temperature, possesses a vastly greater amount | blood. of heat than ice at the same temperature. Davy reasoned thus: "If I, by friction, liquefy ice, a substance will be produced which, according to the material theory, contains a far greater absolute amount of heat than the ice. In this case it cannot with any show of reason be affirmed that I merely render sensible heat which had been previously insensible in the frozen mass. Liquefaction will conclusively demonstrate a generation of new heat." He made the experiment, and liquefied the ice by pure friction. The experiment has been justly regarded as fatal to the material theory. PHYSIOLOGICAL HEAT. During the whole course of our lives we are continually inhaling and exhaling atmospheric air. Now, the nitrogen, which, as we have already learnt, constitutes four-fifths of the bulk of our atmosphere, does nothing toward the support of life. It is solely its companion element that sustains us. When we inhale, the oxygen passes across the cell-walls of the lungs and mixes with the blood, by which it is carried through the body. When we exhale, we pour out from the lungs the carbonic acid produced by the slow combustion of our bodies. To this slow combustion we owe our animal heat. Carbonic acid may be regarded as the rust of the body, which is continually cleared away by the lungs. In every part of the body this combustion is going on. The blood is forced by the heart through the arteries to all parts of the system, and after passing through the capillaries it returns to the heart through the veins. The venous blood is much darker than the arterial blood-an effect due to the deoxidation of the blood. To make room for fresh oxygen, the black venous blood yields up in the lungs the carbonic acid with which, through the combustion of the body, it was previously charged, the red color being thus restored. wrong. Consider, then, all the fires in the world and all the animals in the world continually pouring their carbonic acid into the atmosphere. Would it not be fair to conclude that our air must become more and more contaminated and unfit to support either combustion or life? This seems inevitable, but it would be a conclusion founded upon half knowledge, and therefore A provision exists for continually purifying the atmosphere of its excess of carbonic acid. By the leaves of plants this gas is absorbed, and within the leaves it is decomposed by the solar rays. The carbon is stored up in the tree, while the pure oxygen is restored to the atmosphere. Carbonic acid, in fact, is to a great extent the nutriment of plants; and inasmuch as animals, in the long run, derive their food from the vegetable world, this very gas, which at first sight might be regarded as a deadly constituent of the atmosphere, is the main sustainer both of vegetable and animal life. That the air which comes from the lungs is different in quality from that which goes into them may be shown by a simple experiment. Carbonic acid is warm, and therefore light, when freshly exhaled. It does not readily fall to the bottom of a vessel into which we breathe. But if the breath be chilled by sending it through a metal tube which passes through cold water, the carbonic acid may be collected in an open jar. A single expiration from the lungs suffices to fill a good-sized jar with the gas, which immediately quenches a lighted taper. It is a musical instrument complete in itself, the tremors of which, when they impinge on the nerves, produce the sensation of heat. BY HEAT. I have named the carbonic acid of our bodies "rust," and the reason I have done so is that it is produced by the oxidation of carbon, as iron rust is produced by the oxidation of iron. This latter process is exactly CONTRACTION OF STRETCHED INDIA-RUBBER analogous to the slow combustion within the animal frame; and when the heat thus produced is prevented from wasting itself, it may rise to destructive intensity. By such heat, in all probability, the first Atlantic cable was rendered useless. In 1861 the Messrs. Siemens had charge of the Rangoon and Singapore telegraph cable. Suspecting the injury that might accrue from heat, they had placed in the heart of each cable-coil an instrument capable of indicating any exaltation of temperature. The surmised increase occurred, the temperature augmenting daily by about 3° Fahrenheit. A temperature of 86° was at length shown within the coil when the outside temperature was only 60°. The cable would have been inevitably destroyed in the course of a few days if the generation of heat had been allowed to continue unchecked. The cable was cooled by pouring water at a temperature of 42° Fahrenheit upon the top of the coil. It issued raised to 72° at the bottom. Casting a backward glance over the series of actions here illustrated, we first figure the mutually attracting atoms apart, then rushing together and acquiring, while crossing the insensible interval which separates them, the velocity with which they strike each other. That this velocity is enormous is proved by the amount of heat which it generates. When the atoms clash they recoil, and the consequent tremulous motion is one form of heat. Thus every molecule is animated by a vibratory motion of its constituent parts. Nature is full of anomalies which no foresight can predict, and which experiment alone can reveal. From the deportment of a vast number of bodies, we should be led to conclude that heat always produces expansion and that cold always produces contraction. We have now to notice a first exception to this general rule. If a metal be