Heat Considered as a Mode of Motion

being a Course of Twelve Lectures delivered at the Royal Institution of Great Britain, by JOHN TYNDALL, F. R. S., Professor of Natural Philosophy in the Royal Institution. New York: D. Appleton & Co.

THE readers of the “ Glaciers of the Alps ” have made the acquaintance of Professor Tyndall as an Alpine adventurer, with a passion for frost and philosophy, and a remarkable ability both in describing his mountain -experiences and in explaining the interesting phenomena which he there encountered. All who have read this inimitable volume will testify to its rare attractions. It is at once dramatic and philosophic, poetic and scientific; and the author wins our admiration alike as a daring and intrepid explorer, a keen observer, a graphic delineator, and an acute and original investigator.

In the new work on Heat we are introduced to Professor Tyndall upon the lecturing-platform, where he follows up some of the inquiries started in the “ Glaciers” in a systematic and comprehensive manner. His problem is, the nature and laws of Heat, its relation to other forms of force, and the part it plays in the vast scheme of the universe : an imposing task, but executed in a manner worthy of the gifted young successor of Faraday as Professor of Natural Philosophy in the Royal Institution of Great Britain.

A comparison of the volume before us with any of the previously published treatises on Heat will afford a striking and almost startling proof of the present activity of inquiry, and the rapid progress of scientific research. The topics treated are the same. The first seven lectures of the course deal with thermometric heat, expansion, combustion, conduction, specific and latent heat, and the relation of this force to mechanical processes ; while the remaining five treat of radiant heat, the law and conditions of its movement, its influence upon matter, its relations to other forces, terrestrial and solar radiation, and the thermal energies of the solar system. But these subjects no longer wear their old aspect. Novel questions are presented, starting fresh trains of experiment; facts assume new relationships, and are interpreted in the light of a new and higher philosophy.

The old view of the forces, which regarded them as material entities, may now be regarded as abandoned. Light, Heat, Electricity, Magnetism, etc., which have hitherto been considered under the selfcontradictory designation of “ Imponderable Elements,” or immaterial matter, are now, by common consent, beginning to be ranked as pure forces ; having passed through their material stage, they are regarded as kindred and convertible forms of motion in matter itself. The old notions, that light consisted of moving corpuscles, and that heat, electricity, and magnetism were produced by the agency of various fluids, have done good service in times past; but their office was only provisional, and, having served to advance the philosophy of forces beyond themselves, they must now take rank among the outgrown and effete theories which belong to the infantile period of science. This change, as will be seen, involves the fundamental conceptions of science, and is nothing less than the substitution of dynamical for material ideas in dealing with the phenomena of Nature.

The new views, of which Professor Tyndall is one of the ablest expositors, are expressed by the terms “ Conservation and Correlation of Forces.” The first term implies that force is indestructible, that an impulse of power can no more be annihilated than a particle of matter, and that the total amount of energy in the universe remains forever the same. This principle has been well characterized by Faraday as “ the highest law in physical science which our faculties permit us to perceive.” The phrase “ Correlation of Forces ” is employed rather to express their mutual convertibility, or change from one to the others. Thus, heat excites electricity, and, through that force, magnetism, chemical action, and light. Or, if we start with magnetism, this may give rise to electricity, and this again to heat, chemical action, and light. Or we can begin with chemical action, and obtain the same train of effects.

