THE DOUBLE BOILER
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,
"Certainly not. The test is made with a
ther-  mometer, 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
"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
—the double boiler is better than a
 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."