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"T O avoid the danger of scorching or 'burning on' in cooking food over a fire, the double boiler has been invented. This insures an equable temperature in the process of cooking, with no risk whatever that the temperature will rise above the boiling-point of water, no matter how hot the fire may be under the boiler. Into a kettle partly filled with water is fitted a smaller one with a rim near the top to support it. The inner kettle contains the substance to be cooked, and is provided with a lid. There you have the double boiler. The fire acts directly on the contents of the lower or outer boiler, heating the water, which in turn transmits its heat to the upper or inner boiler. In this manner no excess of heat can by any possibility injure the cooking food, and there is never any crust of burnt matter found clinging to the bottom of the vessel in which the food is contained, simply because no part of that food ever passes the comparatively low temperature of boiling water."
"But if the fire is stirred and more fuel put on," objected Marie, "the water in the under boiler would get hotter and hotter, and the contents of the other surely would burn!"
"No, my child, there you are quite mistaken. The water in a kettle or boiler can never pass a certain limit of heat, let the fire underneath be as hot as you please. Suppose we put a saucepan of water on the fire. The liquid heats gradually and finally begins to boil. Well, when it has once started boiling it is as hot as it ever will be. In vain would you stir the fire and pile on fuel and blow the flame; your energy would be wasted. The water would boil faster, it is true, but it would not become any hotter."
"Do you mean to say," asked Emile, "that a great bed of red-hot coals would not heat water so that it would be any hotter than if there were only a handful of burning sticks under it?"
"It would make it boil faster," was the reply, "and it would generate more steam; but I repeat that the water would not rise a single degree in temperature when once it had begun to boil. Nothing in the world can increase the heat of boiling water so long as the steam is free to escape."
"It is very strange," commented Jules, "that a great fire cannot make more heat than a small one."
"Ah, but you must not fall from one error into another," returned his uncle. "A great fire does, plainly enough, make more heat than a small one; but this excess of heat is not retained by the water, and therefore the latter does not become any the hotter."
"I should like to know," said Claire, "how they make sure that boiling water cannot get any hotter than when it first begins to boil, no matter how hot the fire underneath. They don't put their hands in to find out, I suppose."
"Certainly not. The test is made with a thermometer, the little instrument I have already described to you. If a thermometer is plunged into boiling water the mercury or spirits contained in the bulb and the lower part of the glass tube will be seen to rise until the division marked one hundred (centigrade) is reached—never more, never less, however hot the fire, so long as the water is free from any intermixture of other matter. If the fire burns furiously the water will boil all the faster and will send off great volumes of steam; if the fire is low the water will boil slowly and send off but little steam; but in each case, so long as it boils at all, it will be of exactly the same temperature—one hundred degrees centigrade. In this way it is established beyond question that freely boiling water can never rise above a certain temperature, no matter how hot the fire underneath.
"The usefulness of the double boiler is thus made plain to you. The inner kettle, immersed in boiling water, can never be subjected to a heat greater than that of boiling water. Now, many substances, especially those that serve us as food, suffer no injury from being raised to the boiling-point of water, while they scorch and acquire a bad taste if heated to a higher temperature. Such, for example, is the casein of milk. Therefore in cooking preparations containing milk, it is well to use the double boiler. And in melting butter in order to free it from casein and make it keep better—a subject we have already discussed —the double boiler is better than a single kettle. The casein is deposited at the bottom without risk of burning, and the pure butter is poured off at leisure and stored in pots or jars, which are sealed up and kept from year to year without deterioration of the contents, so far as cooking purposes are concerned."