THE LIQUEFACTION OF GASES 43 



Thus an increase of pressure must lower the 

 melting or freezing-point. On evaporation, the 

 increase in volume occurs with the change from 

 liquid to vapour ; an increase of external pressure 

 therefore makes evaporation more difficult, and 

 consequently produces a rise in the boiling-point. 

 If the change in volume and the amount of heat 

 required to produce the change in state are 

 known, the principles of thermodynamics enable 

 us to calculate the exact amount of alteration in 

 the freezing or boiling-points. 



There is reason to suppose that the three 

 states of solid, liquid, and gas, assumed within a 

 moderate range of temperature and pressure by 

 the familiar substance water, might be obtained 

 with all bodies if we could command temperatures 

 and pressures high enough and low enough. 

 Metals melt and volatilise at high temperatures, 

 while even gases such as air and hydrogen have 

 now been liquefied. 



Several gases, previously unknown in any 

 other form, were liquefied by Faraday. His 

 method consisted in evolving the gas by heating 

 chemical reagents in one limb of a bent glass 

 tube, and cooling the other limb in cold water or 

 a freezing mixture. As the gas is evolved, the 

 pressure rises, and either the gas is liquefied in 

 the cold limb, or the tube bursts. By this simple 

 means chlorine, sulphur dioxide, ammonia, and a 

 few other gases may be liquefied. 



The conditions necessary for liquefaction were 

 not fully understood till Andrews, in 1863, showed 

 that carbonic acid gas could not be liquefied 

 unless its temperature was reduced below a 