It has long been known that machines do not create force, but only communicate, distribute, and apply that which has been imparted to them, and also that a definite amount of fuel corresponds to a definite amount of work performed by the steamengine. This means simply that a fixed quantity of the chemical force of combustion gives rise to a corresponding quantity of heat, and this again to a determinate amount of mechanical effect. Now this principle of equivalency is found to govern the transmutations of all forms of energy. The doctrine of the conservation and correlation of forces has been illustrated in various ways, but nothing has so powerfully contributed to its establishment as the investigation of the relations of heat to mechanical force. Percussion and friction produce heat. A cold bullet, struck upon an anvil by a cold sledge-hammer, is heated. Iron plates, ground against each other by water-power, have yielded a large and constant supply of heat for warming the air of a factory in winter ; while water inclosed in a box, which was made to revolve rapidly, rose to the boiling-point. What, now, is the source of heat in these cases ? The old caloric hypothesis utterly fails to explain it; for to suppose that there is an indefinite and inexhaustible store of latent heat in the rubbing iron plates is purely gratuitous. It is now established, that the heat of collision and of friction depends, not upon the nature of the bodies in motion, but upon the force spent in producing it.

When a moving body is stopped, its force is not annihilated, but simply takes another form. When the sledge-hammer strikes the leaden bullet and comes to rest, the mechanical force is not destroyed, but is simply converted into heat; and if all the heat produced could be collected, it would be exactly sufficient, when reconverted into mechanical force, to raise the hammer again to the height from which it fell. So, when bodies are rubbed together, their surface-particles are brought into collision, mechanical force is destroyed, and heat appears,—the heat of friction. The conversion of heat into mechanical motion, and of that motion back again into heat, may be familiarly illustrated in the case of a railway-train. The beat generated by combustion in the locomotive is converted into motion of the cars. But when it is desired to stop the train, what is to be done ? Its mechanical force cannot be annihilated ; it can only be transmuted; and so the brakes are applied, and the train brought to rest by reconverting its motion into heat, as is manifested by the smoke and sparks produced by the friction. Now, as heat produces mechanical motion, and mechanical motion heat, they must clearly have some common quality. The dynamical theory asserts, that, as they are both modes of motion, they must be mutually and easily convertible. When a moving mass is checked or stopped, its force is not annihilated, but the gross, palpable motion is infinitely subdivided and communicated to the atoms of the body, producing increased vibrations, which appear as heat. Heat is thus inferred to be, not a material fluid, but a motion among the ultimate atoms of matter.

The acceptance of this view led to the highly important inquiry, What is the equivalent relation between mechanical force and heat? or, how much heat is produced by a definite quantity of mechanical force ? To Dr. Joule, of Manchester, England, is due the honor of having answered this question, and experimentally established the numerical relation. He demonstrated that a one-pound weight, falling through seven hundred and seventy-two feet and then arrested, produces sufficient heat to raise one pound of water one degree. Hence this is known as the mechanical equivalent of heat, or “Joule’s Law.”

The establishment of the principle of correlation between mechanical force and heat constitutes one of the most important events in the progress of science. It teaches us that the movements we see around us are not spontaneous or independent occurrences, but links in the eternal chain of forces, — that, when bodies are put in motion, it is at the expense of some previously existing energy, and that, when they come to rest, their force is not destroyed, but lives on in other forms. Every motion we see has its thermal value; and when it ceases, its equivalent of heat is an invariable result. When a cannonball strikes the side of an iron-plated ship, a flash of light shows that collision has converted the motion of the ball into intense heat, or when we jump from the table to the floor, the temperature of the body is slightly raised, — the degree of heat produced in both cases being ascertainable by the application of Joule’s law.

