V THE LIQUEFACTION OF GASES 59 



pressure at o^ for each degree through which the 

 gas is heated. If we call the pressure at o" unity, 



then at 1° the pressure will be i + , at 2° it will 



^ 273 



be I +-?-, and so on. Similarly, if we cool the 



273 ^ , 



gas below the freezing-point, at - 1° the pressure 



I .2 



becomes i , at - 2° the pressure Is i . 



273 273 



If, while we carry on this process, the properties 

 of the gas remain unchanged, as they would were 

 it the ideal gas we have supposed, at a temperature 



of - 273° the pressure will fall to i —, that is, 



273 

 I - 1, or zero. At - 273° C, therefore, the 



pressure of an ideal gas would vanish absolutely, 

 and no further cooling could make it smaller. 

 On the temperature scale which uses the pressure 

 of an ideal gas as the thermometric property, 

 — 273° C. represents an absolute zero, the lowest 

 conceivable degree of cold. But, as we said, 

 such a scale coincides exactly with the true 

 absolute or thermodynamic scale, which, as can 

 be shown, unlike all other temperature scales, is 

 independent of the properties of any particular 

 substance, whether real or imaginary. On the 

 thermodynamic scale also, then, — 273" C. repre- 

 sents the absolute zero. 



We thus see that the idea of an absolute zero, 

 at which all bodies would be deprived entirely of 

 heat energy, is not a mere figment of the mathe- 

 matical imagination, derived from the study of 

 a hypothetical air thermometer. It has a real 

 physical meaning, and the attainment of the 

 absolute zero is, at all events, theoretically possible. 



