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To change Fahrenheit temperatures into their corresponding centigrade values:

Rule II.-Subtract 32° from the temperature, Fahrenheit, multiply by, and the result will be the temperature, centigrade.

Expressing these two rules by means of formulas, let

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EXAMPLE 1.-Change into Fahrenheit temperatures: (a) 100° C.; (b) 4° C.; (c) — 40° C.

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EXAMPLE 2.-Change into their corresponding centigrade temperatures: (a) 60° F.; (b) 32° F.; (c) - 20° F.

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70. Expansion of Bodies by Heat.-The volume of any body-solid, liquid, or gaseous-is always changed if the temperature is changed; nearly all bodies expand when heated and contract when cooled. Although the expansion of solids and liquids is very nearly uniform throughout all the ranges of temperature, water is a marked exception to the general rule. If water is cooled from its boiling point, it continually contracts until it reaches 39.2° F., when it begins to expand, until it freezes at 32° F. On the other hand, if water at 32° F. is heated, it contracts until it reaches 39.2° F., when it commences to expand. Therefore, the density of water is greatest where this change occurs.

In taking the specific gravity of liquids, the temperature of such liquids has to be taken into consideration; in the internal-revenue service, the standard of comparison is water at a temperature of 39.2° F. (4° C.). In determining specific gravities of liquids, however, they are usually kept at a temperature of 60° F. (15.5° C.). As it is impossible in

practical work, such as gauging, etc., to obtain always the exact temperature of 60° F., special tables are computed, which show the necessary corrections that have to be made if the temperature is below or above the standard.

There is, of course, a limit of temperature beyond which the specific gravity of liquids cannot be taken correctly. Alcohol, for instance, boils at about 78° C. (172° F.), while water boils at 100° C. (212° F.), and if a mixture of alcohol and water at a temperature near the boiling point of alcohol were taken, erroneous results would be obtained owing to the unequal expansion of the liquids, due to the different temperatures of their boiling point and the evaporation that takes place.

71. Evaporation.-Evaporation may be defined as the transformation of liquid molecules into gaseous ones, and is due to the absorption of heat that raises the temperature of the liquid molecules to that point of latent heat at which they become gasified. There are two kinds of evaporation: namely, surface evaporation and boiling. The former is entirely due to the influence of the conditions of the atmosphere. A liquid exposed to the atmosphere in an open vessel slowly absorbs at its surface the heat of the atmosphere, which absorption gradually raises the latent heat of the molecules to that temperature at which they change into the state of vapor. It may therefore be stated that surface evaporation depends on decreasing the attraction of the surface molecules caused by the atmosphere surrounding it. The warmer the atmosphere surrounding the molecule, the quicker is the evaporation. It has been further found that surface evaporation depends, to a considerable degree, on the density of the liquid. The less dense a liquid, the quicker does it evaporate; compounds like ether, alcohol, and others having a comparatively low density, evaporate much quicker under the influence of atmospheric conditions than does water, or other liquids of equal or greater densities. The air usually holds in suspension a certain amount of water vapor and owing to this fact has a tendency to absorb moisture from the surface of the

water. The less water vapor held in suspension by the air, or in other words the drier the air, the faster the evaporation. The action of the wind also accelerates evaporation because, as the volume of the air surrounding the liquid becomes saturated, it moves to give place to the drier air. Hot air will absorb more moisture than cold air, and the cause of dew is the depositing of the water held in suspension in the atmosphere by the lowering of the temperature during the night.

Experiments have shown that evaporation: (1) increases with a rise in temperature; (2) increases as the elevation above sea level increases; (3) decreases as the elevation decreases.

72. Evaporation by boiling is produced through artificial heat and consequently takes place more uniformly throughout the whole body of the liquid.

The boiling point of a liquid depends on three conditions; namely, purity, nature of the vessel, and pressure on the surface.

The mixture of a solid substance with a liquid usually elevates the temperature of the boiling point; for example, when salt is dissolved in water, the mixture boils at a higher temperature than does pure water.

The material of which the containing vessel is made affects the temperature of the boiling point of the liquid; for example, water will boil at a lower temperature in an iron vessel than it will in glass.

The temperature of the boiling point of a liquid varies according to the pressure on its surface. As the elevation increases, the pressure decreases and the temperature of the boiling point is lowered; as the elevation decreases, the pressure increases and the temperature of the boiling point becomes higher. For example, water boils at an elevation of 1,000 feet above the sea level at a temperature of 210° F. and at 212.4° F. at 200 feet below the sea level.

73. Distillation.-Distillation is the process of separating, by heat, the more volatile parts of a substance from

the less volatile parts, and subsequently condensing, by cooling, the vapor thus formed. The purposes of distillation vary to a great extent; it may be employed to separate liquids of different boiling points, as in the instance of alcohol and water, fermented liquors, etc. The apparatus employed varies greatly according to its usage, while the principles on which all forms of apparatus depend are, of course, the same. One form of apparatus, called a still, is shown in Fig. 45. It consists essentially of four parts: the body a, a copper

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vessel containing the liquid, the lower part of which fits in the furnace f; the head b, which fits on the body and from which a lateral tube c leads to the worm g, a long spiral tin or copper tube placed in a cistern, or tank d, kept constantly full of cold water. The worm connects the boiler vessel a with the receiving vessel e; in the worm, the vapor condenses again to the liquid form and the distilled liquid passes over into the receiving vessel.

SPIRITUOUS LIQUORS

ALCOHOL

74. Alcohol, as the term is generally understood, signifies vinous spirits of various strengths and is commonly generated in vegetable juices and infusions by a fermentation. Thus the fermented juice of the grape is called wine; of the apple, cider; and the fermented infusion of malt, beer. The nature of the liquids admitting of vinous fermentation, no matter how they differ in other respects, are all similar, in the respect that they contain sugar in some form or other. Experience has demonstrated that the sugar contained in these liquids disappears wholly or in part after vinous fermentations and until recently it was believed that the only new products resulting during the progress of fermentation. were alcohol and carbonic acid, the latter of which passed off. Sugar, alone, will not undergo vinous fermentation, but must be dissolved in water subjected to the influence of a ferment and the mixture kept at a constant temperature. The water gives fluidity to the mixture and the ferment and heat begin and maintain the chemical change.

75. The alcohol is obtained from vinous liquors by successive distillations, each distillation giving a purer product. These products form the different ardent spirits of commerce.

The spirits obtained from the distillation of wine is called brandy; from fermented molasses, rum; from cider, corn, or rye, whisky; from malted barley, rye, potatoes, and rectified from turpentine, common gin; and from fermented rice, arrack.

76. The rectification of alcohol necessitates three series of distillations. The first comprises the distillation of the alcohol at a temperature of 154° F. This first distillation

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