28 MOLECULAR MOTION AND ITS ENERGY 15 



15. Absolute Zero of Temperature 



The law just found allows the position of the so-called 

 absolute zero of temperature for gaseous bodies to be 

 determined. 



If heat is nothing else than the kinetic energy of 

 molecular motion, the temperature at which a gas possesses 

 no more heat must be identical with that at which its 

 molecular motion has disappeared, and all atoms and mole- 

 cules remain in a state of perfect rest. 



The expression that has been found for the molecular 

 speed G shows that this speed vanishes when 



If from the measures of Magnus, 1 Eegnault, 2 Jolly, 3 

 Recknagel, 4 and others, which are all in agreement 5 with 

 each other, we take 0-00367 for the value of a in the case of 

 air when the Centigrade scale is used, and put this in the 

 last equation, we find 



3 = - 272-5 C. 



for the required temperature of the absolute zero. If we 

 reckon temperature, not from the melting-point of ice 

 arbitrarily chosen to start from, but from this absolute zero, 

 then we obtain for the absolute temperature 



= 272-5 + 3; 

 or, in the general case, for all scales in use we have 



e = a + a, 



where the value of the constant a is to be taken as the 

 reciprocal of the coefficient of expansion 6 of air for the 

 scale in question. 



1 Pogg. Ann. Iv. 1842, p. 25. 



2 M6m. de VAcad. de Paris, xxi. 1847, p. 73 ; Pogg. Ann. Iv. and Ivii. 



3 Pogg. Ann. Jubelband, 1874, p. 82. 



4 Pogg. Ann. cxxiii. 1864, p. 127, table i. 



5 Mendelejeff, Ber. d. deutsch. chem. Ges. x. 1877, p. 81. 



6 [This requires definition ; on the Fahrenheit scale, for instance, the co- 

 efficient of expansion is usually denned with reference to an initial volume at 



