THE LIQUEFACTION OF GASES 53 



is to say, on the fire below. In a similar way, 

 liquid air has a definite boiling-point, which, 

 under the normal pressure of the atmosphere, 

 rises from —192° to -i82°C. as evaporation 

 proceeds. This rise is due to the fact that 

 nitrogen is more volatile than oxygen ; and thus 

 the liquid, as it boils away, gradually becomes 

 richer in oxygen. Liquefied air cannot be kept 

 in closed vessels. Its vapour pressure, equal to 

 the pressure of the atmosphere at — 190°, becomes 

 enormously great as heat enters from surrounding 

 objects and the temperature rises. In an open 

 vessel, as heat enters evaporation proceeds, and 

 the heat is used to effect the change of state. 

 Thus, owing to this latent heat of evaporation 

 which is absorbed, no rise of temperature (except 

 the very small change already noted) occurs. 

 But, in a closed vessel, as heat enters the pressure 

 will rise, and the boiling-point will rise with it. 

 The initial temperature being so low, a large rise 

 of temperature is possible, and a consequent very 

 great increase in pressure. As ordinary tempera- 

 tures are approached no vessel would withstand 

 the internal pressure of the evaporating air. 



In order to preserve liquid air for any time in 

 an open vessel, it is clearly necessary to prevent 

 as far as possible the access of heat. Evapora- 

 tion must be proceeding continuously, but, by 

 diminishing the rate at which it goes on, the rate 

 of loss of liquid can be retarded. 



Heat passes from one place to another in 

 three ways : by conduction, when heat flows from 

 one part of a body to another, or between two 

 bodies in contact ; by convection, when air or 

 water, heated by contact with a hot body, rises 



