Entered according to Act of Parliament of Canada, in the year 1880, by william Thomson, Knt., Glasgow, Scotland, in the office ef the Minister of Agriculture. HEAT is a property of matter which first became known to us by one of six very distinct senses.
§ 1. Sense of Heat— The sense of touch, as commonly meant, has two distinct objects—force and heat. If a person stretches out his hand till it meets anything solid, or holds it out while something solid is placed upon it, he experiences a sensation of force. He perceives resistance to the previous motion of his hand in one case, in the other case the necessity of resisting to prevent his hand from being forced downwards; the immediate object of this perception in each case is force. But there is another very distinct sensation, that of heat or cold, which he may or may not perceive in either of those cases, and which he may also perceive, still by what is commonly called the sense of touch, in other cases even when no sense of force is also experienced. Thus, in the first case, if the solid be a fixed can of warm water, or of iced water, the person perceives a sense of heat or of cold; and, supposing him to have performed the operation with his eyes shut, his mind is informed by the double sense of touch that his hand has met with a hot fixed body or a cold fixed body: in the other case he may perceive that a hot heavy solid, or a cold heavy solid, has been laid upon his hand. But if he dips his hand gently into a can of water, or if he holds it towards a fire, or if he exposes it to a gentle current of air, or waves it about through the air, he perceives heat or cold without any accompanying sense of force....
The earliest scientific thoughts respecting these sensations of heat and cold must have led to the true conclusion that there is some property of external matter on which the sensations depend, and a little advance into the natural philosophy of the subject has suggested and proved that this property is also possessed by the living body, and that the sensatiou of heat or cold in the hand, in the instances referred to above, depends on the change produced in the hand in respect to this property by a change of circumstances which preceded the sensation. We now call heat the property of matter concerned in these sensations, and temperature a certain variable quality of matter varying according to its temporary condition in respect to heat.
In the strictest modern scientific language (compare § 3 below) the word heat is used to denote something communicable from one body or piece of matter to another, and temperature a definite variable quality of matter, varying generally in any particular piece of matter when heat is communicated to it or taken from it, varying also as we shall see (§§ 8 and 9 below; also Thermodynamics) in consequence of operations which can take place within the body itself, or which may be performed upon it from without, but which cannot be described as communication of heat to it or drawing off of heat from it.
§ 2. Latent Heat.— There are exceptional cases in which temperature does not vary in a mass of matter when heat is communicated to it from, or taken from it to, external matter. For instance, when the body is ice at the melting point, heat communicated to it does not raise its temperature; or if the body be water at the freezing point with ever so small a piece of ice in it (see Matter, Properties of), heat taken from it does not cause its temperature to fall; or if the whole mass considered be ice and water well mixed, heat may be either communicated to it or taken from it without altering its temperature; or if the body be water at the boiling point in the open air, heat very slowly communicated to it in however great quantities does not raise its temperature sensibly, but causes it to disappear by evaporation from its surface; or if the body be steam in a cylinder with a little water in the bottom and with a frictionless piston above it for roof (fig. 1), under atmospheric pressure, heat taken from it very slowly does not cool it until the whole steam has become condensed into water, and heat communicated to it very slowly does not warm it until the whole water has become evaporated into steam; or if the body be ice (or frozen water), in place of the liquid water of the last case, and if the pressure on the upper side of the piston, instead of atmospheric pressure of about 1033 grammes per square centimetre (14 ’ 7 lb per square inch), be anything less than ∙ 3 1 θth of a gramme per square centimetre, the same statement will still apply with “ice” substituted for water. Black’s celebrated doctrine of latent heat is merely the declaration of a class of phenomena of which the preceding illustrations sufficiently indicate the character. Modern mysticism has been much exercised in respect to the terms sensible heat and latent heat, whether in decrying them, or in continuing to use them, but with aggravating haziness, instead of the clear wrongness of the old doctrine. It has become of late years somewhat the fashion to decry the designation of latent heat, because it had been very often stated in language involving the assumption of the materiality of heat. Now that we know heat to be a mode of motion, and not a material substance, the