208 ENERGY established the dynamical theory of heat upon a strictly scientific basis. After Newton s time the first important step in the history of energy was made by Benjamin Thompson, Count Rumford, and was published in the Phil. Trans, for 1798. Rumford was engaged in superintending the boring of cannon in the military arsenal at Munich, and was struck by the amount of heat produced by the action of the boring bar upon the brass castings. In order to see whether the heat came out of the chips he compared the capacity for heat of the chips abraded by the boring bar with that of an equal quantity of the metal cut from the block by a fine saw, and obtained the same result in the two cases, from which he concluded that "the heat produced could nut possibly have been furnished at the expense of the latent heat of the metallic chips." Rumford then turned up a hollow cylinder which was cast in one piece with a brass six-pounder, and having reduced the connection between the cylinder and cannon to a narrow neck of metal, he caused a blunt borer to press against the hollow of the cylinder with a force equal to the weight of about 10,000 Bb, while the casting was made to rotate in a lathe. By this means the mean temperature of the brass was raised through about 70 Fahr., while the amount of metal abraded was only 837 grains. The cylinder, when it was subsequently removed from the rest of the casting, was found to weigh 113-13 ft>. In order to be sure that the heat was not due to the action of the air upon the newly exposed metallic surface, the cylinder and the end of the boring bar were immersed in 18 7 7 Ib of water contained in an oak box. The temperature of the water at the commencement of the experiment was 60 Fahr., and after two horses had turned the lathe for 2 hours the water boiled. Taking into account the heat absorbed by the box and the metal, Rumford calculated that the heat developed was sufficient to raise 2 6 58 Ib of water from the freezing to the boiling point, and in this calculation the heat lost by radiation and conduction was neglected. Since one horse was capable of doing the work required, Rumford remarked that one horse can generate heat as rapidly as nine wax candles burning in the ordinary manner. Finally, Rumford reviewed all the sources from which the heat might have been supposed to be derived, and concluded that it was simply produced by the friction, and that the supply was inexhaustible. " It is hardly necessary to add," he remarks, " that anything which any insulated body or system of bodies can continue to furnish ivithout limitation cannot possibly be a material substance ; and it appears to me to be extremely difficult, if not quite impossible, to form any distinct idea of anything capable of being excited and communicated in the manner that heat was excited and communicated in these experiments, except it be motion." About the same time that Rurnford s experiments were published, Sir Humphry Davy showed that two pieces of ice could be melted by rubbing them together in a vacuum although everything surrounding them was at a temperature below the freezing point. He did not, however, see that since the heat could not have been supplied by the ice, for ice absorbs heat in melting, this experiment afforded con clusive proof of the dynamical nature of heat. Though we may allow that the results obtained by Rumford and Davy demonstrate satisfactorily that heat is in some way due to motion, yet they do not tell us to what particular dynamical quantity heat corresponds. For example, does the heat generated by friction vary as the friction and the time during which it acts, or is it propor tional to the friction and the distance through which the rubbing bodies are displaced, that is, to the work done against friction, or does it involve any other conditions t If it can be shown that, however the duration and all other conditions of the experiment maybe varied, the same amount of heat can in the end be always produced when the same amount of energy is expended, then, and only then, can we infer that heat is a form of energy, and that the energy consumed has been really transformed into heat. This Joule has done, and his experiments conclusively prove that heat and energy are of the same nature, and that all other forms of energy with which we are acquainted can be transformed into an equivalent amount of heat; and this is the condition ultimately assumed by the energy employed in doing work against friction and similar forces, which energy was in Newton s time supposed to be lost. DEFINITION. The quantity of energy ivhich, if entirely converted into heat, is capable of raising the temperature of the unit mass of water from C. to 1 C. is called t/ie mechanical equivalent of heat. One of the first who took in hand the determination of the mechanical equivalent of heat was Seguin, a nephew of Montgolfier. He argued that, if heat be energy, then, when it is employed in doing work, as in a steam-engine, some of the heat must itself be consumed in the operation. Hence he inferred that the amount of heat given up to the condenser of an engine when the engine is doing work must be less than when the same amount of steam is blown through the engine without doing any work. Seguin was unable to verify this experimentally, but in 1857 Him succeeded, not only in showing that such a difference exists, but in measuring it, and hence determining a tolerably approximate value of the mechanical equivalent of heat. In 1839 Seguin endeavoured to determine the mechanical equivalent of heat from the loss of heat suffered by steam in expanding, assuming that the whole of the heat so lost was consumed in doing external work against the pressure to which the steam was exposed. This assumption, how ever, cannot be justified, because it neglected to take account of work which might possibly have to be done within the steam itself during the expansion. In 1842, Mayer, a physician at Heilbroun, published an attempt to determine the mechanical equivalent of heat from the heat produced when air is compressed. Mayer made an assumption the converse of that of Seguin, asserting that the whole of the work done in compressing the air was converted into heat, and neglecting the possibility of heat being consumed in doing work within the air itself or being produced by the transformation of internal potential energy. Joule afterwards proved (see below) that Mayer s assumption was in accordance with fact, so that his method was a sound one as far as experi ment was concerned, and it was only on account of the values of the specific heats of air at constant pressure and at constant volume employed by him being very inexact that the value of the mechanical equivalent of heat obtained by Mayer was very far from the truth. Passing over Colding, who in 1843 presented to the Royal Society of Copenhagen a paper entitled " Theses concerning Force," which clearly stated the " principle of the perpetuity of energy," and who also performed a series of experiments for the purpose of determining the heat developed by the compression of various bodies which entitle him to be mentioned among the founders of the modern theory of energy, we come to Dr Joule of Manchester, to whom we are indebted more than to any other for the establishment of the principle of the conservation of energy on the broad basis on which it now stands. The best known of Joule s experiments was that in which a brass paddle consisting of eight arms of complicated form arranged symmetrically round an axis was made to rotate
in a cylindrical vessel of water containing four fixed vanes,