HALL AND ATRES. — HEAT CONDUCTION IN IRON. 299 



The difference between the value of ^ found for 20°. 9 and that found 

 for 38°. 9 is so slight that very little importance can be attached to it, in 

 view of the much greater differences between successive measurements 

 of K at or near any one temperature. Making the formal calculation, 

 however, from these values as they stand, we get, as the temperature co- 

 efficient of K between 20°.9 and 38°.9, 



0.1494 — 0.1485 ^ ^^^„„ 



— —0.00033. 



0.1494(38.9 — 20.9) 



Taking the mean of 20°.9 and 38°.9 and the mean of 0.1494 and 0.1485, 

 we have, at 29°. 9, A'= 0.1490. Taking this as a starting point, we 

 find, for the tempeiature coefiicieut of iiT between 29°.9 and 56°.2, 



0.1490-0.1453 __o.ooo94. 



-0.00064. 



0.1490(56.2 — 29.9) 



Similarly, we find between 29°.9 and 74°.7, 



0.1490 — 0.1447 _ 

 0.1490 (74.7-29.9;"" 



So much for the IMay and June numbers. 



From the July and August numbers we get, between 27°. 6 and 56°. 8, 



0.1505—0.1458 



= —0.000107. 



0.1505(56.8 — 27.6) 



The mean of all these estimates of the temperature coefficient of K is 

 — 0.00075, according to which the thermal conductivity of the cast iron 

 disk diminishes about 1% for each 18°. 3 rise of temperature within the 

 limits of the observations above recorded. According to experiments 

 described in Appendix 11. following this paper, the temperature coefficient 

 of electrical conductivity of the same cast iron, between 17° and 67°, 

 is — 0.00118; which means that the electrical conductivity between 

 these limits diminishes at the rate of 1 % for each 8.°5 rise of tempera- 

 ture. At one time during the preparation of this paper it appeared that 

 the two temperature coefficients were very nearly equal. This led to a 

 more careful examination of the evidence than had been made before, 

 and a repetition of certain measurements, with the result given above. 

 It may yet be that the two coefficients are equal. Where both are so 

 small the question of equality or inequality is difficult to settle, although 



