T HEAT A MODE OF MOTION. SELECTED FROM LECTURES ON HEAT. HE aspects of Nature provoke in man the spirit of inquiry. As the eye is formed to see and the ear to hear, so the human mind is formed to explore and understand the basis and relationship of natural phenomena. The human mind began its operations among the powers of Nature, winning first a little knowledge and a little strength, and then turning the knowledge and the strength so won back upon Nature, with the view of winning more. Action and reaction have thus gone on from prehistoric ages to the present time. The result is that stored body of scientific knowledge and that developed power of scientific investigation which have revolutionized philosophy and begotten those marvels of practical science in the midst of which we dwell. pro Whenever friction is overcome heat is duced, and the heat produced is the exact measure of the power expended in overcoming the friction. The heat is simply the original power in another form; and if we wish to postpone this conversion, we must abolish the friction. We place oil upon the surface of a hone, we grease a saw and we saw and we are careful to lubricate the axles of our railway carriages. What is the real meaning of these acts? It is the object of a railway engineer to urge his train from one place to another, and it is not his interest to allow any portion of his force to be applied in a applied in a manner which would not promote the attainment of his object. He does not want his axles heated, and hence he avoids as much as possible expending his power in heating them. He has obtained his force from heat, and it is not his object to reconvert by friction the force thus obtained into its primitive form. For every degree of temperature generated in his axles a definite amount would be withdrawn from been invoked to prove that the observed differences of temperature between sea and air were due solely to mechanical action. Nevertheless, the tradition is an old one, as the following quotation proves: In one of those gales on Sep、 tember 12, Dr. Irving tried the temperature of the sea in that state of agitation, and found it considerably warmer than that of the atmosphere. This observation is the more interesting, as it agrees with a passage in Plutarch's Nat There are friends before me who have stood amid the foam of Niagara, and I have done so myself. Had we dipped sufficiently sensitive thermometers into the water at the top and at the bottom of the cataract, we should have found the latter warmer than the former. The sailor's tradition, also, is theoretically correct: the sea is rendered warmer by a storm, the mechanical dash of its bil-ural Questions-not, I believe, before taken notice of or lows being ultimately converted into heat. * * I say "theoretically correct," because it would require far more care and instrumental delicacy than appear to have confirmed by experiment—in which he remarks that the sea becomes warmer by being agitated in waves.”—A Voyage to the North Pole, undertaken by His Majesty's Commands 1773, by Constantine John Phipps. his urging force. There would be no absolute loss. Could he gather up all the heat generated by the friction and apply it mechanically, he would by it be able to impart to the train the precise amount of speed which it had lost by the friction. Every one of those railway porters whom you see moving about with his can of yellow grease and opening the little boxes which surround the carriage-axles is, without knowing it, illustrating a principle which forms the very solder of Nature. In the long run, however, the generation of heat cannot be avoided. All the force of our locomotives eventually takes this form. To maintain the proper speed, the friction of the train must be continually overcome, and the force spent in overcoming it is entirely converted into heat. An eminent writer has compared the process to one of distillation: the heat of the furnace distils into the mechanical motion of the train, and this motion recondenses as heat in the wheels, axles and rails. So also with regard to the greasing of a saw by a carpenter. He applies his force with the express object of cutting through the wood. He wishes to overcome mechanical cohesion by the teeth of his saw, and when it moves stiffly the same amount of effort may produce a much smaller effect than when the implement moves without friction. But in what sense smaller? Not absolutely so, but smaller as regards the act of sawing. The force not expended in sawing is misapplied, not lost; it is converted into heat. Here, again, if we could collect the heat engendered by the friction and apply it to the urging of the saw, we should make good the precise amount of work which the carpenter, by neglecting the lubrication of his implement, had simply converted into another form of power. We warm our hands by rubbing, and in cases of frost-bite we thus restore animation to the injured parts. By friction a lucifer match is raised to the temperature of ignition. In the common flint and steel the particles of the metal struck off are SO much heated by the collision that they take fire and burn in the air. But the heat precedes the combustion. Hooke proved this, and Davy found that when a gunlock with a flint was discharged in vacuo no sparks were produced, but the particles of steel struck off, when examined under the microscope, showed signs of fusion. Before the safety-lamp