Popular Scientific Recreations in Natural Philosphy, Astronomy, Geology, Chemistry, etc., etc., etc. Gaston Tissandier

HEAT—SPECIFIC HEAT.

      What is Heat?—We will consider this question, and endeavour to explain it before we speak of its effects on water and other matter.

      Heat is now believed to be the effects of the rapid motion of all the particles of a body. It is quite certain that a heated body is no heavier than the same body before it was made “hot,” so the heat could not have gone into it, nor does the “heat” leave it when it has become what we call “cold,” which is a relative term. Heat is therefore believed to be a vibratory motion, or the effects of very rapid motion of matter.

      The Science of Heat, as we may term it, is only in its infancy, or certainly has scarcely come of age. Formerly heat was considered a chemical agent, and was termed caloric, but now heat is found to be motion, which affects our nerves of feeling and sight; and, as Professor Stewart tells us, “a heated body gives a series of blows to the medium around it; and although these blows do not affect the ear, they affect the eye, and give us a sense of light.”

      Although it is only within a comparatively few years that heat has been really looked upon as other than matter, many ancient philosophers regarded it as merely a quality of matter. They thought it the active principle of the universe. Epicurus declared that heat was an effluxion of minute spherical particles possessing rapid motion, and Lucretius maintained that the sun’s light and heat are the result of motion of primary particles. Fire was worshipped as the active agent of the universe, and Prometheus was fabled to have stolen fire from heaven to vivify mankind. The views of the ancients were more or less adopted in the Middle Ages; but John Locke recognized the theory of heat being a motion of matter. He says: “What in our sensation is heat, in the object is nothing but motion.”

      Gradually two theories arose concerning heat;—one, the Material theory—the theory of Caloric or Phlogiston; the other, the Kinetic theory. Before the beginning of the present century the former theory was generally accepted, and the development of heat was accounted for by asserting that the friction or percussion altered the capacity for heat of the substances acted upon. The heat was squeezed out by the hammer, and the amount of heat in the world was regarded as a certain quantity, which passed from one body to another, and that some substances contained, or could “store away,” more of the material called heat than other substances. Heat was the material of fire—the principle of it, or materia ignis; and by these theories Heat, or Caloric, was gradually adopted as a separate material agent—an invisible and subtle matter producing certain phenomena when liberated.

      So the two theories concerning heat arose at the end of the last century. One, as we have said, is known as the Material, the other as the Kinetic theory. The latter is the theory of motion, so called from the Greek kinesis (motion), or sometimes known as the Dynamic theory of heat, from dunamis (force); or again as Thermo-dynamics.

      But any possibility of producing a new supply of heat was denied by the materialists. They knew that some bodies possessed a greater capacity for heat than others; but Count Rumford, at Munich, in 1797, astonished an audience by making water boil without any fire! He had observed the great extent to which a cannon became heated while being bored in the gun factory, and influenced by those who maintained the material theory of heat, paid great attention to the evolution of heat. He accordingly endeavoured to produce heat by friction, and by means of horse power he caused a steel borer to work upon a cylinder of metal. The shavings were permitted to drop into a pan of water at 60° Fahrenheit. In an hour after the commencement of the operation the temperature of the water had risen to 107°: in another half-hour the heat of it was up to 142°: and in two hours had measured 170°. Upon this he says: “It is hardly necessary to add that anything which any insulated body or system of bodies can continue to furnish without limitation cannot possibly be a material substance, and it appears to me to be extremely difficult, if not quite impossible, to form any distinct idea of anything capable of being excited and communicated in these experiments except by motion.”

      A few years later Sir Humphrey Davy made his conclusive experiments, and the Material theory of heat received its death-blow.

      Sir Humphrey Davy—referring to the fact that water at a freezing temperature has “more heat in it” (as it was believed) than ice at the same temperature—said: “If I, by friction, liquify ice, a substance will be produced which contains a far greater absolute amount of heat than ice. In this case it cannot reasonably be affirmed that I merely render sensible heat which had been previously insensible in the frozen mass. Liquification will conclusively prove the generation of heat.

      This reasoning could not be doubted. Sir Humphrey Davy made the experiment. He rubbed together two pieces of ice in the air, and in a vacuum surrounded by a freezing mixture. The ice became liquified, and so the generation of heat by “mechanical means” was proved. Its immateriality was demonstrated, but the Material theory was not even then abandoned by its adherents.

      So things continued, until in 1842–3, Doctor Julius Meyer, of Heilbronn, and Doctor Joule, of Manchester, separately, and by different means, arrived at the conclusion that a certain definite amount of mechanical work corresponds to a certain definite amount of Heat, and vice versâ. Thus was a great support afforded to the Dynamic theory. This fact Doctor Joule communicated to the Philosophical Magazine in 1843, and the conclusions he came to were—

      1. “That the quantity of heat produced by the friction of bodies, whether solid or liquid, is always in proportion to the force expended;

      2. “That the quantity of heat capable of increasing the temperature of a pound of water (weighed in vacuo and taken at between 55° and 60° Fahr.) by 1° Fahr., requires for its evolution the expenditure of a mechanical force represented by the fall of 772 lbs. through the space of one foot.”

      Fig. 76.—Melting a piece of tin on a card.

      This is the “mechanical equivalent of heat.” The first paper written by Mr. Joule demonstrated that the temperature of water rises when forced through narrow tubes; and to heat it one degree, the force of 770 foot pounds was necessary, which means that the 1 lb. of water falling 770 feet, got hotter by one degree when it reached the earth. He subsequently arrived at the more exact conclusions quoted above.

      So heat is now known to be a series of vibrations, or vibratory motions, as sound vibrations, which we cannot hear nor see, but the effects of which are known to us as light and heat.

      

      In considering heat we must put aside the idea of warmth and cold, for they are only different degrees of heat, not the absence of it.

      The study of heat can be briefly undertaken without any complicated apparatus. If we desire a proof of the great conducting power of metals, let us place a fine piece of muslin tightly stretched over a lump of polished metal. On the muslin we put a burning ember, and excite combustion by blowing on it; the muslin is not burned in the least, the heat being entirely absorbed by the metal, which draws it through the material into itself. Fig. 76 represents a similar experiment: it consists of melting some tin on a playing card, held over the flame of a spirit lamp. The metal becomes completely melted without the card being burnt. It is through a similar effect that metals appear cold to us when we take them in our hands; by their conductibility they remove the heat from our hands, and give us the peculiar impression which we do not experience when in contact with substances that are bad conductors, such as wood, woollen materials, etc.

      Fig. 77.—Boiling water in a paper case.

      Fig. 77 shows the method of boiling water in paper. We make a small paper box, such as those made by school-boys, and suspend it by four threads to a piece of wood held horizontally at a suitable height. We fill this improvised vessel with water, and place it over the flame of a spirit lamp. The paper is not burnt, because the water absorbs all the heat into itself. After a few minutes the water begins to boil, sending forth clouds of steam, but the paper remains intact. It is well to perform this operation