We do not know what heat is. Wise men have tried to find out what it is, but they have never been able to do it. But we know some things that heat comes from, and some things that it does, and these I will tell you about.
Most of the heat in the world comes from the sun.
Most of the heat in the world comes from the sun in company with the light. A long way it travels to get here. It is millions and millions of miles that it comes in straight lines to us. Then there is the heat that comes from the fires that we make. Here there is generally light with the heat, just as there is with that which comes from the sun.
Heat and light, when they come together, do not always keep together, but are sometimes separated from each other. If you are standing before a fire and holding a pane of glass before your face, it keeps off the heat—that is, the heat does not come through the glass, or so little of it comes through that you do not feel it. The glass stops the heat, but lets its companion, the light, pass through. Now, if the light of the sun comes through a window, you feel the heat with it. The light and heat come through the glass in company. They are not separated after traveling so many millions of miles together. Why it is different with the fire and the sun we know not. I suppose that the heat and light that come from the sun are in some way more closely united thanthe heat and light that come from the fire, and therefore are not so easily parted.
Heat made in our bodies.
But heat is often made without any light. This is the case with the heat of our bodies. There is a sort of burning every where within us to make the heat, but it is a burning without any flame or light. Our bodies are not made warm by fire and clothing, but they keep themselves warm. The only use of our fires and clothing generally is to keep the heat which is made in our bodies from flying off too fast in the air around us. A great deal of heat is made in the bodies of all animals, and the more active they are the more heat they make. You know that when you play very hard you become very much heated. This is because, when the heart beats so quickly, sending the blood all over the body so rapidly, there is more heat manufactured than when the body is still.
Friction a source of heat.
Heat is also produced by friction without causing any light. Rub two smooth sticks together, and see how warm they become. The woodwork of machinery has been known to take fire from the heat caused by friction; and Indians used often to kindle their fires by rubbing two sticks together.
You know how easily a match takes fire by rubbing it. This is because there is on the end of it a substance that takes fire with a very little heat, and so requires but a little friction to set it on fire. This curious substance is phosphorus. It is mixed with sulphur on the ends of the matches. When once the phosphorus is set on fire with the friction, it burns the sulphur with it.
Lucifer matches.
The tinder-box.
It is not many years since the lucifer matches, as they were atfirst called, were invented. Before this we had a most inconvenient way of getting a light when there was no fire at hand. A flint was struck upon a piece of steel again and again over some tinder. The object was to make a spark which would set fire to the tinder. This was not always readily done, and I remember getting out of patience many a time in working over my tinder-box when I was a student in college.
Heat made in the earth.
There is a great deal of heat made inside of the earth, and it is supposed by some that all the middle of this great round ball that is called the earth is an immense fire like a furnace. The earthquakes are supposed to be caused by the heavings of this fire, and the volcanoes are so many chimneys where the fire of this great furnace gets vent.
Cold not a thing.
Heat is a thing, but there is really no such thing as cold. Any thing is cold when there is but little heat in it. Whether all the heat can get out of any thing we do not know. There is heat even in ice. This has been proved in this way: Two pieces of ice were rubbed together in a very cold day, and some of the ice became melted. How was this? The air all about the ice was too cold to melt it; and it must be, therefore, that it was the heat in the ice, waked up, as we may say, and brought out by the rubbing, that melted the ice.
What feels cold to you may feel warm to another. If, when your hand is very warm, you take hold of some one’s hand that is only moderately warm, it will feel cool to you, and perhaps even cold; but if some one whose hands are quite cold takes hold of the same hand, it will feel to him quite warm.
Experiment with three vessels.
Try a little experiment, which will show the same thing in another way. Take three vessels. Put into one water as hot as your hand can bear, into another ice-cold water, and into the third water that is a little warm, or that has had the chill taken off. Now put one of your hands into the vessel of hot water, and the other into the vessel of cold water. Keep them there a little while. Then take them out, and put both into the vessel that has the water which is slightly warmed. The water in this will feel cold to the hand which was in the hot water, and warm to the hand which was in the cold water.
Drinking water after eating ice-cream.
For the same reason, water standing in a room will feel quite warm to you if you have been handling snow, though it is cold to others. So, also, water that was very cold to you before eating ice-cream, seems, after eating it, to have lost all its coldness.
So you see that heat and cold are not two things separate from each other, of which you can tell where one begins and the other ends. It is convenient to speak of the cold as if it were a thing, just as heat is, though, as I have told you, it is not; and it is well enough to do so if we understand the matter right.
Questions.—What do we know about heat? From what does most of the heat come? What does it come with? What is said about sun-heat and fire-heat? Tell about the making of heat in our bodies. What is the use of our fires and clothing in cold weather? Why do you become so much heated on playing hard? What is said about friction? Explain the operation of Lucifer matches. What is said about tinder-boxes? What is said about the inside of the earth? When is any thing cold? Is there any thing that has no heat in it? How is it proved that there is heat in ice? Does what feels cold to one always feel cold to another? Give the experiment of the three vessels of water. What other things can be explained in the same way?
Questions.—What do we know about heat? From what does most of the heat come? What does it come with? What is said about sun-heat and fire-heat? Tell about the making of heat in our bodies. What is the use of our fires and clothing in cold weather? Why do you become so much heated on playing hard? What is said about friction? Explain the operation of Lucifer matches. What is said about tinder-boxes? What is said about the inside of the earth? When is any thing cold? Is there any thing that has no heat in it? How is it proved that there is heat in ice? Does what feels cold to one always feel cold to another? Give the experiment of the three vessels of water. What other things can be explained in the same way?