148 Prof. Tait on Thermal Conductivity. 



Specific Heat of Iron. 



15° to 100° C 0-1154") Mean. 



0-1127 f . 11M 



0-1158 \ 01152 

 0-1168) 



15° to 150° C &1\§?>\ 



0-1189 V 0-1189 

 0-1186) 



15°to200°C 0*1208} 



0-1214 V 0-1213 

 0-1218) 



15° to 250° C 0-12341 n . 19 o 7 



0-1240 J ° 1261 



15° to 300° C 0-12741 n . 197 . 



0-1276/ 01J75 

 From the first two of these means we find that the specific 

 heat at 15° is 0*109 nearly, and that it increases by y^-Q for 

 each degree. 



Now Forbes 's experiments on iron indicated that the 



quantity -, the ratio of the conductivity to the specific heat, 

 c 



diminishes by about ^-J-^ part for each degree from 0° C. to 



200° C. Hence it is clear that, in this case at least, the 



alteration of specific heat cannot be neglected in estimating 



that of conductivity. For it follows from the numbers just 



given, that the diminution per 1° in the conductivity of iron 



is really only about 2so"o °f the whole amount. My own 



experiments with Forbes's bars gave an average change of 



lc 



- less than that due to the increase of c alone, thus indicating 



c ... ° 



an increase of conductivity with rise of temperature. Ang- 

 strom's result, on the other hand, is considerably greater than 

 that of Forbes ; but the range of temperatures he employed 

 was not above forty degrees. For reasons pointed out in my 

 paper above referred to, I consider Forbes's estimate of the 



lc 

 value of -, from 0° to 150° C, to be probably very near the 



truth. In other metals the change of specific heat is usually 



less than in iron. But so is also that of - It would thus 



c 



appear that we cannot yet state positively that there is any 



metal whose conductivity becomes less as its temperature 



rises; and thus the long-sought analogy between thermal and 



electric conductivity is not likely to be realized. 