was invented the workers in our coal-mines derived their light from showers of sparks generated by the friction. of flint against the edge of a swiftly rotating steel wheel, the sparks having been considered incompetent to ignite the "firedamp." Aristotle refers to the heating of arrows by the friction of the air, and the most probable theory of shooting-stars is that they are small planetary bodies revolving round the sun, which, being caused to swerve from their orbits by the attraction of the earth, are raised to incandescence by friction against our atmosphere. Chladni propounded this view, and Dr. Joule has confirmed it by calculation. He may, moreover, be correct in believing that the earth is spared bombardment through the breaking up of our aerolites by heat. These bodies move at planetary rates. The orbital velocities of the four interior planets are as follows: while the velocity of the aerolites varies from eighteen to thirty-six miles a second. The friction engendered by this enormous speed is no doubt competent to produce the effects ascribed to it. Knowing the velocity and weight of any projectile, we shall subsequently learn how to calculate the amount of heat developed by the destruction of its motion. For example, knowing as we do the weight of the earth and the velocity with which it moves through space, a simple calculation enables us to state the exact amount of heat which | would be developed, supposing the earth to strike against a target strong enough to stop its motion. We could tell, for example, the number of degrees which this amount of heat would impart to a globe of water equal to the earth in size. Mayer, Helmholtz and Thomson have made this calculation and found that the quantity of heat corresponding to this colossal shock would be quite sufficient not only to fuse the entire earth, but to reduce it, in great part, to vapor. Thus, by the simple stoppage of our planet in its orbit, "the elements" might be caused "to melt with fervent heat." The amount of heat thus developed would be equal to that derived from the combustion of fourteen globes of coal, each equal to the earth in magnitude. And if, after the stoppage of its orbital motion, the earth should fall into the sun, as it assuredly would, the amount of heat generated by the blow would be equal to that developed by the combustion of five thousand six hundred worlds of solid carbon. Knowledge such as that which you now possess has caused philosophers, in speculating on the mode in which the sun's power is maintained, to suppose solar light and heat to be caused by the showering down of meteoric matter upon the sun's surface. The life of the human race may be divided into two great periods-the prehistoric and historic. But human beings had done great things before they learnt to write about their doings. Among other things, they had discovered the use of fire both as a means of warming their bodies and cooking their food. Nobody can tell how or when fire was first introduced. Lucretius has a story which ascribes its origin to the rubbing together of dry tree-branches, but this is not a likely source of ignition. Forests are sometimes set ablaze by lightning, and this is a possible origin of our domestic fires. Again, savages have everywhere employed stone implements, shaping pieces of flint with sharp edges for knives, and with sharp points for arrow-heads and spears. Sparks were certainly thus produced, and such sparks may have been the ancestors of our fires. At the present hour the inhabitants of Tierra del Fuego employ two stones, the one a hard flinty pebble, the other a lump of iron pyrites. Friction, however, is the skilful savage's ordinary resource. HISTORIC NOTICES. The tendency to explain the seen by reference to the unseen is continually manifested in the efforts of curious and penetrative minds. to obtain a notion of the nature of heat. They had constant recourse to the scientific imagination. Heat-atoms and fire-atoms were pictured as driving fiercely into the pores of bodies, loosening their molecules and shaking them by mechanical means new heat is called into existence.* In describing one of his experiments he uses the following remarkable language, "It will be convenient to begin with an instance or two of the production of heat wherein there appears not to intervene anything in the part of the agent or patient but local motion and the natural effects of it. When, for example, a smith does hastily hammer a nail or such-like piece of iron, the hammered metal will grow exceedingly hot; and yet there appears not anything to make it so, save the forcible motion of the hammer, which impresses a vehement and variously determined agitation of the small parts of the iron, which, being a cold body before, by that superinduced commotion of its small parts becomes in divers senses hot asunder, thus reducing solids to liquids and liquids to vapors. The notion that heat was a kind of motion was vaguely entertained by Plato, who made Socrates say, "For heat and fire, which generate and sustain other things, are themselves begotten by impact and friction but this is motion. Are not these the origin of fire?" The same thought was clearly formulated by Bacon, who defined heat to be "a motion acting in its strife upon the smaller particles of bodies." His illustrations of this motion were not, however, always happy ones. Descartes, and others in his day, had a clear