44 PHYSICAL SCIENCE 



definite fixed point, which he called the critical 

 point. The critical point of carbonic acid is 

 fairly high, about 30° on the Centigrade scale ; 

 but for other gases, such as air or hydrogen, it is 

 much lower, many degrees below the freezing- 

 point of water. However low it be, unless a gas 

 is cooled to its critical point, no pressure, what- 

 ever be its intensity, can produce liquefaction. 

 Below their critical points, gases may be con- 

 sidered as vapours, and will liquefy if the pressure 

 applied is high enough. The problem of the 

 liquefaction of a refractory gas is thus solved 

 if we can produce cold sufficiently intense to 

 reduce it below its critical point. 



Three methods have been used, either singly 

 or in conjunction, to cool gases below their critical 

 points. The first method depends on the heat 

 which it is necessary to supply in order to 

 evaporate a liquid. A liquid boils when the 

 pressure of its vapour is equal to the pressure of 

 the atmosphere acting upon its surface, and, if we 

 reduce this external pressure, the boiling-point is 

 lowered. Thus, by pumping away the vapour as 

 fast as it is formed, and so keeping the pressure 

 low, a liquid can be boiled at a temperature much 

 below its normal boiling-point. By this method, 

 for example, it is possible to make water boil with 

 no outside supply of heat. The heat necessary 

 for evaporation is then taken from the water itself, 

 which in this way is gradually cooled. If the air- 

 pump is efficient, and if very little heat is allowed 

 to leak in, the cooling may go so far that the 

 remaining water is frozen. Beginning at the 

 normal boiling-point of water, we should then 

 have cooled the system by means of evaporation 



