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begin to boil vigorously, showing that if the pressure is reduced the boiling point is lowered.

The effect of reduced pressure upon the boiling point is

FIG. 253

seen upon high mountains, where water boils at so low a temperature that food cannot be cooked by boiling. Advantage is taken of this effect in the making of sugar, where vacuum pans are used to evaporate the solution without burning it. In the extraction of glue from bones and hides the pressure is increased and the boiling point correspondingly raised.

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297. The Spheroidal State. Whenever water is thrown upon a very hot metallic surface, a condition called the spheroidal state is set up. The effect of the heated surface is to vaporize a little of the liquid, so that the remainder does not rest directly upon the metal, but upon a cushion of steam. This by its constant movement keeps the liquid in rapid vibration. The liquid takes the globular form because of surface tension, and changes into vapor at a rate faster than evaporation and slower than boiling. If the metal cools and the liquid comes in contact with it, a sudden production of steam is the result. Steam boiler explosions have resulted from the water in the boiler getting low, and then cold water being suddenly turned on after the boiler had become red-hot above the water line.

Demonstration. Place a smooth tin or brass plate upon a tripod and heat it with a burner. Drop a few drops of water upon it with

a pipette. After the spheroidal condition is set up, remove the flame and let the plate cool. What occurs when the water touches the plate? Why?

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298. Condensation of Vapors; Distillation. The condensation of a vapor to a liquid is brought about by lowering its temperature or by increasing the pressure upon it, or by both. In condensing, it gives off as much heat as was required to vaporize it. It is possible to separate a liquid

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from substances with which it is mixed, or which it holds in solution, by boiling the mixture or solution and condensing the vapor. This process, which depends upon the difference in the boiling point of liquids, is called distillation, and the liquid that has been purified by it is called the distillate. The condenser, in which the condensation takes place, may be in the form of a straight tube, as in Fig. 254, or in the form of a spiral tube, which is called a worm. Each form is surrounded by a water jacket to keep the tube cool. The purity of the distillate is increased by redistillation. In a mixture

of alcohol and water, for example, a little water comes over with the alcohol in the first distillation, but less in the second.

Demonstration. Arrange apparatus as shown in Fig. 254. Pass cold water in at the lower end of the water jacket and let it run out at the top. Put a mixture of equal parts of alcohol and water in the flask. Boil it until about a third of the mixture is condensed in the beaker. Remove the flame and test the distillate by setting it on fire. Test the mixture remaining in the flask in the same way.

299. Fractional Distillation. - Since every liquid has its own boiling point, it is possible to separate a mixture of several that have different boiling points, by the process called fractional distillation. When crude petroleum is distilled, as soon as the boiling begins it is kept at the same temperature until no more distillate comes over. This most volatile part of the oil having been removed, the temperature is now raised a few degrees, boiling begins again, and the petroleum is kept at the new temperature as long as any vapor comes over. The process is repeated a number of times. The product that comes off last is the highest grade of all. That is it ignites at the highest temperature.

When air is liquefied and allowed to stand in a flask, the nitrogen will boil off first, since the boiling point of nitrogen is 195°, while that of oxygen is 183°. After the flask has stood for some time, the nitrogen will have boiled away and the liquid left will be oxygen.

300. Critical Temperature. The pressure required to reduce a gas to a liquid increases as the temperature rises. Moreover, there is for every gas a temperature above which it cannot be liquefied, however great the pressure. This temperature is called the critical temperature.

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Figure 255 is a graphical representation of the relation

between the pressure, tempera-
ture, and physical state of am-
monia.1 This shows that ammo-
nia can be reduced from a gas
to a liquid by pressures varying
from 115 atmospheres at the
critical temperature, 130°, to 1
atmosphere when the tempera-
ture is reduced to - 33°.
also shows that ammonia can
be reduced to a liquid at the
ordinary temperature of the air
by pressure alone.

It

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PHYSICAL
STATE

PRES- TEMPER

SURE

ATURE, C.

Always a GAS

Any

Pressure

Pressure

From 1 to 115 atmospheres

Pressure

1 atmosphere

130°

Critical Temperature

A GAS or
a LIQUID,
dependent
on pressure

-33°

-Boiling Point

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FIG. 255-Physical State of
Ammonia

the water to be frozen is poured into cans set in a large tank containing brine (Fig. 256). Coils of pipe are placed in the tank, and in these coils ammonia which has been liquefied by pressure is allowed to vaporize. The pressure is reduced by pumping the ammonia vapor back into the compressor. By the vaporization of the liquid and the rapid expansion of the vapor, the temperature of the brine is lowered to about

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10° C. The freezing of the water in the cans goes on rapidly, the crystals of ice extending from all sides. The heat produced in compressing and liquefying the ammonia is allowed to dissipate before the liquid ammonia is returned to the coils. The cooling of the compressed ammonia is brought about by causing cold water to drip over the pipes leading from the compression cylinder, before they connect with the expansion cylinder. This cools the ammonia before it enters the expansion cylinder and thus secures a lower temperature in the brine tank.

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