—— 
PROFESSOR TAIT ON THERMAL AND ELECTRIC CONDUCTIVITY. 13D 
which the thermal conductivity of iron is a minimum,—but this is quite 
uncertain. 
COPPER. 
Crown. C. 
0 0:076 0:054 
100 0-079 0:057 
200 0:082 0-060 
300 0:085 0:063 
I have already (§ 13 above) stated that uncertainty must attach to all these 
determinations of conductivity of copper at high temperatures on account of 
the different amounts of oxidation of the short bars and of different parts of 
the long bars. The small increase of conductivity with rise of temperature, 
here shown, may depend upon too great a rate of cooling having been adopted 
for the hotter parts of the long bars. 
GERMAN SILVER. 
0 0:0088 
100 0:0090 
200 00092 
300 00094 
The several experiments on German silver, both statical and dynamical, did 
not show so satisfactory an agreement as those on the other bars. <A set of 
mean values is therefore given. 
LEAD. 
0 00152 
100 0:0160 
The experiments on lead have not been conducted through a sufficient range 
of temperature to make the change here indicated certain. 
The experiments on gas-coke proved a failure. ‘The method is not adapted 
to substances of such low conductivity. 
To convert these numbers to the usual unit of conductivity, they must be 
multiplied by the specific gravity and the specific heat of each substance: and 
also by the number of pounds in a cubic foot of water, if heat is to be 
measured in the usual thermal unit. The former constants I have as yet 
determined only roughly, and not for very great ranges of temperature. I 
need scarcely, therefore, add that in the calculations no heed has been taken 
of the change of specific heat with temperature. This would increase the 
values of & at higher temperatures, and thus reduce the change in conductivity 
in iron, and increase the small changes indicated for the other substances. 
§ 16. As the above results, though the outcome of a very protracted investi- 
gation, are, for reasons already stated, only provisional, I do not think it neces- 
sary to print the details of the observations, graphical constructions, or calcula- 
