Harper's New Monthly Magazine, Vol. 2, No. 8, January, 1851

by Various

"And water," observed Mr. Bagges, "would be robbed of a very valuable property."

"Good again, uncle. Now, if we could see through the kettle, we should be able to observe the water boiling in it which is a curious sight when looked into. To examine water boiling, we must boil the water in a glass vessel—a long tube is the best—heated with a spirit lamp. Then first you see the water in motion, and the air-bubbles being driven off by the heat. As the water gets hotter, other bubbles appear, rising from the bottom of the tube. They go up for a little way, and then they shrink, and by the time they get to the top of the water, you can hardly distinguish them. These are bubbles of steam, and they get smaller as they rise, because at first the water is colder above than below, in proportion to the distance from the flame, and the cold gradually condenses the bubbles. But when the water gets thoroughly hot, the bubbles grow larger and rise quicker, and go of the same size right up to the top of the water, and there escape—if you choose to let them. And steam was allowed to escape so for many, many ages, wasn't it, uncle, before it was set to work to spin cotton for the world, and take us to America within a fortnight, and whirl us over the ground as the crow flies, and almost at a crow's pace?"

"For all which," remarked Mr. Bagges, "we have principally to thank what's his name."

"Watt was his name, I believe, uncle. Well; heat turns water into steam, and I dare say I need not tell you that a quantity of water becoming steam, fills an immense deal more space than it did as mere water. Cold turns the steam back into water, and the water fills the same space as it did before. Water, in swelling into steam and shrinking back into water again, moves, of course, twice, and mighty motions these are, and mighty uses are made of them, I should rather think."

"I believe you, my boy," said Mr. Bagges.

"And now," asked Harry, "have you any idea of what a deal of heat there is in steam?"

"It is hot enough to scald you," answered his mamma; "I know that."

"Yes; and hot enough, too, to cook potatoes. But there is much more heat in it than that comes to. Take a kettle of cold water. See at what degree the thermometer stands in the water. Put the kettle on the fire, and observe how long it takes to boil. It will boil at two hundred and twelve degrees; and therefore, during the time it has taken to boil, there has gone into it the difference of heat between two hundred and twelve degrees and the degree it stood at when first put on the fire. Keep up the same strength of fire, so that the heat may continue to go into the water at the same rate. Let the water boil quite away, and note how long it is in doing so. You can then calculate how much heat has gone into the water while the water has been boiling away. You will find that quantity of heat great enough to have made the water red-hot, if all the water, and all the heat, had remained in the kettle. But the water in your kettle will have continued at two hundred and twelve degrees to the last drop, and all the steam that it has turned into will not have been hotter—according to the thermometer—than two hundred and twelve degrees; whereas a red heat is one thousand degrees. The difference between two hundred and twelve degrees and one thousand degrees, is seven hundred and eighty-eight degrees; and what has become of all this heat? Why, it is entirely contained in the steam, though it does not make the steam hotter. It lies hid in the steam, and therefore it is called latent heat. When the steam is condensed, all that latent heat comes out of it, and can be felt, and the quantity of it can be measured by a thermometer. The warmth that issues from steam-pipes used to warm a house, is the latent heat of the steam that escapes as the steam turns back to water."

"Latent, heat! latent heat!" repeated Mr Bagges, scratching his head. "Eh? Now, that latent heat always puzzles me. Latent, lying hid. But how can you hide heat? When the zany in the pantomime hides the red-hot poker in his pocket, he cauterizes his person. How—eh?—how can heat be latent?"

"Why, the word heat has two meanings, uncle. In the first place, it means hotness. Hotness can not be latent, as the clown finds when he pockets the poker. In the second place, heat means a something the nature of which we don't know, which is the cause of hotness, and also the cause of another effect. While it is causing that other effect, it does not cause hotness. That other effect which heat causes in the instance of steam, is keeping water in the form of steam. The heat that there is in steam, over and above two hundred and twelve degrees, is employed in this way. It is wholly occupied in preserving the water in an expanded state, and can't cause the mercury in the thermometer to expand and rise as well. For the same reason, it could give you no feeling of hotness above what boiling water would—if you had the nerve to test it. While it is making steam continue to be steam, it is latent. When the steam becomes water again, it has no longer that work to do, and is set free. Free heat is what is commonly understood by heat. This is the heat which cooks our victuals, the heat we feel, the heat that singes Mr. Merriman. Latent heat is heat that doesn't warm, singe, or cook, because it is otherwise engaged. If you press gas suddenly into a fluid, the latent heat of the gas is set free. You seem to squeeze it out. Indeed, the same thing happens, if you violently force any substance into a closer form all at once. Every thing appears to have more or less latent heat in it, between its little particles, keeping them at certain distances from each other. Compress the particles within a smaller compass, and a part of the latent heat escapes, as if it were no longer wanted. When a substance in a compressed state expands on a sudden, it draws in heat, on the other hand. When a lady bathes her forehead with eau-de-Cologne to cure a headache, the heat of the head enters the eau-de-Cologne, and becomes latent in it while it evaporates. If you make steam under high pressure, you can heat it much above two hundred and twelve degrees. Suppose you let off steam, so compressed and heated, by a wide hole, from the boiler, and put your hand into it as it rushes out—"