compressed, heat is developed; but if a wire be stretched, cold is the result. Dr. Joule and others have worked experimentally at this subject and found this fact all but general. One striking exception to the rule (there are probably many others) has been known for a great number of years. The sheet of india-rubber now handed to me has been placed in the next room to keep it quite cold. Cutting from this sheet a strip three inches long and an inch and a half wide, and turning our thermopile upon its back, I lay upon its exposed face the strip of india-rubber. The deflection of the needle proves that the rubber is cold. Laying hold of the ends of the strip, I suddenly stretch it, and press it, while stretched, on the face of the pile. The needle moves with energy, showing that the stretched rubber has heated the pile. But one deviation from a rule always carries other deviations in its train. In the physical world, as in the moral, acts are never isolated. In many of his investigations Dr. Joule has been associated with Sir William Thomson, who, when made aware of the deviation of india-rubber from an aggrega almost general rule, suggested on theoretic | are pushed asunder; and on the relation of grounds that the stretched india-rubber these two antagonistic powers the might shorten on being heated. The test was applied by Joule, and the shortening was found to take place. THE SOLID, LIQUID AND GASEOUS FORMS OF MATTER. On the occasion of our first meeting here a sledge-hammer was permitted to descend upon a lump of lead, which was heated by the blow. Formerly it was assumed that the force of the hammer was simply lost by the concussion. In elastic bodies it was supposed that a portion of the force was restored by the rebound, but in the collision of inelastic bodies it was taken for granted that the force of impact was lost. We now admit no loss, but assume that when the motion of the descending hammer ceases it is simply a case of transference instead of annihilation. The motion of a mass has been transferred into molecular motion. Here the imagination must help us. In the case of solid bodies, while the force of cohesion still holds them together, you must conceive a power of vibration, within certain limits, to be possessed by their atoms. And the greater the amount of heat imparted to the body, or the greater the amount of mechanical action invested in it by percussion, compression or friction, the greater will be the rapidity of some, and the wider the amplitude of other, atomic oscillations. As already indicated, the atoms or molecules thus vibrating, and ever as it were seeking wider room, urge each other apart, and thus cause the body of which they are the constituents to expand in volume. By the force of cohesion, then, the molecules are held together; by the force of heat they tion of the body depends. Every fresh increment of heat pushes the molecules more widely apart, but the force of cohesion, like all other known forces, acts more and more feebly as the distance through which it acts. is augmented. As, therefore, the heat grows strong, its opponent grows weak, until, finally, the particles are so far loosened from the thrall of cohesion as to be at liberty not only to vibrate to and fro across a fixed position, but also to roll or glide around each other. Cohesion is not yet destroyed, but it is so far modified that the particles, while still offering resistance to being torn directly asunder, have their lateral mobility over each other's surfaces secured. This is the liquid condition of matter. In the interior of a mass of liquid the motion of every molecule is controlled by the molecules which surround it; but when we develop heat of sufficient power, even within the body of a liquid, the molecules break the last fetters of cohesion and fly asunder to form bubbles of vapor. If. moreover, one of the surfaces of the liquid be quite free-that is to say, uncontrolled either by a liquid or a solid-it is easy to conceive that some of the vibrating superficial molecules will be jerked entirely away from the liquid, and will fly with a certain velocity through space. Thus freed from the influence of cohesion, we have matter in the vaporous or gaseous form. This conception of gaseous molecules is now generally accepted as expressing the truth of nature. Such molecules are supposed to be always flying in straight lines through space. The hypothesis has been developed in our day by Joule, Krönig, and Maxwell, but chiefly in a series of admirable papers by Clausius. The quickness with which the perfume of an odorous body fills a room might seem to harmonize with the idea of direct projection. It may, however, be proved that if the theory of rectilinear motion be true, the molecules must move at the rate of several hundred feet a second. Hence it might be objected that, according to the above hypothesis, odors ought to ought to spread much more rapidly than they are observed to do. The answer to this objection is that the odoriferous molecules have to make their way through a crowd of air atoms, with which they come into incessant collision. On an average, the distance through which such a molecule can travel without striking against an atom of air is infinitesimal, the propagation of a perfume through air being thus enormously retarded by the air itself. When a free communication is opened between the surface of a liquid and a vacuum, the vacuous space is almost instantaneously filled with the vapor of the liquid. greater amount of heat would be expended in the lifting of the weight. Now, the atoms of