9 8 



GENERAL PROPERTIES OF GASES 



sure alone, some gases can be easily transformed into liquids 

 by temperature alone ; on the other hand, many gases are so 

 difficult to liquefy that both pressure and low temperature are 

 needed to produce the desired result. If a gas is cooled and 

 compressed at the same time, liquefaction occurs much more 

 surely and easily than though either factor alone were de- 

 pended upon. The air which surrounds us, and of whose ex- 

 istence we are scarcely aware, can be reduced to the form of 

 a liquid, but the pressure exerted upon the portion to be lique- 

 fied must be thirty-nine times as great as the atmospheric 

 pressure and the temperature must have been reduced to a 

 very low point. 



93. Artificial Ice. Ammonia gas is liquefied by strong pres- 

 sure and low temperature and is then allowed to flow into pipes 

 which run through tanks containing salt water. The reduction 

 of pressure causes the liquid to evaporate or turn to a gas, 

 and the fall of temperature which always accompanies evapo- 

 ration means a lowering of the temperature of the salt water 

 to 1 6 or 1 8 below zero. The brine does not freeze, however, 

 because it is kept in constant motion and has a low freezing 

 point. But immersed in the salt water are molds containing 

 pure water, and since the freezing point of water is o C., the 



FIG. 56. Apparatus for making artificial ice. 



water in the molds freezes and can be drawn from the mold as 

 solid cakes of ice. 



