« AnteriorContinuar »
movement to matter, and to imagine a species of movement constituting heat ? Never did savant, who had painfully learned to observe what surrounds him, entertain that thought. For him, the cause of heat is of the same order with that of the fall of bodies to the surface of the earth. It is a force, an abstract principle, which it is not his mission to fathom. And if, having become philosopher, he aims to ascend higher in the scale of causes, he must advance with an extreme sagacity, under the penalty of encountering the most mortifying failures. Few men are endowed with those qualities of mind which are congruous to the philosopher, and those who carry into this rugged enterprise the science and the modesty of the sage are apt to arrive, in their conclusions, at principles of the purest spiritualism.
PRINCIPLES OF THE MECHANICAL THEORY OF HEAT.
BY DR. Joh. MÜLLER,
(COMMUNICATED BY THE AUTHOR, IN GERMAN, TO THE SMITHSONIAN INSTITUTION, FROM HIS LEHRBUCH
DER PHYSIK, AND ACCOMPANIED BY HIS ALTERATIONS AND CORRECTIONS.)
1.-DEVELOPMENT OF HEAT BY MECHANICAL MEANS.
It is well known that by compression of the air heat is disengaged; and under certain circumstances, as, for example, by means of the fire syringe, may be rendered so considerable as readily to kindle combustible matter. Such development of heat, however, also takes place through the compression of a solid body. To how high a degree the hardest bodies may be heated by violent compression may be observed in the hammering of metals and the coining of money.
But among all mechanical means of generating heat none is more available than friction; and it is this which is almost universally employed when fire is to be provided anew. Every one knows that, for this purpose, uncivilized tribes make use of two pieces of wood—Fig. 1, for example, shows an arrangement of which the Dakota Indians avail themselves. A staff, a b, of hard wood, about
six inches long and somewhat sharpened at both ends, is inserted in a small cavity of the board A, is pressed on the upper end by the board B, and, in the manner represented in the figure, is thrown into rapid revolution. As soon as fire makes its appearance, a piece of tinder, applied by a second person, effects the desired purpose. The kindling of fire by steel and flint depends likewise on the application of heat, developed by attrition, which suffices to kindle to a glow some of the small detached particles of steel, and the now widely-employed
friction match derives its great utility from the heat supplied by a slight rubbing of the kindling matter.
The first scientific experiments on the development of heat by friction were those which Count Rumford instituted in the cannon foundry of Munich, (Gilbert's annal. XII, p. 554.) In order that the muzzle of the cannon, which in founding is uppermost, may not become porous, a massive cylindrical piece of metal is cast thereon, usually called “the rejected head," (der verlorene Kopf.) In one experiment Count Rumford inclosed the rejected head of a six-pounder in an oblong wooden box, open at the top and filled with water. Through one end of this box was passed, water-tight, the narrow neck which united the head to the cannon; and through the other, also water-tight, the stem of a steel borer. The cylindrical head was 9.8 inches long and 7.75 inches in diameter. The box was charged with 184 pounds of water; the arrangement being horizontal, the cannon and the attached head were made to revolve by horse-power at the rate of 32 revolutions per minute, the borer, at the same time, being pressed against the end of the head. The temperature of the water was raised after one hour 41° C.; after one and a half hour, 61° C.; after two hours, 81° C. To the wonder of the spectators the water, at the expiration of two and a half hours, was actually boiling. The cylinder and the spindle of the borer were also heated to the same temperature. During the two and a half hours 4,145 grains (about 17 half-ounces) of metal shavings had been turned out.
The experiment not being easily repeated in the form above given, a very elegant and commodious apparatus for exhibiting the same result has been devised by Professor Tyndall. On the axis of rotation of a small wheel (driven by a large one) is screwed a glass tube a, (Fig. 2,) open above and closed below, and having a length of about 12 centimetres, with a diameter of rather more than two centimetres. This tube is not quite filled with water, and is held firmly
between two boards of hard wood, provided with semi-circular grooves, the boards being connected by a hinge on one side, and on the other pressed against the tube with the hand. If the fly-wheel be now thrown into rapid rotation, so strong a friction is exerted upon the circumference of the tube that the temperature of the water is quickly raised, as may be easily shown by a thermometer, and finally attains the boiling point. If the tube be made air-tight, with a cork not too firmly fixed, the latter will be thrown out through the elasticity of the enclosed
vapor. Should the experiment require too much time, the tube may be at first filled with warm water.
Sir. H. Davy succeeded in melting two pieces of ice by rubbing them together in a spaco exhausted of air and cooled below the freezing point, while Mayer first showed (1842) that heat is developed by the friction of water against solid bodies, having, by simple agitation, raised its temperature from 12° C. to 13° C. (Annal. der Chem. und Pharm., May, 1842.)
II.—THE NATURE OF HEAT. As regards the explanation of the phenomena of heat, two contrary hypotheses have stood, from an early period, in opposition to one another. According to the
these phenomena proceed from an imponderable element, which, filling up the intervals between the separate atoms of matter, operates as a repulsive principle. Through an augmentation of the particles of heat in a body, its temperature is raised, its constituent atoms still further separated from one another, and thus its volume increased, while cohesion becomes more and more enfeebled and the conditions of aggregation are changed; solid bodies melting, and fluids passing into vapor. This mode of explanation has, till the most recent times, formed the basis of the doctrine of heat as presented in most popular works on the subject, without any positive assertion however, as to the correctness of such a theory. It was employed, in the interim, for want of a better, in order more easily to combine the various phenomena of heat under a common point of view.
