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?
CHAPTER XXIII.
THE DIFFUSION OF HEAT.
Experiment showing how heat spreads.
Heat always tries to spread itself in all directions. If you put the end of a poker in the fire and hold it there, you do something more than heat that end. You heat the whole of it up to the end that you hold in your hand. The reason is, that the heat that comes into the end of the poker which is in the fire spreads through all of it to the other end.
This figure represents an experiment that you can try, which shows how the heat spreads through any thing solid. A rod or bar of iron is taken, and small balls of wood are fastened to it, as you see, by some wax. Now, on heating one end of the bar with a lamp, as the heat spreads along the bar, the balls one after another drop off, because the wax that holds them melts.
Heat spreads from one thing to another when it can get a chance to do it. If one thing that has a good deal of heat in it touches or is near by another that has less heat in it, it parts with some of its heat, and lets it go into the other thing, and after a little while one will be as warm as the other. For this reason, in a warmed room, all the furniture, the tables, the bureaus, the carpet, and the walls of the room become heated alike. The heat from the fire spreads through them all. It takes some time to do this, but it is done.