Man's physical universe

xanabras

THE ABSORPTION OR EVOLUTION OF HEAT 229

If liquid air remains in contact with the skin very long, it freezes it

and leaves a bad burn similar to that produced with a red-hot iron.

Flowers become so brittle in liquid air that they can be broken like

thin glass. A rubber tube can be broken into pieces with a hammer,

while a rubber ball cooled in liquid air will break into many pieces when

thrown against a wall.

Mercury may be frozen to a solid in liquid air, as may many other

liquids such as kerosene, alcohol, and glycerine.

Natural gas is readily liquefied by running it into a flask cooled by

liquid air.

Liquid air is widely used in scientific work to obtain low temperatures.

The Heat of Vaporization Has Important Applications.

When water is vaporized, heat must be applied. As water is heated,

part of the heat is

used up in raising the temperature of the water;

but when the boiling-point has been reached, all of the heat is used in

vaporizing the water, the temperature of the water remaining constant.

Highly volatile liquids, like ether, will quickly vaporize when poured

on the hand, which will itself be cooled by the process because the heat

required is taken from the hand. Each different kind of liquid has its

own heat of vaporization, which is defined as the amount of heat {in

calories)

required to vaporize one gram of a liquid at its boiling-point.

Heat of Vaporization

Water 538

Ammonia 294

Ethyl alcohol 205

Mercury 68

Chloroform 58

Refrigeration units are cooled by the vaporization of a liquid in the

cooling coil. Liquids of high heat of vaporization are naturally most

efficient for refrigeration purposes and for transmitting heat.

The heat of vaporization of water is higher than that of any other

known liquid, nearly six times that of liquid sulfur dioxide and nearly

ten times that of liquid carbon dioxide.

This high heat of vaporization of water Is of great importance in

understanding many of the weather changes which will

be described

in later sections of this Unit. It results in the maximum amount of heat

with the return to the boiler of a minimum weight of condensed material

in steam-heating systems, because it gives off more heat than an

equal weight of any other liquid would when it is condensed. A mixture

of diphenyl and diphenyl oxide does not exert such dangerously high

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