this one, we may be inclined to think it less chimerical than it' would at first sight appear.

It is well known that when a body is expanded heat disappears. By the old theory, it is absorbed and becomes latent; by the new, it is consumed in producing the motion of the atoms in expanding, or it is used up in interior work in changing the state of aggregation of the atoms; that is, in converting a solid into a liquid or a liquid into a gas. Thus latent heat is only heat changed into motion. It therefore disappears as heat. But when the body is condensed, the same motion of translation is changed back into that of vibration, and appears again as sensible heat.

In all cases of friction the same theory applies. Here the motion, destroyed by friction, shows itself in the form of heat or molecular motion. A railway train furnishes a good illustration. "It is the object of the railway engineer to urge his train bodily from one place to another, say from London to Edinburgh, or from London to Oxford, as the case may be; he wishes to apply the force of his steam, or of his furnace, which gives tension to the steam, to this particular purpose. It is not his interest to allow any portion of that force to be converted into another form of force which would not further the attainment of his object. He does not want his axles heated, hence he avoids, as much as possible, expending his power in heating them. In fact, he has obtained his force from heat, and it is not his object to reconvert the force thus obtained into its primitive form. For, for every degree of temperature generated by the friction of his axles, a definite amount would be withdrawn from the urging force of his engine. There is no force lost absolutely. Could we gather up all the heat generated by the friction, and could we apply it all mechanically, we should by it be able to impart to the train the precise amount of speed which it had lost by friction. Thus 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 so doing he is unconsciously affirming both the convertibility and indestructibility of force. He is practically asserting that mechanical energy may be converted into heat, and that when so

converted it cannot still exist as mechanical energy, but that for every degree of heat developed a strict and proportional equivalent of the locomotive force of the engine disappears. A station is approached say at the rate of thirty or forty miles an hour; the brake is applied, and smoke and sparks issue from the wheel on which it presses. The train is brought to rest. How? Simply by converting the entire moving force which it possessed at the moment the brake was applied into heat."-Page 21.

By the consideration of these examples, and others of like character, we are forced to recognize the doctrine of "correlation of forces," or the mutual convertibility of the various forms of force. This doctrine is now universally recognized as a principle of science, and has already been productive of many astonishing results. Not only is motion convertible into heat, but as every mechanical force is measured by the motion it produces or the work it performs, so every form of mechanical energy may be ultimately expressed in terms of heat.

The establishment of this principle, now become axiomatic, was a grand step in the advancement of science; but another step yet remained to be taken in the same direction. The quantitative determination of the mechanical equivalent of heat remained to be made. For even after the point that heat and mechanical force were qualitatively convertible was reached, the question yet remained, How much mechanical energy can we get out of a given quantity of heat? or a fixed amount of mechanical energy being given, how much heat will it produce? This practical question has been answered by two dif ferent persons, working independently of each other, and by entirely different processes, yet arriving at the same result. In 1842 Dr. Mayer, of Heilbronn, Germany, determined the mechanical equivalent of heat by a calculation based upon the known constitution of elastic fluids and their rate of expansion under a constant pressure. The result of Mayer's calculation, though entirely theoretical, was corroborated by the experimental determination of the same question in 1843 by Dr. Joule, of Manchester. By agitating water and other fluids by paddles turned by measurable forces, by causing metallic disks to rotate against each other, and by forcing water through capillary tubes, he determined in each case the exact quantity

of heat generated and the amount of force expended. By these experiments, patiently carried on for years, he not only established the correctness of the principle before advanced theoretically only, that under all circumstances the quantity of heat generated by a given amount of force is fixed and invariable, but also obtained the exact measure of it. This mechanical equivalent of heat, as obtained both by Mayer's calculation and Joule's experiments, may be stated thus: The heat sufficient to raise the temperature of one pound of water one degree Fahrenheit will generate force enough to raise seven hundred and seventy-two pounds one foot high, or one pound seven hundred and seventy-two feet high; and con-⚫ versely, if one pound falls seven hundred and seventy-two feet, it will generate heat enough to raise the temperature of one pound of water one degree. Or in other words, if we consider the heat required to raise one pound of water one degree Fahrenheit as the unit of heat, and the force necessary to raise one pound weight one foot high as the unit of force, we find that one unit of heat is equivalent to seven hundred and seventyseven units of force. This is called "Joule's Law."