The principle thus demonstrated has given a new interest and a vast impulse to the science of Thermotics. It is the fundamental and organizing conception of Professor Tyndall’s work, and in his last chapter he carries out its application to the planetary system. The experiments of Herschel and Pouillet upon the amount of solar heat received upon the earth’s surface form the starting-point of the computations. The total amount of heat received by the earth from the sun would be sufficient to boil three hundred cubic miles of ice-cold water per hour, and yet the earth arrests but 1/2,300,000,000 of the entire thermal force which the sun emits. The entire solar radiation each hour would accordingly be sufficient to boil 700,000,000,000 cubic miles of ice-cold water ! Speculation has hardly dared venture upon the source of this stupendous amount of energy, but the mechanical equivalent of heat opens a new aspect of the question. All the celestial motions are vast potential stores of heat, and if checked or arrested, the heat would at once become manifest. Could we imagine brakes applied to the surface of the sun and planets, so as to arrest, by friction, their motions upon their axes, the heat thus produced would be sufficient to maintain the solar emission for a period of one hundred and sixteen years. As the earth is eight thousand miles in diameter, five and a half times heavier than water, and moves through its orbit at the rate of sixty-eight thousand miles an hour, a sudden arrest of its motion would generate a heat equal to the combustion of fourteen globes of anthracite coal as large as itself. Should it fall into the sun, the shock would produce a heat equal to the combustion of five thousand four hundred earth-globes of solid coal,— sufficient to maintain the solar radiation nearly a hundred years. Should all the planets thus come to rest in the sun, it would cover his emission for a period of forty-five thousand five hundred and eighty-nine years. It has been maintained that the solar beat is actually produced in this way by the constant collision upon his surface of meteoric bodies, but for the particulars of this hypothesis we must refer to the book itself.

Professor Tyndall opens the question in his volume respecting the share which different investigators have had in establishing the new theory of forces, and his observations have given rise to a sharp controversy in the scientific journals. The point in dispute seems to have been the relative claims of an Englishman and a German — Dr. Joule and Dr. Mayer—to the honor of having founded the new philosophy. Tyndall accords a high place to the German as having worked out the view in an a priori way with remarkable precision and comprehensiveness, while he grants to the Englishman the credit of being the first to experimentally establish the law of the mechanical equivalent of heat. But his English critics seem to be satisfied with nothing short of an entire monopoly of the honor. The truth is, that, in this case, as in that of many others furnished us in the history of science, the discovery belongs rather to an epoch than to an individual. In the growth of scientific thought, the time had come for the evolution of this principle, and it was seized upon by several master-minds in different countries, who worked out their results contemporaneously, but in ignorance of the efforts of their fellow-laborers. But if individual claims are to be pressed, and each man accorded his aliquot share of the credit, we apprehend that America must be placed before either England or Germany, and for the explicit evidence we need look no farther than the volume of Professor Tyndall before us. The first clear connection and experimental proof of the modern theory was made by our countryman Benjamin Thompson,— afterwards knighted as Count Rumford by the Elector of Bavaria. He went to Europe in the time of the American Revolution, and, devoting himself to scientific investigations, became the founder of the Royal Institution of Great Britain. Davy was his associate, and, so far as the new views of heat are concerned, his disciple. He exploded the notion of caloric, demonstrated experimentally the conversion of mechanical force into heat, and arrived at quantitative results, which, considering the roughness of his experiments, are remarkably near the established facts. He revolved a brass cannon against a steel borer by horse-power for two and one-half hours, thereby generating heat enough to raise eighteen and three - fourths pounds of water from sixty to two hundred and twelve degrees. Concerning the nature of heat he wrote as follows, the Italics being his own: — “ What is heat ? Is there any such thing as an igneous fluid ? Is there anything that with propriety can be called caloric ? We have seen that a very considerable quantity of heat may he excited by the friction of two metallic surfaces, and given off in a constant stream, or flux, in all directions, without interruption or intermission, and without any signs of diminution or exhaustion. In reasoning on this subject, we must not forget that most remarkable circumstance, that the source of the heat generated by friction in these experiments appeared to be inexhaustible. It is hardly necessary to add, that anything which any insulated body or system of bodies can continue to furnish without 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 any thing capable of being excited and communicated in these experiments, except it be MOTION.”

In style, Professor Tyndall’s work is remarkably clear, spirited, and vigorous, and many of its pages are eloquent with the beautiful enthusiasm and poetic spirit of its author. These attractions, combined with the comprehensiveness and unity of the discussion, the range and authenticity of the facts, and the delicacy, originality, and vividness of the experiments, render the work at once popular and profound. It is a classic upon the subject of which it treats.