The hypothesis of which we speak, namely, that the phenomena of heat result from the quiescent presence of an inponderable calorific element, and which, on that account, we will call briefly the material theory, stands opposed to another, according to which heat is the result of a vibratory motion of the minutest particles of bodies, and which thus refers the explanation of the phenomena to mechanical principles; on this account we shall designate the latter in our further discussion of those principles as the mechanical theory of heat. It was long ago said by Locke that “heat is a nost active concussion of the imperceptibly small particles of a body, which produces in us the feeling we term warmth; the cause of our perception of heat is, in reality, therefore only a motion.” There is nothing, in fact, which argues more conclusively in favor of the mechanical explanation of the phenomena of heat than its production through mechanical forces, as exhibited in preceding paragraphs. Certainly, neither the experiment of Rumford nor that of Davy gives the smallest countenance to the conduction of calorific matter from without.
The adherents of the material theory sought to explain the development of heat by the agency of compression, on the assumption that the capacity of bodies for heat decreases with their density, whence a body, when its density is increased, must give out heat. The difference between the specific heat of gases under constant pressure and with constant volume seemed to argue in favor of this hypothesis till Regnault had proved that the specific hcat of a given weight of gas is independent of its density. With regard to the development of heat by friction, the material theory endeavored to account for it by assuming that friction is always attended by a corresponding compression, and by the diminution of specific heat thereby occasioned. But more exact investigation showed that the specific heat of the shavings, which fall away from the cannon in boring, differed not sensibly from that of the metal before the boring; while in the experiment of Davy a body is formed, namely, water, whose specific heat is not only not smaller than that of ice, but is actually twice as great. Here, then, the development of heat in nowise admits of being referred to a diminution of the specific heat.
Rumford, as well as Davy, had instituted their experiments with a view to prove the necessity of having recourse to a mechanical explanation of heat. But though they had certainly indicated the right course for answering the question as to the nature of heat, that course was, for the time at least, not followed up;
not so much perhaps because the generality of physicists had declared, in opposition to the two English philosophers, for the material theory, as that the scien. tific inquirers of that period scarcely occupied themselves at all with the question. Still, however, the material in hand did not cease to accumulate, which, when thrown at a later period into the scale, was destined to give an unquestionable preponderance to the mechanical theory.
The study of radiant heat had taught that every heated body in a colder medium sends forth on all sides calorific rays in like manner as a luminous body distributes rays of light. And as the luminous rays pass through air and other transparent bodies, without communicating to them the property of luminosity, so the rays of heat traverse the air and other diathermanous substances without imparting to them any sensible warmth. The rays of heat are then only converted into perceptible heat when they are absorbed by some body upon which they strike, in the same way that certain bodies (phosphorus, for instance) become themselves luminous under the influence of strong rays of light.
Like the rays of light, the rays of heat are propagated with a velocity which, in relation to terrestrial distances, may be termed instantaneous. They follow the same laws of reflection and refraction as the rays of light. In the rays of heat just such differences appear as those which, in the case of the rays of light, determine the diversity of colors. In a word, it is now fully recognized that the rays of light and heat are, in their nature, identical, and that if any modification distinguishes them, it can only be of a quantitative nature; whence it follows that the phenomena of light and heat must be referred in principle to the same explanation. Since, then, in regard to the phenomena of light, the theory of vibration has triumphantly vindicated its claims against the theory of emanation, no doubt can any longer be properly entertained that the phenomena of heat also are to be referred to mechanical principles.
A body is luminous when its several atoms oscillate with a sufficient degree of intensity and velocity about their position of equilibrium. These atomic vibrations call forth in the surrounding ether an undulatory movement, by which the rays of light are propagated, and hence many analogies exist between sound and light. While sound is generated through the vibratory motion of elastic bodies, light arises from a far more rapid oscillatory motion of the minutest or ethereal particles of matter. As sound is propagated through an undulatory movement of the air, so is light through an undulatory movement of the ether. Like the diversity of tones, so the diversity of colors arises from a difference in the duration of the oscillations of the conducting medium. But, seeing that the rays of light and of heat emitted by a body in combustion are identical, can we avoid the conclusion that the cause of its light and its heat is the same; that the heat of bodies proceeds, also, only from an oscillatory movement of its atoms ?
Perhaps it may be objected that non luminous bodies also emit heat; that the sun's light, as well as electrical light, is accompanied in large proportion by invisible rays of heat. It might hence seem that a difference exists between the rays of light and those of heat. But more exact investigation has shown that it is only a quantitative difference which is here in question. The obscure rays of heat are not different in their intrinsic nature from those which are at the same time luminous; there are rays which are endued with a greater amplitude of oscillation than the red, and whose period of vibration, therefore, exceeds the limit to which the organization of the eye restricts its visual perceptions.
Thus the study of radiant heat has led to the same consequences which Rumford and Davy had deduced from their experiments on the production of heat by friction. But, though the majority of physicists shared the views of the two philosophers, and entertained the conviction that the emanation theory was thenceforth untenable, for a long time nothing further was done to bring this question to a decision until some 24 years ago it was again taken in hand with great energy and prosecuted with ardor in various quarters. The first by whom