Isotherm

“ Thus, when a gas is absorbed by water, much heat is evolved, and we may suppose the result to be due in part to chemical combination, and in part to the condensation of the gas into a liquid, by which means its latent heat is rendered sensible. On the other hand, when a liquid unites with a solid, or when two solids unite with one another, and the product is a liquid, we have very often the absorption of heat, the heat rendered latent by the dissolution of the solid being more than that generated by combination. ”

“ When a body passes from a state of solidity to that of fluidity, it absorbs a quantity of heat, which becomes chemically combined with it, and insensible to the touch or the thermometer; in the same manner, when it passes from a fluid state to that of vapour or gas, it combines with a still larger quantity of heat, which remains latent in it, so long as it continues in the state of gas, but when it returns to the liquid or solid state, it gives out the heat which was combined with it, which, being set at liberty, flows into the surrounding bodies, and augments their temperature. ”

“ Through the science of thermodynamics, heat has a place in many of the industrial processes that bring us the conveniences of modern life. But as recently as 200 years ago, the true nature of heat was not understood. In the early days of science the phenomena associated with heat were ascribed to a mystical and intangible fluid called "caloric." This fluid was believed to have the power of penetrating and expanding materials, sometimes melting or dissolving them, and converting some substances to vapor. ”

“ The gas is imprisoned in small thin globes of ice which shatter when they strike the rug, and melt in a few seconds, leaving no trace other than a few drops of water which quickly evaporate or are absorbed by the rug fibres. These globes are kept in a small refrigeration plantwhich is just above my head, and probably quite thoroughly insulated against heat. The intense cold in this plant is produced by a substance which is not new to science, but the use of which for this particular purpose is quite new. ”

“ By such expansion heat is absorbed and the temperature of the surroundings is lowered. From the coolers the gas returns to the absorber, from which it is pumped, in liquid form, into the retort, to be again heated, the gas expelled and the process repeated. As the gas passes through the different processes, being heated under pressure, cooled, expanded again, more or less decomposition takes place, presumably from a combination of a small portion of the nitrogen with vegetable, animal, or mineral matter that finds its way into the system. ”

“ This we know will happen when compound substances are raised to a very high temperature. We shall suppose that the heat has been sufficient to separate each particle of water into its constituent atoms of oxygen and hydrogen; we shall suppose that the heat has been sufficient to decompose even lime itself into its constituent parts, and exhibit them in the form of vapour. The heat is to be so great that even carbon itself, the most refractory of substances, has had to yield, so that after passing through a stage of dazzling incandescence it has melted and ultimately dissolved into vapour. ”

“ Now, water expands very much when under the influence of heat, like all liquids; solids and gases also expand upon the application of heat.
We can easily establish these statements. A metallic ring when heated is larger than when cool. A small quantity of air in a bladder when heated will fill the bladder, and water will boil over the vessel, or expand into steam, and perhaps burst the boiler. So expansion is the tendency of what we term heat.
We make use of this quality of heat in the thermometer, by which we can measure the temperature not only of liquids or solids, but of the atmosphere. ”

“ There will, however, be somewhere an elevation at which the heat evolved by the joint compression of the elementary and combined gases will be just sufficient to dissociate the latter, and here will be the meeting surface of the combined and the uncombined constituents of water. There will be a sphere containing combined oxygen and hydrogen surrounded by an atmospheric envelope containing large quantities of aqueous vapor, and the temperature at this limiting surface will be equal to that of the oxyhydrogen flame under a corresponding pressure. ”

“ Now if the earth’s crust will thus retain the cold why may it not as well retain the heat? In that case any excess of heat escaping from earth, over and above the [Pg 43] heat it constantly receives from the sun, may be readily accounted for. It is claimed that the earth has large quantities of sunlight and heat stored in a liquid state, as petroleum; in a gaseous state, as natural gas; and in a solidified form, as coal; some of which are found at a depth of 1600 or more feet, although coal is sometimes found near the surface, and even upon the mountains. ”

“ Thus, when a solid melts, or when a liquid is rendered gaseous, a considerable amount of heat is spent in the process, which does not become sensible, that is to say, does not affect the thermometer. Thus, in order to melt a kilogramme of ice, heat is required sufficient to raise a kilogramme of water through 80° C., and yet, when melted, the water is no warmer than the ice. ”

“ When the ice is all melted potential energy is no longer stored, but is manifested in the sensible heating of water, the degree of which is measurable by the thermometer, until it reaches the boiling point, where it is again arrested. All of the surplus heat above that temperature is consumed in rending the liquid water into moisture globules that float away into the air, each one of them charged with a store of potential energy. Let us follow this vapor spherule as it floats into the upper regions of the atmosphere. ”

“ Gases in many cases undergo a remarkable condensation when chemically combined. In slaking lime, the water becomes solid in the molecules of the hydrate of lime formed, and the intense heat produced arises from the liberation of that caloric which had been employed to keep the water liquid. When a solid passes into the liquid state, cold is produced by the abstraction from surrounding objects of the heat required to effect fluidity. An alteration of temperature occurs whenever chemical change takes place, as we have already shown, with a few trivial and uncertain exceptions. ”

