292 PHENOMENA DEPENDENT ON MOLECULAR PATHS 108 



specific heat are known. This has been already done by 

 Maxwell and Clausius, the first founders of the theory, 

 long before any measurements of the conductivity of a gas 

 were experimentally made. Clausius concluded from his 

 theory that air conducts heat 1,400 times less well than 

 lead, and Maxwell predicted that the conductivity of air 

 is 3,500 times smaller than that of iron ; and both these 

 predictions have been since confirmed in striking fashion by 

 Stefan's experiments. 



The formula that leads to these conclusions, viz. 



f = 1-6027 TJC, 



has yet to be proved. We have to calculate the theoretical 

 value of the conductivity of different gases from the observed 

 values of their coefficient of viscosity 77 and of their specific 

 heat at constant volume c, and compare them with the 

 observed values of the conductivity. 



The earlier attempts, and even that made in the first 

 edition of this book, to carry out this calculation and com- 

 parison led to no perfectly satisfactory results ; the calculated 

 and observed numbers exhibited no general agreement to- 

 gether. A satisfactory agreement was obtained only in the 

 case of gases which contain not more than two atoms in 

 the molecule, while with all other gases no approach to 

 agreement was found. This result led to the hypothesis of 

 assuming two different kinds of conduction in gases ( 105). 

 This faulty attempt to account for the contradiction be- 

 tween theory and experiment was refuted by Wiillner, 

 who saw and proved that the failure to obtain agreement 

 was only due to the faultiness of the values assigned in the 

 formula to magnitudes, the variation of which with tem- 

 perature was at that time not known with sufficient accuracy, 

 the values employed having been determined at quite 

 different temperatures. 



Wiillner, 1 whose memoir has already been several 

 times mentioned in 55-58 of Chapter V., determined anew 

 the ratio of the two specific heats for a series of gases, and 

 investigated the dependence of its value on the temperature. 



1 Wied. Ann. 1878, iv. p. 321. 



