Man's physical universe



nitrogen, hydrogen, oxygen, and a few other gases resisted all attempts

at liquefaction by increase of pressure.

Finally it was discovered that

the critical temperatures of these gases were lower than ordinary temperatures.

Then it became a simple thing to liquefy them by lowering

the temperature at the same time that the pressure was increased.

Figure 71 shows the boiling- and freezing-points of some common


In order to liquefy any gas or vapor, one need only cool

it below the boiling-point at standard pressure. Thus steam may be

liquefied at 100° C. Likewise, if one should cool air to a temperature

of— 190° C, it would liquefy. This is a very difftcult process, however,

and it has been found that air will liquefy at much higher temperatures

by raising the boiling-point with a great increase in pressure. In order

to liquefy air, therefore, it is only necessary to compress it and allow

a portion of it to expand. This expansion process cools the rest of the

air which has been compressed below the critical temperature. The

cooling effect is an example of the general principle that a gas absorbs

heat when it expands.

Liquid Air Is Extremely Interesting.

The experiments which can be performed with liquid air are

extremely interesting because they enable one to observe the properties

of various kinds of matter at very low temperatures.

The word "zero" when referring to the Fahrenheit temperature

scale brings to the minds of most people visions of a midwinter cold

spell. A temperature of 20° or 30° below zero Fahrenheit reminds us

of Siberian or Canadian winters, while 50° below zero calls to mind the

worst features of Polar expeditions. Liquid air, however, boils at 377°

below zero; it will boil merrily on a cake of ice.

A teakettle containing liquid air when heated over a flame will form

a white deposit of solid carbon dioxide and water ice, which results

from cooling the products of the combustion of gas.

Liquid air must be kept in open containers protected by a vacuum

jacket; a thermos bottle meets these conditions.

In even the best of

these containers sufificient heat is absorbed to keep the liquid air evaporating.

The pressure produced by the evaporation of liquid air in a

closed container would eventually blow it to pieces. This is well demonstrated

by placing a cork in a heavy glass tube containing liquid air.

Toy steam engines will run furiously when liquid air is placed in

their boilers.

Toy rubber balloons may be inflated by attaching them to a tube

This illustrates the fact that liquid air expands

containing liquid air.

eight hundredfold upon vaporization.

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