“ By losing heat it contracts, but the heat generated by the contraction is more than sufficient to keep the temperature from falling. A gaseous mass, losing heat by radiation, must, therefore, at the same time, grow both smaller and hotter, until the density becomes so great that the ordinary laws of gaseous expansion reach their limit, and condensation into the liquid form begins. ”

“ 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. ”

“ Thus, the rocks, earths, metallic oxides, water, carbonic dioxide, and nitrogen are extremely stable, and resist decomposition, or chemical union with other substances, with great energy. With regard to all substances this law holds good, that the tendency is to fall back from a less stable to a more stable condition, and that such a falling back is always attended with an evolution of heat; while, on the other hand, heat is always absorbed and disappears whenever the elements of a more stable substance are made to enter into a less stable condition. ”

“ No, my dear; vapour is, indeed, an elastic fluid, and bears a strong resemblance to a gas; there are, however, several points in which they essentially differ, and by which you may always distinguish them. Steam, or vapour, owes its elasticity merely to a high temperature, which is equal to that of boiling water. And it differs from boiling water only by being united with more caloric, which, as we before explained, is in a latent state. When 183 steam is cooled, it instantly returns to the form of water; but air, or gas, has never yet been rendered liquid or solid by any degree of cold. ”

“ Hence we are fairly entitled to extend our conclusion to those which we have not yet been able to succeed with; and thus we are led to regard it as a general fact, that the liquid and aëriform or vaporous states are entirely dependent on heat; that were it not for this cause, there would be nothing but solids in nature; and that, on the other hand, nothing but a sufficient intensity of heat is requisite to destroy the cohesion of every substance, and reduce all bodies, first to liquids, and then into vapour. ”

“ The third essay, ‘On Evaporation,’ showed the quantity of water evaporated in a given time to be strictly proportional to the force of aqueous vapour at the same temperature, and to be the same in air as in vacuo. The fourth, ‘On the Expansion of Gases by Heat,’ announced the law (arrived at almost simultaneously by Gay-Lussac) ‘that all elastic fluids expand the same quantity by heat’ (p. 537) . ”

“ We have here, then, two processes—one of evaporation, another of condensation; and these, by an express and admirable provision, have a constant tendency to limit each other's operations. Evaporation is increased by heat, but it produces cold in proportion to the rapidity with which it is carried on. Condensation is produced by cold, and at the same time heat is liberated. ”

“ According to the theory developed here, we therefore have to imagine an energy store in the interior of each body, whose amount is so enormous that the usually observed heating and cooling processes, and even quite deep invasive chemical transformations associated with considerable heat effects, changes it by only an imperceptible fraction. This is valid down to the lowest attainable temperatures: for both the specific heat of a body as well as the reaction enthalpy of chemical processes keep up their magnitude close to absolute zero. ”

“ The distinction of Formal Thermotics and Physical Thermotics,—of the discovery of the mere Laws of Phenomena, and the discovery of their causes,—is applicable here, as in other departments of our knowledge. But we cannot exhibit, in any prominent manner, the latter division of the science now before us; since no general theory of heat has yet been propounded, which affords the means of calculating the circumstances of the phenomena of conduction, radiation, expansion, and change of solid, liquid, and gaseous form. ”

“ This is not boiling, as may be proved by trying the temperature with the finger. What, then, is it?
It is the yielding back of the atmospheric gases which the water has dissolved or condensed within itself. These bubbles have been collected and by analysis proved to consist of oxygen, nitrogen, and carbonic acid, obtained from the air; but in the water they exist by no means in the same proportions as originally in the air, nor in constant proportions in different samples of water. ”

“ When the heavier penetrates to a sufficient depth, the interior being still warm, it is converted into gas and driven back to the surface, only to be recondensed on reaching the upper air. Thus it may happen that two rains composed of separate liquids may fall together. There being but little of any other atmosphere, much of it consists of what you might call the vapour of hydrogen, and many of the well-known gases and liquids on earth exist only as liquids and solids ”

“ By friction, two pieces of ice may be made to melt each other; and could we, by mechanical pressure, force water into a solid state, an immense quantity of heat would be set free. By the condensation of hydrogen and oxygen gases, pulverulent platinum will become glowing red-hot, and, with certain precautions, even the compact metal, platinum, itself; the heat being derived from the gases, the union of which it has effected. A body passing from the solid to the fluid state absorbs heat from all surrounding substances, and hence a degree of cold is produced. ”

“ If we push the process still further, with a sufficient application of energy we can separate the vapor molecules into their original elements, oxygen and hydrogen.
Let us go back now to the freezing point of water and see what is becoming of the heat that is consumed in melting the cake of ice, and still does not produce any effect upon the mercury in the thermometer. Sensible heat, [Pg 183] as before stated, is a movement of the atoms of matter, and temperature, as it affects the thermometer, is a measure of the intensity of motion exhibited by these atoms. ”

“ 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. ”

“ To the gases which form the component parts of the present atmospheric air—namely, nitrogen, oxygen, and carbonic acid—to enormous masses of watery vapour, must be added vast quantities of mineral substances, metallic or earthy, reduced to a gaseous state, and maintained in that state by the temperature of this gigantic furnace. The metals, the chlorides—metallic, alkaline, and earthy—sulphur, the sulphides, and even the silicates of alumina and lime; all, at this temperature, would exist in a vaporous form in the atmosphere surrounding the primitive globe. ”