bodies, though we cannot suppose them to be in contact, exert enormous attractions. It would require an almost incredible amount of ordinary mechanical force to augment the distances intervening between the atoms of any solid or liquid so as to increase its volume in any sensible degree. It would also require a force of great magnitude to squeeze the particles of a liquid or a solid together so as to make the body sensibly less in size. I have vainly tried to augment permanently the density of a soft metal by pressure. Water, which yields so freely to the hand plunged in it, was for a long time regarded as absolutely incompressible. Great force was brought to bear upon it, but sooner than shrink it oozed through the pores of the metal sphere which contained it, and spread like a dew on the surface. This is a classical experiment which was long ascribed to an erroneous source. Bacon is its author. half a century after him a similar experiment was described by the secretary of the Accademia del Cimento, and it thus came to be called "The Florentine Experiment." Ba About ENERGY OF MOLECULAR POSITION-SPECIFIC con's own account of his experiment is this: HEAT. We must, as usual, turn these conceptions regarding sensible masses to account in forming conceptions regarding insensible masses. As an intellectual act, it is quite as easy to conceive the separation of two mutually attracting atoms as to conceive the separation of the earth and our lead weight. If that weight had been lifted by a steam-engine, an amount of heat equivalent to the work done would have been consumed; and if the force of gravity were far greater than it is, a far 66 Now it is certain that rarer bodies (such as air) allow a considerable degree of contraction, as has been stated, but that tangible bodies (such as water) suffer compression with much. greater difficulty and to a less extent. How far they do suffer it I have investigated in the following experiment: I had a hollow globe of lead made capable of holding about two pints and sufficiently thick to bear considerable force; having made a hole in it, I filled it with water, and then stopped up the hole with melted lead, so that the globe be came quite solid. I then flattened the two opposite sides of the globe with a heavy hammer, by which the water was necessarily contracted into less space, a sphere being the figure of largest capacity; and when the hammer had no more effect in making the water shrink, I made use of a mill or press, till the water, impatient of further pressure, exuded through the solid lead like a fine dew. I then computed the space lost by the compression, and concluded that this was the extent of compression which the water had suffered, but only when constrained by great violence." By refined and powerful means we can now compress water, but the force necessary to accomplish this is very great. When, therefore, we wish to overcome molecular forces, we must attack them by their peers. Heat accomplishes what mechanical energy, as usually wielded, is incompetent to perform. Bodies, when heated, expand, and to effect this expansion the molecular attractions must be overcome, and where the attractions to be surmounted are so vast we may infer that the quantity of heat necessary to overpower them will be commensurate. A moment's further attention devoted to this wonderful substance, water, will repay our pains. First we have its constituents as free atoms of oxygen and hydrogen, which attract each other and combine. The mechanical value of this atomic act is easily determined. The heating of 1 lb. of water 1° C. is equivalent to 1390 foot-pounds; hence the heating of 34,000 lbs. of water 1° C. is equivalent to 34,000 x 1390 footpounds. We thus find that the concussion of our 1 lb. of hydrogen with 8 lbs. of oxyis equal, in mechanical value, to the gen raising of 47,000,000 pounds one foot high. It was no over-statement, then, on my part, when I affirmed that the force of gravity, as; exerted near the earth, is almost a vanishing quantity, in comparison with these molecular forces. The distances which separate the atoms before combination are so small as to be utterly immeasurable; still, it is in passing over these distances that they acquire a velocity sufficient to cause them to clash with the tremendous the tremendous energy here indicated. After combination the substance is in a state of vapor which sinks to 100° C. and afterward condenses to water. In the first instance, the atoms fall together to form the compound; in the next instance, the molecules of the compound fall together to form a liquid. The mechanical value of this act is also easily calculated: 9 lbs. of steam, in falling to water, generate an amount of heat sufficient to raise 537.2 x 94835 lbs. of water 1° C., or 967 x 98703 lbs. 1° F. Multiplying the former number by 1390, or the latter by 772, we have, in round numbers, a product of 6,720,000 foot-pounds as the mechanical value of the mere act of condensation. The next great fall is from the state of water to that of ice, and the mechanical value of this act is equal to 993,564 foot-pounds. Thus, our 9 lbs. of water, at its origin and during its progress, falls down three great precipices: the first fall is equivalent, in energy, to the descent of a ton weight down a precipice 22,320 feet high; the second fall is equal to that of a ton down a precipice 2900 feet high; and the third is equal to the fall of a ton down a precipice 433 feet high. The stone avalanches of the Alps are sometimes seen to smoke and thunder down the declivities with a vehemence |