380 Heat not material. 562. Although heat is known to be abundant in the sunbeam, and to radiate copiously from a blazing fire, we cannot arrest or detect it in its progress except by allowing it to enter, and remain, in some ponderable substance. We know hot iron, hot water, or hot air, but nature nowhere presents to us, nor has art succeeded in exhibiting to us, heat alone. If we balance a quantity of ice in a delicate weigh-beam, guarded against air-currents, and then leave it to melt, the equilibrium will not be in the slightest degree disturbed. Or if we substitute for the ice, boiling water or red-hot iron, and leave these to cool, there will be no difference in the result. If we place a pound of mercury in one scale of the balance, and a pound of water in the other, and then either heat or cool both through the same number of degrees, although (as will be explained below) about thirty times more heat enters or leaves the bulky water than the dense mercury, the two substances will still remain perfectly equipoised.* Again, a broad sunbeam, with its intense light and heat, may be concentrated by a powerful lens or mirror, and be made to fall upon the scale of a most delicate balance placed in a vacuum, but will produce no depressing effect on the scale, such as would follow if what constitutes the beam had the least weight or momentum forwards. * These facts appear to show that the presence of heat in a body cannot be determined by the most delicate balance. The same may be said of electricity, for a Leyden jar charged with electricity, sufficient to destroy the life of an animal, weighs no more than it did before it received the electricity. In describing these and other forms of matter as imponderable, we must not overlook the fact that our means of determining the presence of matter by gravitation are still very imperfect. The most delicate balance will not with certainty indicate the presence of a smaller quantity than the thousandth part of a grain. The mote in a sunbeam is imponderable, but it is still visible and material. Professor Tyndall has lately shown by experiments that these motes can only be detected in the atmosphere by passing through a closed glass vessel containing air, a powerful beam of light in a darkened room. Light is therefore a more delicate test of the presence of matter than any balance yet constructed. A grain of matter may be split into a million or a billion of parts (see Art. 2), and thus rendered imponderable. Sir Humphry Davy long ago observed that if heat was as much lighter than hydrogen, as hydrogen is lighter than platinum, i.e., bulk for bulk, 230,000 times, it would be far beyond the reach of any balance, although it might still be material. Such were among the facts which led modern physicists to reject the old material theory or separate existence of a matter of heat called caloric, and to hold heat to be merely motion of a certain kind among the material particles of bodies. "The Radiometer." 563. The crucial character of this test of the materiality of heat disappears, however, in the face of more delicate modern experiments, especially the recent experiments of Mr. Crookes on the "attraction and repulsion resulting from radiation." Mr. Crookes suspends a fine glass stem, with a sphere or disc of pith, or paper, or metallic foil at each end, by means of a single cocoon fibre, inside a glass bulb of three inches diameter blown at the end of a glass tube eighteen inches long. By means of a Sprengel mercury pump (see Art. 468) the air is exhausted from within the tube and bulb with the greatest possible perfection. Strange to say, a beam of sunlight has a most powerful effect on this tiny balance; not only so, but the mere heat of the finger, or of a candle, impinging upon either disc, suffices to wheel it round through a quarter of a turn or more. With a large apparatus of the same kind, and a pith bar suspended, a lighted candle placed about two inches from the globe causes the pith bar to oscillate to and fro, and then to make, as by cumulation of the heating effect, several complete revolutions, till the twist of the suspending fibre stops the rotation and finally reverses it; and this movement is kept up with great energy and regularity as long as the candle burns. The action of a piece of ice or other source of cold is the reverse of this, causing the index to follow the block of ice as a needle follows a magnet. Mr. Crookes has also suspended a lump of magnesium by means of a fine platinum wire within a long tube, so as to form a pendulum 39°14 inches in length (a seconds' pendulum). On making a very good vacuum within the tube, he has found that a ray of sunlight allowed to fall once on the pendulum and then cut off, is sufficient to set it swinging. Several other forms have been given to the apparatus, but the principle is much the same in all, and the results are quite uniform in their character if the vacuum is perfect. When air or any gas is admitted the results are gradually modified, attraction in air taking 382 Mr. Crookes' Radiometer. the place of repulsion in vacuo. The barometric position or the density of the gas, at the neutral point which divides attraction from repulsion, seems to vary with the density of the disc on which the radiation falls, and on other physical conditions, which Mr. Crookes has investigated. He "is inclined to believe that the true action of radiation is repulsion at any pressure, and that the attraction observed when the rarefaction is below the neutral point is caused by some modifying circumstances connected with the surrounding gas, but not of the nature of air currents. The sensitiveness of this apparatus is extraordinary, being much superior to that of an ordinary thermometric pile. It remains for future experiments in the same direction to prove whether bare luminosity, apart from any heating rays, has such a sensible effect. Mr. Crookes' experiments seem to show that heatrays act with the same effect on white or black pith discs, but that luminous rays sifted out from their heating companions have a different action and seem to repel black surfaces more than white. Taking advantage of this fact, he has constructed an instrument of extreme interest and delicacy which he calls a radiometer. It consists simply of four arms