Popular Science Monthly 



917 



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 withdrawn, it rap- 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. 



