Curiosities of Heat

by Lyman Beecher Tefft

“Ansel, will you state the theories which have been held touching the nature of heat?”

“I will do it as well as I can. The ancient philosophers supposed fire to be one of the four elements of which all bodies were composed. The three other elements were earth, air, and water. These four elements were mingled in various proportions. Of these, fire was esteemed the purest and most ethereal; this constantly tended upward to the empyrean, the highest heaven, where the element of fire and light was supposed to exist unmingled and pure. In the seventeenth century, Beccher and Stahl, two German chemists, brought forward what is known as the phlogistic hypothesis. They supposed that every combustible body held in composition a pure, ethereal substance which they called phlogiston, a Greek word which signifies burned, and that in combustion this phlogiston escaped. Flame was supposed to be this escaping phlogiston. These were the notions held about fire and combustion, but they are hardly worthy to be called theories of heat. The discovery of oxygen by Dr. Priestley of England, in 1774, and the introduction of the balance by Lavoisier of France, joined with the ever-enlarging circle of facts to be explained, rendered the phlogistic hypothesis untenable, and it was thrown aside.

“Until a few years since the caloric theory was generally received. According to this theory, heat is a substance, a subtle ether, diffused through all bodies and surrounding their atoms. This ether has been supposed to have a strong attraction for the atoms of every other substance, while between its own atoms a strong repulsion exists. In solid bodies each atom of matter, or in compound bodies each cluster of atoms, has been supposed to be surrounded by a little atmosphere, so to speak, of caloric, which prevented the atoms from coming into absolute contact. According to this theory, heat expands bodies by increasing and deepening these minute atmospheres, thus pressing the atoms farther from each other.”

“You need not explain this theory farther,” said Mr. Wilton; “we have hardly time to go into the history of theories. Tell us the latest received theory.”

“The theory now commonly believed is called the mechanical or dynamic theory. According to this theory, the essence of heat is motion. A hot body is one whose atoms are in a state of rapid and intense motion or vibration; and the sensation of heat on touching a hot body arises from the impact, or rapid blows, of the agitated atoms, communicating the same atomic vibration to the flesh and nerves of the hand.”

“Very well stated, Ansel. This is the theory now more commonly received. The caloric theory, like the crude notions of the old Greek philosophers about fire, and like the phlogistic hypothesis, has been rejected because it failed to explain the phenomena of heat. Whether the dynamic theory is destined to share the same fate remains to be seen. It seems, however, to have a better foundation than its predecessors. The dynamic theory, though recently made popular, is by no means a recent conception. It was advocated by such men as Bacon, Newton, Rumford, Davy, Locke, and others. Locke, the distinguished intellectual philosopher who lived in the latter half of the seventeenth century (born 1632, died 1704), said, ‘Heat is a very brisk agitation of the insensible parts of an object, which produces in us that sensation from which we denominate the object hot, so that what in our sensations is heat in the object is nothing but motion.’ Benjamin Thompson, an American gentleman who went to Europe in the time of our revolution, and for his scientific fame was made Count Rumford, and became the founder of the Royal Institution of England, declared that he could form no conception of the nature of heat generated by friction unless it were motion.

“A beautiful generalization has been made to show how well this idea of heat harmonizes with the entire plan of the universe. In the whole boundless universe each system of worlds, like our solar system, may be regarded as a molecule, or complex atom. These cosmical molecules, or complex atoms of the universe, are in motion through unmeasured space. In these systems of worlds the planets, with their satellites, are the molecules, and they are in motion—indeed, they commonly have several motions. Our earth, for example, rotates upon its axis once each day; it revolves in its orbit around the sun once each year, and the axis of the earth has a slow wabbling motion which produces the precession of the equinoxes, requiring 25,868 years for a complete revolution. The earth also is made up of parts, and all these are in ceaseless motion. As said the old Greek philosopher, ‘All things flow’—that is, everything is in a state of change. Solomon has well described this perpetual movement and change: ‘One generation passeth away, and another generation cometh. The sun also ariseth, and the sun goeth down, and hasteth to his place whence he arose. The wind goeth toward the south, and turneth about unto the north. It whirleth about continually, and the wind returneth according to his circuits. All the rivers run into the sea; yet the sea is not full; unto the place from whence the rivers come, thither do they return again. All things are full of labor; man cannot utter it. The eye is not satisfied with seeing, nor the ear filled with hearing. The thing that hath been, it is that which shall be; and that which is done is that which shall be done, and there is no new thing under the sun.’ Eccles. i. 4-9. It is certainly in harmony with this universal movement that the atoms of matter, though they seem so closely packed, should in their inconceivable smallness through inconceivably minute spaces vibrate, or rotate, or revolve through an orbit, never at rest. Intensity of heat we may think of as intensity of this atomic motion—a wider swing, so to speak, in their vibration or revolution. This, of course, requires a wider separation of the atoms and a consequent expansion of bodies. A feebler atomic motion permits the atoms to approach each other. In this manner we explain the enlargement of bodies by heat and their contraction by decrease of temperature. ‘The ideas of the best-informed philosophers are as yet uncertain regarding the exact nature of the motion of heat, but the great point at present is to regard it as a motion of some kind, leaving its more precise character to be dealt with in future investigation.’ This is the most we can do at present.”

“What is the evidence,” asked Samuel, “that the dynamic theory of heat is true?”

“The evidence that any theory is true is its ability to explain the facts or phenomena with which it has to do. If it explains all the facts and contradicts no known principles, it is regarded as true, or at least no objection can be made to it. Let me illustrate. Astronomers had long inquired what force or law controlled the movements of the heavenly bodies. At length Newton answered, A force of attraction between bodies which decreases in proportion as the square of the distance between them increases. This explanation has been found sufficient to explain all the known facts in the working of the heavenly bodies. Upon the basis of this theory astronomers calculate the positions of planets and comets for years and centuries to come.

“This theory led to the discovery of the planet Neptune, the last discovered of the primary planets. For thirty years irregularities in the motion of Uranus had been noticed. These variations were so slight that if another planet had revolved in the proper orbit of Uranus they would have seemed to the naked eye, throughout their course, one and the same star. This slight irregularity of motion was so nicely measured that the place of the unseen planet which caused it was almost exactly calculated from the estimated force and direction of its attraction. This theory of a universal attraction of gravitation so well explains all the facts in the case, and has become so universally received, that we are liable to forget that, after all, it is nothing but a theory.