202 Dr. 0. J. Lodge on the Variation of the Thermal 



the correctness of the theory. At any rate it does not seem 

 to agree very well with the results of experiment, except in 

 the case of iron. The experiments of Principal Forbes* estab- 

 lished the fact that the thermometric conductivity of wrought 

 iron is nearly inversely proportional to the absolute tempera- 

 ture ; but the agreement is not quite perfect, as the following 

 Table shows. 



Centigrade 

 tempera- 

 ture, 

 t. 



Conductivity 



at the temp. 



t, as found 



by Forbes, 



reduced to 



C.G.S. units, 



h 



cp 



Product of 

 conductivity 

 and absolute 

 uemperature, 



(274+0 L 

 cp 



Product which 



is more nearly 



constant, 



(400+0 -• 



CP 



Product, 



(308+0 -• 

 cp 



o 

 

 50 

 100 

 150 

 2< J 

 250 



•2331 

 ■1995 

 •1764 

 •1629 

 •1528 

 •1440 



63-87 

 64-65 

 65-98 

 69-08 

 72-41 

 75-43 



93-24 



89-78 

 88-20 

 89-60 

 91-68 

 93-60 



72-26 

 71-82 

 72-32 

 74-93 



The third column contains the numbers which ought to be 

 constant if the theory were accurate. The fourth column con- 

 tains numbers calculated on the hypothesis that the conducti- 

 vity varies inversely as the absolute temperature increased by 

 some constant, say by 126. These numbers agree with one 

 another rather better than those in the preceding column; 

 but still there is a regular divergence perceptible between the 

 hypothesis and the experimental results, especially at high 

 temperatures. 



The results at the higher temperatures, however, do not 

 seem to have been regarded by Principal Forbes as equally 

 dependable ; for he gives an empirical formula for the con- 

 ductivity at any Centigrade temperature t which does not 

 agree very closely with the experimental results at high tem- 

 peratures, saying, " I have assumed that the most trustworthy 

 part of the observational curves are those between the actual 

 temperatures of 40° and 160°, and that within moderate limits 

 the conductivity may be represented in terms of the tempera- 

 ture by such a formula as " 



cp 



A + Bt + Ct 2 , 



Trans. Eoy. Soc. Edinb. vols, xxiii. & xxiv. 