This determination completed one of the greatest triumphs of modern science. The introduction of the balance into chemistry by Lavoisier afforded the means of proving the indestructibility of matter; but the experimental discovery of the exact value of the unit of heat has led as inevitably to the adoption of this higher principle of the conservation or persistence of force. This doctrine, it is true, had already been recognized as a legitimate deduction from the indestructibility of matter; for, as we have no knowledge of matter except by the force it exerts, so if matter is indestructible, the force behind it, of which it is but the manifestation, must also be indestructible. But now it is firmly established on the basis of experiment, and must be admitted in all our hypotheses regarding the action of matter as a physical principle equally with the invariability of gravity and the indestructibility of matter. Faraday calls it "the highest law in physical science which our faculties permit us to perceive." "No hypothesis," he says, "should be admitted, nor any assertion of a fact credited, that denies the principle. No view should be inconsistent or incompatible with it."

While this principle of the correlation and conservation of physical forces is universally admitted by scientific men, it is considered premature as yet to attempt to extend it to vital forces, though the attempt has been made by Mayer, Helmholtz, Carpenter, and others, and with much ability and ingenuity. The phenomena of life are not yet well enough understood to admit of much more than a merely speculative application of it. This, however, is one of the new fields of investigation which this new philosophy has opened to science, and a rich harvest of interesting and useful results may soon be expected from it.

Let us now consider some of the practical applications of Joule's law. This mechanical equivalent of heat being known, it is easy to apply it to the measurement of any form of physical force in nature. "From these considerations," says Professor Tyndall, "I think it is manifest that if we know the velocity and weight of any projectile, we can calculate with ease the amount of heat developed by the destruction of its moving force. For example: knowing as we do the weight of the earth and the velocity with, which it moves through space, a simple calculation would enable us to determine the exact amount of heat which would be developed, supposing the earth to be stopped in her orbit. 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 and Helmholtz have made this calculation, and found that the quantity of heat generated by 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 a simple stoppage of the earth 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 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."-Page 57.

We have no better or more striking application of this law than in the calculation of the mechanical energy exerted in the passage of water through the various stages of its existence.

When one pound of hydrogen combines with eight pounds of oxygen to form nine pounds of steam, the concussion is found to be equivalent in mechanical value to the raising of fortyseven million pounds one foot high. The force with which the atoms of these nine pounds of steam fall together to produce water is sufficient to lift six million seven hundred and eighteen thousand seven hundred and sixteen pounds one foot high. Again, in changing to ice, the nine pounds of water have a fall whose mechanical value is nine hundred ninetythree thousand five hundred and sixty-four foot-pounds. "Thus our nine pounds of water, in its origin and progress, falls down three great precipices; the first fall is equivalent to the descent of a ton weight, urged by gravity down a precipice twenty-two thousand three hundred and twenty feet high; the second fall is equal to that of a ton down a precipice two thousand nine hundred feet high; and the third is equal to the descent of a ton down a precipice four hundred and thirty-three feet high. I have seen the wild stone avalanches of the Alps, which smoke and thunder down the declivities with a vehemence almost sufficient to stun the observer; I have also seen snow-flakes descending so softly as not to hurt the fragile spangles of which they were composed; yet to produce from aqueous vapor a quantity of that tender material which a child could carry, demands an exertion of energy competent to gather up the shattered blocks of the largest stone avalanche I have ever seen, and pitch them to twice the height from which they fell."-Page 164.

Thus far heat has only been considered as existing in the bodies in which it was generated. We are yet to consider the method by which it is transferred from one body to another with which it is not in contact. That it is so transferred is proved by our sensations on approaching a hot body. This is radiant heat. By the corpuscular theory, radiation of heat was easily explained. On the supposition that heat was matter whose particles were self-repellant, it was easy to understand how these particles must be driven off in straight lines in every direction through space by their own repulsion. But when we reject the materiality of heat, and consider it nothing but the vibrations of the atoms of matter, the question at once arises, How are these vibrations to be transmitted through