"Eh?" said Mr. Bagges, "why, what?—no—I did know something about it the other day—but I've such a memory!—and—eh?—no—I've quite forgotten it."
"By radiation, you know. All warm bodies are constantly giving off rays of heat, as shining ones are giving off rays of light, although the heat-rays are invisible."
"How do we know that?" asked Mr. Bagges.
"Get a couple of concave mirrors—a sort of copper basins, polished inside. Stand them face to face, some yards apart. Put a hot iron ball—not red hot—in the focus of one mirror. Put a bit of phosphorus in the focus of the other. The phosphorus will take fire; though without the mirrors you might place it much nearer the hot iron, and yet it would not burn. So we know that there are rays of heat, because we can reflect them as we can rays of light. Some things radiate better than others. Those that have bright metal surfaces radiate worst, though such are what are used for reflectors. If their surfaces are blackened or roughened, they radiate better. A bright kettle gives off fewer rays of heat than a black one, and so far, is better to keep water hot in. But then, on the other hand, it yields more heat to the air, or the hob or hearth that it stands upon—if colder than itself. The bright kettle gives off heat in one way and the black in another. I don't know at what comparative rate exactly."
"Six of one, and half-a-dozen of the other," Mr. Bagges suggested.
"Now look at the wonderful relations of the kettle, uncle!"
"Relations?—Eh?—what the pot and the saucepan?" said Mr. Bagges.
"Oh, oh, uncle! No; its relations to the pressure of the atmosphere and every cause that affects it—to the conveyance, and conduction, and radiation of heat—to latent heat or caloric, to the properties of water, to chemical decomposition—and to steam and its astonishing marvels, present and to come!"
"Well," said Mr. Bagges, "it is wonderful; and the kettle certainly is very respectably connected. Eh? And I hope to profit by the subject of our conversation; and so, I say, pour me out a cup of tea."