conception that the sensation of heat arose from a kind of motion communicated to the nerves, and some of these early writers were also clear as to the fact that the sensation was derived from the molecular motion of the warm body.first, in a more lax acceptation of the word They, however, for the most part assumed a special igneous matter, which produced the molecular motion. The illustrious Robert Boyle, for example, affirmed heat to be molecular motion, but to account for fire he assumed a special igneous matter. Euler and Newton curiously changed places with regard to their respective notions of light and heat. Euler was one of the most ardent defenders of the undulatory theory, which ascribes light to vibratory motion, but he regarded heat as a kind of matter. Newton supported the emission theory, which assumed light to be a kind of matter, while he considered heat to be vibratory motion. Hobbes, it may be added, was very distinct in his affirmation that heat is motion, and with regard to solar heat he avows his disbelief that anything material is emitted by the sun. Robert Boyle appears to have seen as clearly as we do to-day that when heat is generated in reference to some other bodies, in respect. of whom it was cold before, and then sensibly hot, because this newly-gained agitation surpasses that of the parts of our fingers. And in this instance it is not to be overlooked that oftentimes neither [both ?] the hammer by which nor [and?] the anvil on which a cold piece of iron is forged continue cold after the operation is ended, which shows that the heat acquired by the forged piece of iron was not communicated by the hammer or anvil as heat, but produced in it by motion, which was great enough to put so small a body as the piece of iron into a strong and confused motion of its parts. without being able to have the like operation upon so much greater masses of metal as the hammer and the anvil. And now I *On this point Bacon also was perfectly clear. "Heat," he says, "is produced by the motion of attrition, without any preceding heat." am put in mind of an observation that seems to contradict, but does indeed confirm, our theory—namely, that if a somewhat large nail be driven by a hammer into a plank or piece of wood, it will receive divers strokes on the head before it grows hot; but when it is driven to the head, so that it can go no farther, a few strokes will suffice to give it a considerable heat; for whilst at every blow of the hammer the nail enters farther and farther into the wood, the motion that is produced is chiefly progressive, and is of the whole nail tending one way; whereas, when that motion is stopped, then the impulse given by the stroke, being unable either to drive the nail farther on or destroy its entireness, must be spent in making a various, vehement and intestine commotion of the parts among themselves, and in such an one we formerly observed the nature of heat to consist." After "the nimble hammering of iron by three lusty men" accustomed to the work, Boyle found the metal so hot that it could not be safely touched. To the wonder of the bystanders, it was able to ignite the sulphur of gunpowder and to cause it to burn with a blue flame. He also refers to the heat produced in cold iron by a rough file causing an intestine commotion of its parts. Nothing can be clearer or more to the point than these utterances and illus trations. Among the philosophers of the seventeenth century none, however, possessed a greater power of symbolizing the phenomena of heat than Robert Hooke. His illustration of the manner in which fluidity is produced by the motion of heat is a fine example of his penetration. "First," he First," he says, "what is the cause of fluidness? This I conceive to be nothing. else but a certain pulse or shake of heat; for, heat being nothing else but a very brisk and vehement agitation of the parts of a body (as I have elsewhere made probable), the parts of a body are thereby made so loose from one another that they easily move any way and become fluid. That I may explain this a little by a gross similitude, let us suppose a dish of sand set upon some body that is very much agitated and shaken with some quick and strong vibrating motion, as on a millstone turned round upon the under stone very violently whilst it is empty, or on a very stiff drumhead, which is vehemently or very nimbly beaten with the drumsticks. By this means the sand in the dish, which before lay like a dull and unactive body, becomes a perfect fluid; and ye can no sooner make a hole in it with your finger but it is immediately filled up again and the upper surface of it levelled. Nor can you bury a light body, as a piece of cork, under it but it presently emerges or swims as 'twere on the top; nor can you lay a heavier on the top of it, as a piece of lead, but it is immediately buried in sand, and (as 'twere) sinks to the bottom. Nor can you make a hole in the side of the dish but the sand shall run out of it to a level. Not an obvious property of a fluid body, as such, but this does imitate; and all this meerly caused by the vehement agitation of the conteining vessel, for by this means each sand becomes to have a vibrative or dancing motion, so as no other heavier body can rest on it, unless sustained by some other on either side; nor will it suffer any body to be beneath it, unless it be a heavier than itself." By this power of making the seen the |