EXPERIMENT 79: MIXTURES FOR FREEZING.—Prepare 2 cupfuls of cracked ice. Place 1 cupful of the cracked ice in each of two bowls. To one bowl of ice add 1/3 cupful of rock salt, and mix thoroughly. Insert thermometers into both bowls and note temperature. What effect does the salt have upon the temperature of the ice?

Allow the ice mixture to stand a few minutes, then observe the difference in the condition of the ice in the two bowls. Besides lowering the temperature, what does the salt do to the ice?

FREEZING WITH ICE AND SALT.—When a solid substance is changed to a liquid, heat is absorbed from surrounding materials. When solid ice liquefies, heat is absorbed from surrounding materials. Salt makes ice liquefy at a lower temperature, thus absorbing more heat from its surroundings. Since foods must be cooled, i.e. heat drawn from them, in order to freeze them, a mixture of ice and salt rather than ice is used in freezing.

If ice and salt surround a tin can containing substances to be frozen, from what is the heat absorbed when the ice is changed to liquid form? Explain why it is that a mixture of ice and salt, rather than ice alone, is used to freeze a dessert.

CONDUCTORS OF HEAT.—In Experiment 2 and in, The Principle of Fireless Cookery, it is shown that some materials are better conductors of heat than others. Which is a better conductor of heat, wood or metal? Explain why it is that most freezers consist of an inner can of metal and an outer bucket of wood. A few freezers have an outside metal bucket. Such freezing devices have been found more satisfactory when heavy paper is tied around the outer metal bucket.

COOLING BY EVAPORATION.—If a few drops of alcohol, ether, or gasoline are poured in the palm of the hand and allowed to evaporate, the hand feels cold. During evaporation, the liquid takes heat from the hand. When any liquid evaporates, heat is absorbed from surrounding materials. Water may be cooled by placing it in a porous jar and hanging it in a breeze.

When there is no ice, this principle of cooling by the rapid evaporation of a liquid may be applied to the cooling of butter and other foods. Wrap butter in an oiled paper and place it in a flower crock or any porous jar. Place the crock in a draft; put a bowl of water beside it. Wrap a wet cloth about the crock and place one end of it in the bowl of water. The continuous evaporation of the moisture keeps the food cool.

PREPARING AND PACKING THE FREEZER.—Scald the can, the cover, and the dasher of the freezer; cool it before the mixture that is to be frozen is placed in it. Adjust the can carefully in the bucket; put in the dasher; pour in the mixture, cover; adjust the crank. Crush the ice for freezing by placing it in a strong bag and pounding it with a wooden mallet. Mix the ice with rock salt in the proportion given below. Then pour the ice and salt mixture around the can of the freezer. The ice and salt mixture should be higher around the can than the level of the mixture inside.

For freezing ice creams and most ices use three parts of cracked ice to one of rock salt. If ice of coarse grain is desired, use a greater quantity of salt. The less salt in

proportion to ice used, the finer the grain; the process of freezing, however, takes place very slowly when little salt is used.

For mixtures which are frozen by merely packing in ice and salt but are not stirred, such as mousse or parfait, use two parts of cracked ice to one of rock salt.

For packing frozen mixtures after freezing, use four parts of cracked ice to one of rock salt.

FREEZING.—If a dessert of fine texture is desired, turn the crank slowly and steadily until the mixture is rather stiff, then turn more rapidly. In making water ices, it is considered advisable by some to turn the crank steadily for 5 minutes, then allow to stand 5 minutes, turn again 5 minutes, and continue until freezing is completed. Do not draw off the salt water while freezing the mixture, unless the salt water stands so high that there is danger of its getting into the can.

When the mixture is frozen, remove the ice and salt around the top of the can; wipe the cover and top; uncover; and remove the dasher. Then stir the frozen mixture thoroughly; place thin paper or paraffin paper over the can; cover; place a cork in the hole of the cover. Drain off all the water which has collected in freezing; repack the freezer with ice and salt mixture in the proportion given above; cover with carpet, blanket, or newspapers; and allow to stand in a cold place several hours.

FRUIT ICE

4 cupfuls water 2 3/4 cupfuls sugar 3 oranges 3 lemons 3 bananas 1/4 teaspoonful salt

Make a sirup of the sugar and water, and then cool it. Extract the juice from the lemons and oranges; crush the peeled and scraped bananas with a wooden potato masher. Mix the fruits and salt immediately with the sirup. Freeze at once. When frozen, remove the dasher and repack as directed above.

A less expensive but more mildly flavored ice may be prepared by using 3 pints of water (instead of 4 cupfuls). When the greater quantity of water is used, 3 1/4 cupfuls (instead of 2 3/4 cupfuls) of sugar should be used.

These recipes for Fruit Ice are modifications of the popular recipe termed
"Five Threes."

QUESTIONS

Explain why it is necessary to scald the can, cover, and dasher of an ice cream freezer (see Care of Milk).

What harm sometimes results when an ice cream freezer has been carelessly prepared?

Why should not the salt water be drawn from the freezer during freezing (see Experiment 79)?

What is the purpose of placing paper over the can when packing the frozen mixture?

What is the purpose of covering the packed freezer with carpet, blanket, or newspapers (see The Principle of Fireless Cookery)?

Why is it well to tie heavy paper around an outside metal bucket of a freezer?

Why should "Fruit Ice" mixture be frozen at once after preparing the fruit?

Heat aids chemical action. Can you explain why acid mixtures are not acted upon by the metal and consequently discolored when frozen in a tin or iron can?