321. Why does friction produce heat?

Because all bodies contain latent heat, that is, heat that lies hid in their substance, and the rubbings of two bodies against each other draws the latent heat to the excited surfaces.322. Why does the rubbing of two surfaces together attract latent heat to those surfaces?

Because it is a law of nature that heat shall always attend motion; and it is generally found that the intensity of heat bears a specific relation to the velocity of motion.323. What are the sources of heat?

The rays of the sun, the currents of electricity, the action of chemicals, and the motion of substances.324. Why does water freeze?

Because its latent heat is partly drawn off by the surrounding air.325. Why does ice melt?

Because the heat, once latent in the water, but drawn off by the air, has returned to it, and restored the water to its former condition.

"So teach us to number our days, that we may apply our hearts unto wisdom." Psalm xc.

326. Why does water become steam?

Because a larger amount of heat has entered into it than can remain latent in water. The water therefore expands and rises in the form of vapour, or water attenuated by heat.327. How many degrees of heat are latent, or hidden, in the different states of water?

In thawing ice, 140 deg. of caloric become latent; and in converting the water into steam, 1,000 deg. more of caloric are be taken up. Therefore, ice requires to take up 1,140 deg. of latent caloric before it becomes steam.328. What is the most modern theory of heat?

It is this—that caloric, which produces heat, is an extremely subtile fluid, of so refined a nature that it possesses no weight, yet is capable of diffusing itself among the particles of the most solid bodies.

It is also believed that—all bodies are subject to the action of two opposing forces: one, the mutual attraction of their particles; the other, the repulsive force of caloric—and that bodies exist in the Æriform, fluid, or solid state, according to the predominance of either the one or the other of these opposing forces.329. How do we measure the quantity of caloric in any substance?

It is impossible to determine the amount of caloric which any body contains. Our sensations would obviously be deceptive, since, if we dipped the right hand in snow, and held the left hand before the fire, and then immersed both hands in cold water, the water would feel warm to the right hand and cold to the left hand.

But, as caloric uniformly expands substances that are under its influence, one of the bodies most sensitive to calorific effects has been selected to be the indicator of the amount of caloric. This substance is quicksilver; and the scale of measurement, and the apparatus for exhibiting the rise or fall of the quicksilver, constitute the thermometer.330. If it is impossible to measure the amount of caloric in any substance, how can it be said that ice absorbs 140. deg. in becoming water?

Those figures simply record the amount of calorie indicated by the thermometer. The instrument will show with sufficient accuracy the relative amount of caloric in various bodies, or in the same bodies under different circumstances, but it can never determine the precise amount of caloric in any one body.

"Great is the Lord, and greatly to be praised in the city of our God, in the mountain of his holiness."—Psalm xlviii.

331. Why, if a hot and a cold body were placed near to each other, would the cold one become warmer, and the hot one cooler?

Because free caloric (that is, caloric that is not latent,) always exhibits a tendency to establish an equilibrium. If twenty bodies, of different temperatures, were placed in the same atmosphere, they would all soon arrive at the same temperature. The caloric would leave the bodies of those of the highest, and find its way to those of the lowest temperature.332. How does caloric travel?

It travels in parallel rays in all directions with a velocity approximating to that of light; and it passes through various bodies with a rapidity proportionate to their power of conduction.333. Why does melted metal run like a stream of fluid?

Because caloric has passed into its substance, and, repelling its particles, has separated them to that degree which produces fluidity.334. How do we know that it is caloric passing into the substance of the metal which produces this effect?

Because, as soon as a bar of metal begins to be heated, it expands and lengthens. It continues to do so, until the heat arrives at that point which causes the metal to melt.335. Why does the iron of an ironing-box sometimes become too large for the box to receive it?

Because caloric has passed into the substance of the iron, and repelled its particles, by which it has become expanded.336. Why does the iron enter the box when it has become partially cooled?

Because a portion of the caloric has left the iron, the particles of which have drawn closer together, and contracted the mass.

"Cast thy burden upon the Lord, and he shall sustain thee; he shall never suffer the righteous to be moved."—Psalm lv.