old “impressive, clear, and wrong” statements regarding latent heat, evolution and absorption of heat by compression, specific heats of bodies and quantities of heat possessed by them, are summarily discarded. But they have not yet been generally enough followed by equally clear and concise statements of what we now know to be the truth. A combination of impressions surviving from the old erroneous notions regarding the nature of heat with imperfectly developed apprehension of the new theory has somewhat liberally perplexed the modern student of thermodynamics with questions unanswerable by theory or experiment, and propositions which escape the merit of being false by having no assignable meaning. There is no occasion to give up either “sensible heat” or “latent heat”; and there is a positive need to retain the term latent heat, because if it were given up a term would be needed to replace it, and it seems impossible to invent a better. Heat given to a substance and warming it is said to be sensible in the substance. Heat given to a substance and not warming it is said to become latent. These designations express with perfect clearness the relation of certain material phenomena to our sensory perception of them. Thus when heat given to a quantity of water warms it, the heat becomes sensible to a hand held in the water. When a basin of warm water and a basin of water and ice are placed side by side, a hand dipped first in one and then in the other perceives the heat. If now the warm water be poured into the basin of ice and water, and stirred for a few seconds of time (unless there is enough of warm water to melt all the ice), the hand perceives no warmth; on the contrary, it perceives that the temperature is the same as it was in the basin of ice and water at the beginning. Thus the heat which was sensible in the basin of warm water has ceased to be sensible in the water that was in that basin, and has not become sensible in the other. It is therefore well said to have become latent.
Calorimetry.
§ 3. Calorimetry by Latent Heat.— The doctrine of latent heat leads us very smoothly to a most important measurement in thermal science, the measurement of quantities of this wonderful property of matter which we call heat; and this without our knowing anything of what the nature of heat is, —whether it be a subtle elastic fluid, or a state of motion, or possibly some modification of matter related to action of force. Without, in the first place, admitting into our minds any definite idea as to the nature of heat, we may agree to measure quantities of heat by quantities of ice melted into water without change of temperature. Thus if a kilogramme of ice is melted by a large quantity of water at a lukewarm temperature, or by a comparatively small quantity of very hot water, the same quantity of heat has certainly gone from the warm water to the ice in each case, supposing that the result in each case is the ice and warm water left all in a state of ice-cold water. The measurement of quantities of heat, whether thus by the melting of ice, or by any other means, received the name of “Calorimetry,” when the essence of heat was supposed to be a fluid, and this fluid called caloric. The name calorimetry is still by general consent retained to designate measurement of quantities of heat, as distinguished from thermometry, or the measurement of temperature ( § 10-67). As long as the truth or falsity of the materialistic hypothesis seemed an open question, the word caloric was held to imply the materiality of heat. Thus Davy, after discussing some of the fundamental dogmas of the “Calorists,” as he called them, and describing his own experiments, which proved beyond all doubt the falsity of their fundamental hypothesis that heat is matter, varied the statement of his conclusion by saying, “or caloric does not exist.” While accepting Davy’s conclusion, however, we need not accept this way of stating it; and as most of our best modern writers still use the word calorimetry, and as French writers have, in comparatively recent times, introduced the word “calorie” to designate a unit quantity of heat, it is decidedly convenient still to retain the name caloric to denote definitely the measurable essence of heat. This is convenient scientifically as tending to give precision to language and ideas respecting the two classes of measurement, calorimetry and thermometry; and it has the advantage of leaving the more popular word heat available for that somewhat lax general usage, from which we cannot altogether displace it, in which it may sometimes mean high temperature, as when we speak of great heat, or summer heat, or blood heat, sometimes a measurable quantity of heat, as in the term latent heat, and sometimes a branch of study or science dealing with the transference of heat by conduction and radiation, as in the title of Fourier’s great work Théorie analytique de la Chaleur, or the whole province of science concerned with heat, including calorimetry and thermometry, and conduction and radiation of heat, and generation of heat, and dynamical relations of heat, as in English titles of separate books such as Dixon’s, Balfour Stewart’s, and Maxwell’s, or of chapters or divisions of larger treatises, such as even the present article.
