HOUSEHOLD REFRIGERATION BY THE USE OF LIQUID SO2 The compressing pump P run by the electric motor E liquefies by pressure the gas SO2 in the compressor coils C, the heat of condensation being partially removed by the fan F. The liquid SO2 escaping through the expansion valve V into the evaporating coils immersed in the brine tank B rapidly evaporates under the reduced pressures in these coils and maintains the whole tank below the freezing temperature. The thermostat T opens and closes the circuit to the electric motor as needed to maintain constant temperature. The brine tank takes the place of the cake of ice in the old refrigerator. In some systems the brine tank is absent and the evaporating coils themselves take the place of the ice. (Courtesy of the Kelvinator Corporation)

systems carbon dioxide is used instead of ammonia, but the principle is in no way altered. Sometimes, too, the brine is dispensed with, and the air of the rooms to be cooled is forced by means of fans directly over the cold coils containing the evaporating ammonia or carbon dioxide. It is in this way that theaters and hotels are cooled. (See "Electric refrigeration," opposite page 198.) 229. Distillation. Let water holding in solution some aniline dye be boiled in B (Fig. 175). The vapor of the liquid will pass into the tube T, where it will be condensed by the cold water which is kept in continual circulation through the jacket J. The condensed water collected in P will

be seen to be free from all traces of the color of the dissolved aniline.

B

FIG. 175. Distillation

We learn, then, that when solids are dissolved in liquids, the vapor which rises from the solution contains none of the dissolved substance. Sometimes it is the pure liquid in P which is desired, as in the manufacture of alcohol, and sometimes the solid which remains in B, as in the manufacture of sugar. In the whitesugar industry it is necessary that the evaporation take place at a low temperature, so that the sugar may not be scorched. Hence the boiler is kept partially exhausted by means of an air pump, thus enabling the solution to boil at considerably reduced temperatures.

230. Fractional distillation. When both the constituents of a solution are volatile, as in the case of a mixture of alcohol and water, the vapor of both will rise from the liquid. But the one which has the lower boiling point, that is, the higher vapor pressure, will predominate. Hence, if we have in B (Fig. 175) a solution consisting of 50 per cent alcohol and 50 per cent water, it is clear that we can obtain in P, by evaporating and condensing, a solution containing a much larger percentage of alcohol. By repeating this operation a number of times we can increase the purity of the alcohol.

This process is called fractional distillation. The boiling point of the mixture lies between the boiling points of alcohol and water, being higher the greater the percentage of water in the solution.

Gasoline and kerosene are separated from crude oil, and different grades of gasoline are separated from each other by fractional distillation.

231. Cooling by solution. Let a handful of common salt be placed in a small beaker of water at the temperature of the room and stirred with a thermometer. The temperature will fall several degrees. If equal weights of ammonium nitrate and water at 15° C. are mixed, the temperature will fall as low as 10° C. If the water is nearly at 0° C. when the ammonium nitrate is added, and if the stirring is done with a test tube partly filled with icecold water, the water in the tube will be frozen.

These experiments show that the breaking up of the crystals of a solid requires an expenditure of heat energy, as well when this operation is effected by solution as by fusion.

232. Freezing points of solutions. If a solution of one part of common salt to ten of water is placed in a test tube and immersed in a "freezing mixture" of water, ice, and salt, the temperature indicated by a thermometer in the tube will not be zero when ice begins to form, but several degrees below zero. The ice which does form, however, will be found, like the vapor which rises above the ocean or any other salt solution, to be free from salt, and it is this fact which furnishes a key to the explanation of why the freezing point of the salt solution is lower than that of pure water. For cooling a substance to its freezing point simply means reducing its temperature, and therefore the mean velocity of its molecules, sufficiently to enable the cohesive forces of the liquid to pull the molecules together into the crystalline form. Since in the freezing of a salt solution the cohesive forces of the water acting to form the ice are obliged to overcome the attractions of the salt molecules as well as the motions of the water molecules, the motions must be rendered less, that is, the temperature

must be made lower, than in the case of pure water in order that crystallization may occur. From this reasoning we should expect that the larger the amount of salt in solution the lower would be the freezing point. This is indeed the case. The lowest freezing point obtainable with common salt in water is -22° C., or -7.6° F. This is the freezing point of a saturated solution.

233. Freezing mixtures. If snow or ice is placed in a vessel of water, the water melts it, and in so doing its temperature is reduced to the freezing point of pure water. Similarly, if ice is placed in salt water, it melts and reduces the temperature of the salt water to the freezing point of the solution. This may be one, or two, or twenty-two degrees below zero, according to the concentration of the solution. Therefore, whether we put the ice in pure water or in salt water, enough of it always melts to reduce the whole mass to the freezing point of the liquid, and each gram of ice which melts uses up 80 calories of heat. The efficiency of a mixture of salt and ice in lowering temperature is therefore due simply to the fact that the freezing point of a salt solution is lower than that of pure water.

The best proportions are three parts of snow or finely shaved ice to one part of common salt. If three parts of calcium chloride are mixed with two parts of snow, a temperature of 55° C. may be produced. This is low enough to freeze mercury.

SUMMARY. The heat of vaporization of a liquid is the number of calories of heat absorbed in the formation of 1 gram of vapor or liberated in its condensation. For water at 100° C. it is 539 calories.

The boiling temperature of a liquid is the temperature at which bubbles of its vapor form and rise as such through the body of the liquid; or, it is the temperature at which the pressure of the saturated vapor first becomes equal to the pressure on the surface of the liquid.

Distillation is a process of vaporization and condensation by which a liquid is separated from a solid dissolved in it, or by which intermingled liquids having different boiling points are separated.