suspended on a steel pivot, and rotating horizontally like a miniature wind-gauge, with pith discs at the ends of the four arms, painted black on one side, the black sides all facing one way. The whole is enclosed in a glass tube from which the air is exhausted to the highest attainable degree, the tube being then hermetically sealed. The mere light of a candle at a distance of one or two feet causes the arms of such an apparatus to rotate slowly, while the speed increases proportionally by reducing the distance, according to the well-known law of luminous intensity, being fourfold for half the distance, &c.; or the speed increases by keeping the light at the same distance and adding more candles. In full daylight the arms keep up a constant rotation at the rate of from thirty to forty turns per minute; while in full sunshine the rate increases to three or four turns per second, or even more, according to the lightness and delicacy of the individual radiometer. Mr. Crookes, in assigning this mechanical effect to the radiant force or ethereal momentum, has estimated that a candle at a distance of six inches has a mechanical effect equal to '00172 grain. Experimenting on the strength of solar radiation, Mr. Crookes makes out its mechanical effect to be equal to 32 grains on the square foot, or The Energy-doctrine of Heat. 383 57 tons on the square mile, or three billion tons on the whole earth, -a force which, but for gravitation, would drive our globe into space.* Mr. Cunnington has shown by various experiments that heat, and not light, is most probably the motive power,f as in the experiments above described. Admitting this statement, modern physicists look upon light and radiant heat as similar forces, differing only in the number of vibrations. "Heat a form of Energy." 564. In another part of this work a place was assigned to Heat among forms of energy, thereby virtually removing it from a classification among substances; it now remains to explain more fully the reasons for stating, and the real meaning of the statement, that "Heat is a mode of motion," or rather of energy. Bacon, among his keen-sighted aphorisms in his Novum Organum asserts that "heat is a species of the genus motion," that it is not merely a result of motion, but is in its essence motion and nothing else. Apart, however, from experimental proof, the speculative reasonings of even such an acute mind as that of Bacon are comparatively valueless. Experiment is the only sure foundation upon which to build our arguments as to the causes of natural phenomena. We shall, therefore, give a short account of the experimental data, to accord with which the only reasonable hypothesis is that heat is a form of energy. In 1798 there was read before the Royal Society, by Count Rumford, an essay on the generation of heat by friction, in which it was contended that heat could not be a substance or material, but was in all probability motion. He was led to the conclusion by the results of a number of experiments he had made, which had been suggested by his observation of the very intense heat generated by the boring of cannon in the arsenal at Munich. His most important experiment was made with a hollow cylinder like a cannon, having a blunt steel borer which pressed against the bottom, the cylinder being turned round the borer by two horses. The cylinder turned water-tight in the centre of a deal box containing about 2 gallons of water: at starting, the temperature of the apparatus and the water was 60°F. Exactly at the end of 2 hours, the water was actually raised to the boiling point by the heat generated · Proc. R. S.,' April, 1875-6. Pop. Science Review,' April, 1876. 384 The mechanical theory of Heat. by friction against the borer. Calculating the amount of heat produced in the metal cylinder and borer, and adding this to that produced in the liquid, Count Rumford concluded that by converting into friction the motion of two horses, turning for 24 hours, sufficient heat had been generated to raise the temperature of 261⁄2 lbs. of water from the freezing to the boiling point. A constant stream of heat had thus been given out in all directions without any signs of exhaustion, and it was altogether incomprehensible that a limited mass could be capable of evolving an inexhaustible supply of any material substance, or of anything else than motion supplied to it from some other inexhaustible source of motion. The explanation of this production of heat by friction given by those who, under the name caloric, regarded heat as a substance, was that abrasion lessened the capacity of bodies for heat, and so liberated the heat which lay concealed in the intermolecular spaces. of the unabraded mass. Experiment, however, proves that the capacity of a body for heat remains the same, whether it be in mass or in powder; in other words, that a pound of solid copper and a pound of copper dust thrown (at a temperature of 60° F.) into a pound of boiling water, absorb the same amount of heat from the water or diminish its temperature by the very same amount. Sir Humphry Davy demonstrated the inconsistency of this hypothesis of latent heat by showing that water, with double the capacity for heat compared with that of ice, can be generated from ice by merely rubbing two ice-blocks together at a temperature below 32° in vacuo-an incontestable proof that the molecular excitement due to the friction is the soie cause of the difference between the liquid and the solid forms of water. 565. In every case of friction, of percussion, or of compression, heat is produced in greater or less quantity according to the mechanical force expended; and the simple and most reasonable interpretation of the fact is, that the sensible heat generated, is merely the transference of the molar or locomotive motion to the molecules of the body, in which it may be conceived to exist as an exceedingly rapid vibration, or non-locomotive motion, which is revealed to us as temperature by the thermometer. It has been proved further, that an exaci relation holds between the amount of heat generated and the amount of mechanical force expended in producing it There are many difficulties surrounding the experimentai determination of this relation. In the first place only simple mechanical means must be employed, such as are |