550

Work of a Steam-engine.

2160 pounds of lead resting on it and no air, the result would be the same.

Then, knowing further the weight of a cubic foot of air, and the the so-called capacity for heat of air at different densities, as compared with an equal weight of water, there will be no difficulty in determining the quantity of heat absorbed or expended in lifting this vast weight. The ascertained fact is, that the amount of heat which warms one pound of water one degree of Fahrenheit, (the so-called thermal unit) has force to lift one pound weight of anything 772 feet, or a weight of 772 pounds one foot. This expansive force of the unit of heat, as compared with the downward force of gravitation, is called the mechanical equivalent of heat.

774. Persons, before studying this subject, would be far from thinking that so small an amount of heat could produce such a powerful mechanical effect, but now there is no room for doubt. This fact, when once understood, explains many other marvels; for instance, the force of a single steam-engine doing the water-pumping work of six hundred strong horses. Then it is the force of the sun's hot rays which is daily lifting to the clouds, in evaporation from the water-surfaces below, the enormous quantity of water which falls again all over the globe as rain and snow, ultimately to fill the countless river-channels, great and small, which return it to the sea. Then it is the heat of the sun which produces all the motions of the atmosphere called winds, including the variable breezes of temperate climates, the strong and steady trade-winds of the tropics all round the earth, and the furious hurricanes which occasionally sweep over wide regions. And, lastly, the whole surface of the ocean is constantly heaving in waves, low or lofty, produced by the force of the wind blowing upon it.

In the last paragraphs are set forth phenomena in which heat acts mechanically to produce motion in heavy masses, with a force exactly proportioned to its quantity. We may now state, in addition, that motions of masses so produced can, when suddenly arrested by obstacles, reproduce the exact amount of heat which caused them, either of the two forms of force being thus convertible into the other.

775. It had long been familiarly known that certain motions or masses, while exhibiting the phenomena of friction, percussion, condensation, and others, generated heat, but the relations were not accu rately determined. Thus a soft iron nail laid upon an anvil, and receiving in rapid succession powerful blows of a hammer, becomes hot

Heat from Percussion and Friction.

551 enough to light a match. In the case of the mutual percussion of flint and steel in the old gun-lock, small portions of one or both are struck off in a state of white heat, and the minute particles of the iron become incandescent and burn in passing through the air. The heat produced by rubbing strongly against each other two pieces of dry wood, has been elsewhere described as the means commonly used among savages to light their fires. Men warm their cold hands in winter by rubbing them against each other, or against their coatsleeves. The axles of a heavily-laden waggon, or of the rapidly revolving wheels of a railway-carriage, if left without grease or oil, may be so heated by friction as to inflame the wood and set the carriage on fire. The line attached to a whale-harpoon, as it runs over the side of the boat, when the whale dives after being struck, requires to have water constantly thrown upon it to prevent it setting fire to the boat. The cable of a ship drawn very rapidly through the hawse-opening, produces there intense heat and smoke. When a great ship is launched from the builder's yard, and glides along the sloping beams to the water, a dense smoke usually rises from the points of rubbing contact.

Other examples present themselves in daily life. The friction of the wheel of a heavily laden waggon on a paved road, when the wheel is locked and prevented from revolving, generates intense heat, indicated by the production of smoke and vapour. A man cannot venture to touch the iron skid of a wheel which has been used in the long descent of a hill. Another remarkable instance of the conversion of the force of percussion into heat, is seen in firing a leaden bullet against an iron screen. The heat generated is such as often to carry it to the point of fusion, which is not less than 600°.

776. In all these cases, however, the amount of heat produced is variable, and it had not been accurately measured until the experiments of Joule, Mayer, and others, proved that the quantity of heat evolved is definite and exactly proportioned to the mechanical force expended. Thus, a body falling from a height, and arrested by collision with another body on the ground, produces just as much heat shared between the two as, if again used to dilate air or steam, would lift the body to the height from which it fell; nearly as the momentum of a pendulum acquired by descending from one side of its arc to the centre, just suffices to lift it to the same height on the other side. Any liquid poured out and falling into a vessel below, becomes heated in exact proportion to the height of fall. Any liquid

552

Experiments of Mayer and Foule.

as water, oil, or mercury, driven round or churned in a vessel by a paddle-wheel moved by a falling weight, is warmed just in proportion to the weight and the height of its descent. There are similar relations and correspondence between heat and other forces and actions as of electricity, chemical combination or decomposition, and muscular power; and there is reason to believe that all these are convertible into one another in fixed proportions.

