Page:Popular Science Monthly Volume 89.djvu/931

 Popular Science Monthly

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��from the molecules and have caused them to return to their natural degree of vibra- tion. If we continue to cool the liquid, we still further paralyze the motion of the molecules, until they become so crowded together that we have a solid — ice.

To Change a Gas into a Liquid — Cool It

Now, then, in the light of the knowledge imparted in the foregoing paragraphs, if we wish to change a gas to a liquid we must cool it. This is true. If sulphur dioxide (a gas obtained by burning sulphur) is cooled to a few degrees below zero, it con- denses into a liquid. As soon as the artificial means of

The tin cup on the right was frozen by immersion in liquid air, after which it was easily broken

cooling the gas is withdrawn, it rap- idly assumes its natural state, as gas, by evapora- tion.* Now to get back to its natural needs a specific amount of heat to make its molecules vibrate at a definite rate, that which nature deter- mined. Where does it get this heat.'' It abstracts it from its surroundings so rap- idly that a still further de- gree of coldness is realized as the gas is formed from the liquid and passes off carrying with it its natural amount of heat which it has greedily robbed from material in con- tact with it. For commercial purposes liquid carbon diox- ide is stored under great pressure in durable steel cylinders. If the jet on the cylinder is opened, the liquid evaporates so rapidly that the temperature of the con- tainer is soon lowered far below zero, and a solid

���Part of condenser in a laboratory refrigerating apparatus. The chamber is so cold that frost is formed. Yet liquid air would boil briskly if placed on the tubes

At left: What remains of a large rubber cork after it has been frozen and struck with a hammer

At left below: Two screweyes frozen into a I block of mercury so solidly that they sus- tain the weight of two flatirons suspended from a great height

formation of carbon dioxide appears on the mouth of the jet.

Professor Dewar liquefied h^"drogen and helium in the laboratory of the Royal So- ciety by a different method from that of rapid evapora- tion. The principle applied by him is based on the fact that a compressed gas al- lowed to expand freely great- ly lowers its own tempera- ture. Lord Kelvin made known this fact early in his career, and it was commer- cially utilized by Linde, a German scientist, and by Hampson, an English physi- cian. Both workers were laboring independently of each other.

���*This is assuming that the gas is not stored under pressure, which prevents evaporation.

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