f 4. Calorimetry by Meltiny of Ice.— Calorimetry was first practised by means of the melting of ice as explained above, and the first thermal unit, or unit quantity of heat, or “calorie,” although not then called calorie, was the quantity of heat required to melt unit weight of ice. This, for example, is the unit on which Fourier founds his reckoning illustratively when he explains the fundamental principles of his theory of the conduction of heat. Ice seems to have been first used for calorimetry by Wilcke, a Swede. For the systematic application of this method for the measurement of quantities of heat in various physical inquiries Laplace and Lavoisier constructed an instrument, the first to which the name of calorimeter was applied, and described it in the memoirs of the French Academy of Sciences for 1780. 1 Though in the hands of Laplace and Lavoisier it gave good results, it had a great inconvenience, which with less careful and less scientific experimenters might lead to great inaccuracies, on account of the water adhering by capillary attraction to the broken ice, instead of draining away from it completely and showing exactly how much ice had been melted. To avoid this evil Sir John Herschel suggested that, instead of draining away the water from the ice, the water and ice should all be kept together, and the whole bulk measured. The diminution of bulk of the whole thus gives an accurate measurement of the quantity of ice melted, because ice melting into water comes to occupy just 91'675 per cent, of its original volume. This suggestion is admirably carried out by Bunsen 2 in his ice-calorimeter, an instrument possessing also other novel features of remarkable beauty and scientific interest. It is particularly valuable for the measurement of small quantities of heat. Its inventor, for example, by means of it succeeded in making satisfactory determinations of the specific heats of some of those rarer metals, such as indium, of which only a few grammee have been obtained.
§ 5. Calorimetry by the Evaporation of Water.— By another application of Black’s doctrine of latent heat, the evaporation of water may be used for calorimetry with great advantage in many scientific investigations. It is used generally in engineering practice, particularly for testing the heating power of different qualities of coal and the economy of various forms of furnaces. The thermal unit, which presents itself naturally in this system, is the quantity of heat required to evaporate unit weight of water when the pressure of the atmosphere as measured by the barometer is of some conventional standard amount, such as that called one atmosphere, or one atmo, being that for which the barometer, with its mercury column at zero centigrade (or the temperature at which ice melts), stands at 76 centimetres in the latitude of Paris, 48° 50', 3 or at
76 × (1 + ∙00531 sin 2 48° 50 , )
1 + ‘00531 sin e l in any latitude Z. This thermal unit (see Thermodynamics) is, according to Regnault’s observations, equal to 6’8 times the ice-calorimetric unit.
§ 6. Thermometric Calorimetry.— The most prevalent mode of calorimetry in scientific investigation has been hitherto, however, neither that by the melting of ice, nor that by evaporation of water, nor indeed anything founded on the doctrine of latent heat at all. It has been founded on the elevation of temperature produced in water by the communication to it of the heat to be measured; and, for the sake of distinction from calorimetry by latent heat or otherwise, it maybe called thermometric calorimetry. We can only consider it now in anticipation, as we have not yet reached the foundation of any thermometric scale; but even now we can see that, if in any way we fix upon any two particular determinate temperatures, the quantity of water warmed from the lower to the higher of them by the heat to be measured is a perfectly definite measure for the quantity of this heat. The two temperatures chosen for thermometric calorimetry are those marked 0° and 1° on the centigrade scale. The first of these we can understand at present, being the temperature at which ice melts under ordinary atmospheric pressure. The second is fully defined in §§ 35, 37, 51, 67. The quantity of heat required to raise unit mass of water (1 kilogramme, or 1 gramme, or 1 milligramme, or 1 lb, as the case may be) from zero to 1° C. is called the thermal unit centigrade, and sometimes, especially by French writers, the “calorie.”