By the terms mechanical equivalent of heat, we are, therefore, to understand the quantity of heat required to produce a certain amount of work or certain mechanical results. It is susceptible of measurement and expression in a formula, as it has been elsewhere explained (Art. 192).

777. In 1842, Dr. Mayer, of Heilbronn, made a calculation of the equivalent of heat based upon certain theoretical grounds. In the following year Dr. Joule, of Manchester, performed many. experiments on this subject, the results of which are considered to establish, in a conclusive manner, the exact mechanical relation or equivalence of heat. He caused paddles to turn in vessels of water, of oil, and of mercury, by means of a weight falling through a given height; and, measuring the increase of heat generated by the agitation of the liquids, he found within the legitimate limits of experimental accuracy, that from the same amount of mechanical energy the same amount of absolute heat was in every case produced, account being of course taken of the fact, that the sensible expression by the thermometer of the heat communicated to the water, would be only one-thirtieth of that in the case of mercury.

Percussion, as it has been already observed, offers a simple and direct example of the conversion of mechanical force into heat. If a ball of iron or lead be allowed to fall from a height of say ten feet upon a block of iron or lead, the moving force expressed from gravity would be stopped and apparently destroyed by the percussion. In reality, however, it has been converted into an equivalent non-locomotive or internal motion, which for such a small height of fall might escape observation, but which, for a great height or after repeated falls, would be very appreciable.

The experiments of Joule, Mayer, and others, show that, with a double height of fall, a double degree of heat is created; also that, knowing the weight of any body and the height from which it fell, we can at once determine the amount of heat that would be generated by percussion from that height. The average of a great number of experiments made by Joule, gives the quantity of heat

Heat from the Collision of Bodies.

553 generated by one pound in falling from a height of 772 feet, and thus converting all its moving power into heat by percussion or friction, to be such as would raise one pound of water by one degree (Fahr.), or one pound of mercury by thirty degrees.*

778. It follows, then, that, knowing the weight of any body and the rate at which it is moving, and having deduced the height from which it would have to fall, in order to produce the same velocity, we can estimate what amount of heat would be produced, if its motion were to be suddenly converted into heat by collision or percussion. A fall of 772 feet corresponds to a velocity of about 223 feet per second (Art. 139); a fall of 4, 9, &c., times 772 feet corresponds to a velocity of 2, 3, &c., times 223 feet per second, but to an increase of 4, 9, &c., times the quantity of heat if the fall be stopped by percussion. Hence, just as by doubling the velocity of a cannon-ball we quadruple its penetrating energy, so we quadruple the heat-motion or energy derivable from friction or percussion.

On the other hand, if we know the amount of heat generated and the weight of the colliding masses, we can calculate the velocity of collision. The chemist explains the burning of a candle, or of common gas, as the union, in virtue of chemical affinity or attraction, of the carbon particles of the candle or gas with the oxygen particles of the air: the physicist goes one step farther and sees, in the atomic attraction of the chemist, an intense atomic projectile force, as it were, which produces the heat and flame in exactly the same way as the percussion or collision of large visible masses produces heat in them; and if we knew the absolute weights of oxygen and carbon-particles, and the degree of heat generated by a given number of them, we could at once tell the velocity with which they collide. This subject is, however, at present beyond the reach of experiment. It is demonstrable that masses of matter produce heat by percussion or friction; and it is inferred that where heat is evolved, as in the chemical union of substances, the ultimate atoms suddenly acquire an intensely rapid movement among themselves, so as to become incandescent, and emit heat as well as light; in other words,

* This, as it has been already explained, constitutes a unit of heat. On the Centigrade scale, the figures would of course differ, as one degree of this scale corresponds to 18° of Fahrenheit. It would require the fall of one pound through 1390 feet to raise the temperature of one pound of water by one degree, or one pound of mercury by thirty degrees.

554

Heat from Chemical Combination.

that chemical union operates by producing a violent mechanical motion among atoms which they did not previously possess. In the production of heat by friction or percussion, the substances undergo no change; they are the same after as before. In its production by chemical agency, however, the substances are no longer the same; they are converted into new forms of matter, and are so completely changed in their properties as to be no longer recognizable in the compounds produced.