Science for the School and Family, Part I. Natural Philosophy

by Worthington Hooker

271. Nature of Heat.—There are two theories in regard to the nature of heat. One is that heat is an imponderable (§ 16), and of course a very subtile substance, which pervades all matter. Its particles are supposed to repel each other strongly, and hence they have a tendency to diffuse themselves, and to separate the particles of matter from each other. It is in this way that they are supposed to occasion the expansion of substances. The other supposition, which is most commonly received, is that heat is a vibration of the particles of bodies, and that it passes from these to bodies less warm through a subtile fluid called ether, supposed to fill all space. You see that if this be the true theory, there is some analogy between heat and sound.

Fig. 191.

272. Sources of Heat.—The principal of the sources of heat on our earth is the sun, though that body is ninety-five millions of miles distant from us. As the heat, in traveling all this long journey, is becoming more and more diffused; or, in other words, as its rays are all the way separating from each other more and more, we can have no conception of the concentrated heat that exists in the sun itself. We can, however, approximate to the idea by observing the effects of heat when some of its separated rays are gathered to a point by a powerful lens, as represented in Fig. 191. A lens which concentrated the heat ten thousand times melted platinum, gold, quartz, etc., in a few seconds. And as the heat at the sun is supposed to be thirty times more concentrated than this, none of the most solid substances of our earth would remain solid if they were there, but would be some of them liquid, and others even in a state of vapor. The heat which the sun constantly radiates to the earth pervades all substances, producing motion, and awakening life every where, so that, in the expressive language of the Bible, "There is nothing hid from the heat thereof."

Another source of heat is within the earth itself. It has been found as we go down into the earth there is a constant increase of temperature the farther we go. This internal heat is attributed in part to subterranean fires and various chemical actions. We see here and there external evidences of the operation of these causes in the eruptions of volcanoes, the boiling springs, the jets of steam and sulphureous vapors, etc. But that the heat in our earth which comes from these subterranean sources is small compared with that which comes from the sun, is seen in the fact that the rate of increase of heat at great depths is much less than it is nearer the surface. This would seem to show that although fires within the earth may have considerable influence in heating its crust, on which we live, it derives the most of its heat from the sun, at least to a very great depth.

How great a source of heat electricity is we know not, but that considerable heat comes from this source is evident from the melting and burning effect which we often see resulting from the passage of the electric fluid.

Another very common source of heat is chemical action. We see it continually produced in chemical experiments. Combustion, which, as will be shown to you in the Second Part of this Series, is nothing but an example of chemical action, is the most common of all the chemical sources of heat. Animal heat is also, for the most part, a result of chemical action.

Mechanical action is a common source of heat. The rubbing of a match producing heat enough to occasion flame is a familiar example. The spark produced in what is called striking fire is the burning of a particle of steel set on fire by the blow. The Indian was accustomed to light his fire by the rubbing together of two dry sticks till he learned an easier way from civilized neighbors; and the blacksmith, previous to the invention of phosphorus matches, often lighted his fire by touching a sulphur match to a nail made red-hot by rapid and continued hammering. Machinery has sometimes been set on fire by friction, and the water around a mass of metal has been so heated by boring as even to boil. If you stretch a piece of India rubber several times in quick succession, and then apply it to your lips, you will perceive that the motion has warmed it.

273. Relations of Heat and Light.—Heat is sometimes alone, and is sometimes in intimate union with light. All substances have some amount of heat, and it passes from them to other bodies in their neighborhood that happen to have less heat in them. In doing this it may or may not have the company of light. In the radiation of heat from a stove, unless it be heated to redness, there is no light with the heat; but from an open, burning fire the light and heat come together. But the rays of the sun give us the best example of the union of light and heat. Traveling together at an equal pace they are most curiously mingled, as you will see when I come to speak particularly of light.