This effect is frequently observed by females in domestic life, who, when they are ironing, or using the Italian irons, find that the heated metal has been too much expanded to enter the box or tube. They find it necessary to wait until the cooling of the iron has had the effect of reducing its dimensions. The expansion of bodies by heat is one of the grandest and most important laws of nature. We are indebted to it for some of the most beautiful, as well as the most awful, phenomena. And science has gained some of its mightiest conquests through its aid. Yet frequently, though quite unthought of, in the hands of the humble laundress, will be found a most striking illustration of this wonderful force of caloric.337. Are there any instances in which the abstraction of latent heat will reduce the hulk of bodies?

Yes, there are several. But the most familiar one is that which is exhibited by mixing a pint of the oil of vitriol with a pint of water. A considerable amount of heat will be evolved; and it will be found that the two pints of fluid will not afterwards fill a quart measure.338. Is there any latent heat in air?

Yes: a considerable amount. In a pint measure of air, though in no way evident to our perceptions, there lurks sufficient caloric to raise a piece of metal several inches square to glowing redness.339. How do we know that caloric exists in the air?

It has been positively demonstrated by the invention of a small condensing syringe, by which, through the rapid compression of a small volume of air, a spark is emitted which ignites a piece of prepared tinder.340. What is the cause of the spark when a horse's shoe strikes against a stone?

The latent heat of the iron or the stone is set free by the violent percussion. The same effect takes place when flint strikes against steel, as in the old method of obtaining a light with the aid of the tinder-box.

"The waters are laid as with a stone, and the face of the deep is frozen."—Job xxxviii.

What an eloquent lecture might be delivered upon the old-fashioned tinder-box, illustrated by the one experiment of "striking a light." In that box lie, cold and motionless, the Flint and Steel, rude in form and crude in substance. And yet, within the breast of each, there lies a spark of that grand element which influences every atom of the universe; a spark which could invoke the fierce agents of destruction to wrap their blasting flames around a stately forest, or a crowded city, and sweep it from the face of the world; or which might kindle the genial blaze upon the homely hearth, and shed a radiant glow upon a group of smiling faces; a spark such as that which rises with the curling smoke from the village blackmith's forge—or that which leaps with terrific wrath from the troubled breast of a Vesuvius. And then the tinder—the cotton—the carbon: What a tale might be told of the cotton-field where it grew, of the black slave who plucked it, of the white toiler who spun it into a garment, and of the village beauty who wore it—until, faded and despised, it was cast amongst a heap of old rags, and finally found its way to the tinder-box. Then the Tinder might tell of its hopes; how, though now a blackened mass, soiling everything that touched it, it would soon be wedded to one of the great ministers of nature, and fly away on transparent wings, until, resting upon some Alpine tree, it would make its home among the green leaves, and for a while live in freshness and beauty, looking down upon the peaceful vale. Then the Steel might tell its story, how for centuries it lay in the deep caverns of the earth, until man, with his unquiet spirit, dug down to the dark depths and dragged it forth, saying, "No longer be at peace." Then would come tales of the fiery furnace, what Fire had done for Steel, and what Steel had done for Fire. And then the Flint might tell of the time when the weather-bound mariners, lighting their fires upon the Syrian shore, melted silicious stones into gems of glass, and thus led the way to the discovery of the transparent pane that gives a crystal inlet to the light of our homes; of the mirror in whose face the lady contemplates her charms; of the microscope and the telescope by which the invisible are brought to sight, and the distant drawn near; of the prism by which Newton analysed the rays of light; and of the photographic camera in which the sun prints with his own rays the pictures of his own adorning. And then both Flint and Steel might relate their adventures in the battle-field, whither they had gone together; and of fights they had seen in which man struck down his fellow-man, and like a fiend had revelled in his brother's blood. Thus, even from the cold hearts of flint and steel, man might learn a lesson which should make him blush at the "glory of war;" and the proud, who despise the teachings of small things, might learn to appreciate the truths that are linked to the story of a "tinder-box."