§ 7. Comparison of Calorimetric Units.— Observations by Prevostaye and Desains, and by Régnault, on the latent heat of fusion of ice, show it to be 79’25 thermal units centigrade, a result differing but little from Black’s original determination, which made it 142 thermal units Fahr.,— this being equal to 78’9 thermal units centigrade. Thus if one kilogramme of ice be put into 79-∣ kilogrammes of water at 1° C., and left till the whole is melted (the process may be accelerated by not too violent stirring, § 9), the result will be 801 kilogrammes of water at 0° C.
Regnault’s experiments on the latent heat of steam show that the quantity of heat required to convert into steam unit 1 The instrument itself is preserved in the Obnsm·a&n're des Artset Métiers in Paris. It is described and explained in Maxwell’s Theory of Heat, chap. iii.
2 Pogg. Ann., Sept. 1870, and Phil. Mag., 871; Maxwell’s Theory of Heat, p. 61.
3 This is chosen because all the most accurate experimental determinations depending on a conventional standard for atmospheric pressure, such as measurements of thermal expansions and specific heats of gases, of latent heat of melting solids in terms of a calorimetric unit depending on the centigrade thermometric scale, of latent heats of vapours, and thermal expansions of mercury and glass, and comparisons of mercury and air thermometers, are those of Regnanlt, and were made in Paris and calculated and given to the world according to an arbitrary standard atmosphere corresponding to 76 centimetres of mercury there.
mass of water at the boiling temperature, uuder standard atmospheric pressure (§ 5), is 536^ thermal units centigrade. This number, which is no doubt very accurate, differs but little from Watt’s final result, 960 thermal units Fahr, (equal to 533 ’ 3 thermal units centigrade), obtained by him, in a repetition in 1781 of experiments which he had commenced in 1765 at the invitation of Black, whose pupil he was.
§ 8. Preliminary regarding the Nature of Heat.—Dynamical Calarimetry.— From the dawn of science till the close of last century two rival hypotheses had been entertained regarding the nature of heat, each with more or less of plausibility, but neither on any sure experimental basis: — one that heat consisted of a subtle elastic fluid permeating through the pores or interstices among the particles of matter, like water in a sponge; the other that it was an intestine commotion among the particles or molecules of matter. In the year 1799 Davy, in his first published work entitled An Essay an Heat, Light, and Combinations of Light f conclusively overthrew the former of these hypotheses, and gave good reason for accepting as true the latter, by his celebrated experiment of converting ice into water by rubbing two pieces of ice together, without communicating any heat from surrounding matter. A few years earlier Rumford had been led to the same conclusion, and had given very convincing evidence of it in his observation of the great amount of heat produced in the process of boring cannon in the military arsenal at Munich, and the experimental investigation on the excitation of heat by friction with which he followed up that observation. He had not, however, given a perfect logical demonstration of his conclusion, nor even quite a complete experimental basis on which it could be established with absolute certainty. According to the materialistic doctrine it would have been held that the heat excited by the friction was not generated, 3, but was produced, squeezed out, or let flow out like honey from a broken honeycomb, from those parts of the solid which were cut or broken into small fragments, or rubbed to powder in the frictional process. If this were true, the very small fragments or powder would contain much less heat in them than an equal mass of continuous solid of the same substance as theirs. But unhappily the caloristic doctrine, besides its fundamental hypothesis, which we now know to be wrong, had given an absurd and illogical test for quantity of heat in a body, of which a not altogether innocuous influence still survives in our modern name “specific heat and Rumford actually, in trying to disprove the materialistic doctrine, was baffled by this sophism. That is to say, he measured the specific heat or “capacity for heat” of the powder, and he found that the powder took as much heat to warm it to a certain degree as did an equal mass of the continuous solid, and from this he concluded that the powder did not contain less heat than the continuous solid at the same temperature. This conclusion is so obviously unwarranted by the premises that it is difficult to imagine how Rumford could have for a moment put forward the “capacity for heat” experiment as proving it, or could have rested in the conclusion without a real proof, or at least the suggestion of a real proof. All that Rumford’s argument proved was that the fundamental hypothesis of the “calorists” and their other altogether gratuitous doctrine of equality of “specific heat” as a test for equality of whole quantities of...
