in combustion it is liberated, and we have heat as a result. The atom of radium or of any other radio-active substance, as it disintegrates, generates heat. "Every hour radium generates sufficient heat to raise the temperature of its own weight of water, from the freezing point to the boiling point." ”

“ The capacity of a body for heat is called its specific heat. With but few exceptions the specific heats of liquids are much greater than those of solids or gases. It requires ten times the quantity of heat to raise a pound of water one degree that it does a pound of iron. Ice has the greatest specific heat of any of the solids, except paraffin and wood.
When a solid is melted or a liquid vaporized a large amount of heat becomes latent, insensible to the touch; it disappears as heat. ”

“ But because radiation intensity increases with temperature, the warmer object therefore loses heat, and its temperature decreases—part of its heat is transferred to the other object. Conversely, the cooler object receives more radiation than it is emitting and its amount of heat and temperature increases. Eventually, both objects attain the same temperature—their radiant energy gain and loss just balances. Thus, if a substance receives more radiation than it is losing, it gains heat and becomes warmer, but if it is radiating more energy than it is receiving, it loses heat and becomes cooler. ”

“ The thermostat controls the temperature merely by regulating the combustion of fuel; and with the advent of mild weather we let the fires go out altogether. In the body the fires are always burning, the briskness with which they burn—or, to drop the metaphor, the rate at which bodily heat is generated—depending, above all, upon muscular activity, but also upon other causes. It is true that we possess a nervous mechanism, analogous to the thermostat,317 which tends to adjust the production of animal heat in such a manner as to offset the cooling effect of our environment ”

“ Our argument, however, with regard to the increase of heat as we look back is the very opposite of this. It is the essence of a cooling body to lose heat more rapidly in proportion as its temperature is greater. Thus though the one-thousandth of a degree may be all the fall of temperature that our earth now experiences in a twelvemonth, yet in those glowing days when the surface was heated to incandescence, the loss of heat per annum must have been immensely greater than it is now. ”

“ 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 fall of caloric produces more motive power at inferior than at superior temperatures.
Thus a given quantity of heat will develop more motive power in passing from a body kept at 1 degree to another maintained at zero, than if these two bodies were at the temperature of 101° and 100°.
The difference, however, should be very slight. It would be nothing if the capacity of the air for heat remained constant, in spite of changes of density. ”

“ Before this equality of temperatures, however, is reached, the hotter body will radiate more heat waves than it absorbs, while the colder body will absorb more heat waves than it emits. All bodies, whatever their temperature, are incessantly radiating heat waves. This may be proved experimentally with proper apparatus, as for example with an instrument known as the thermopile. When, however, the total heat waves radiated out by a body are less than it absorbs, the body gets gradually colder, and the temperature decreases. ”

“ A generation or so ago physicists were content to say that aqueous vapor turns to drops of water in the air merely on account of being cooled. The question of how the drops get their start, or why the moisture forms drops at all, does not seem to have troubled them.
One way in which air or any other gas is cooled is by expanding against pressure. Some of the energy in the gas, originally manifesting itself as heat, is applied to the work of expansion, and thus ceases to be heat. Hence the temperature of the gas falls. ”

“ That is to say, as evaporation always diminishes temperature, so the perspiration, as it passes off in the form of fine vapor, cools the surface. Accordingly, in hot weather this function is much more active, and the cooling influence increases in proportion. When the air is already charged with moisture, and does not readily receive this vapor of the body, the heat of the atmosphere apparently increases, and the discomfort therefrom is relatively greater. ”

“ The vapor of water employed in an inverse manner to that in which it is used in steam-engines can also be regarded as a means of destroying the equilibrium of the caloric. To be convinced of this we need but to observe closely the manner in which motive power is developed by the action of heat on vapor of water. Imagine two bodies A and B, kept each at a constant temperature, that of A being higher than that of B. These two bodies, to which we can give or from which we can remove the heat without causing their temperatures to vary, exercise the functions of two unlimited reservoirs of caloric. ”

“ For the air resting on the heated surfaces is warmed by convection, rises and gives place to cooler particles, causing a constant heating of its particles by movement. Thus in a room with an open fire, the person is warmed in part by radiation from the fire and the surrounding walls and furniture, and in part by the warm air surrounding the body.
In regard to the warmth of air, the thermometer is not an exact index of its temperature. For all bodies are constantly radiating their heat to cooler adjacent surfaces until all come to the same temperature. ”

“ The capacity of space to hold moisture increases with its temperature, and hence any lowering of this temperature will reduce the capacity. If lowered sufficiently, the point of complete saturation will be reached and further cooling must result in precipitation. Hence, anything which forces an air current to rise into more rarefied zones above, will lower the pressure upon it and so bring about a cooling effect in which no heat is abstracted. ”

“ Materials with high heat capacities can absorb and lose large quantities of heat without much temperature change. Conversely, relatively little heat is needed to change the temperature of materials with low heat capacity. At ordinary temperatures, the heat capacity of air is about 0.017 Btu/ ft3/°F—little heat is needed to change its temperature. ”

“ If one pint pot of boiling water be placed in out-of-door air that is cold as freezing water, and another be placed at the same time in a room where the air has the warmth of a mild summer day, the former will be deprived of all its excess of heat much sooner than the latter.
Warm bodies lose their excess of heat in two ways. They shoot it off into surrounding space. This is what learned men call “raying” or “radiating” it away. The sun, you know, shoots or rays its heat off to the earth, and so does the fire to your body when you stand before it. ”

“ When the shortwave radiation strikes a solid object, such as the earth's surface or something on it, a large part of the radiant energy is converted to heat, warming the object. The heated object radiates energy too, but mostly in long waves because of the object's relatively low temperature. Water vapor and carbon dioxide in the air can absorb the longwave radiation, thereby raising the temperature of the air. ”

“ One could even conceive the bodies A and B maintaining the same temperature, although they might lose or gain certain quantities of heat. If, for example, the body A were a mass of steam ready to become liquid, and the body B a mass of ice ready to melt, these bodies might, as we know, furnish or receive caloric without thermometric change. ”

“ Now, if the sun were a cool, solid, or even liquid mass, the sudden accession of merely this quantity of heat would undoubtedly produce an enormous rise of temperature and a great increase of radiation. But, constituted as the sun is—mainly a mass of gas and vapor—the effect would be entirely different, the energy being principally expended in producing expansion and evaporation, with comparatively little increase of temperature or radiation. If one stirs up the fire under an open kettle, the water gets no hotter—it only boils faster. ”

“ 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. ”