740 PROFESSOR TAIT ON THERMAL AND ELECTRIC CONDUCTIVITY. 
2. At temperatures above 150° C. the diminution of conductivity of iron is 
less rapid than that assigned by Forses. The conductivity seems to reach a 
minimum somewhere about red-heat. 
3. The thermal conductivity of copper and lead changes much less than that 
of tron with rise of temperature, and probably in the sense of increase instead 
of diminution. The same is true of German silver. 
4. Electrically bad copper conducts heat worse than electrically good copper— 
but not in the same ratio. 
5. The metals examined have the same order as conductors of heat and of 
electricity. The alloy violates this arrangement. 
Postscript.—As I have not given the experimental data for the first part of 
this paper, I may state here the peculiarity upon which the above deductions 
chiefly depend. 
The law of cooling is nearly the same (to a constant factor) for iron and 
the two kinds of copper throughout the range of temperatures employed. 
But the statical curve for iron differs considerably from that for copper. 
The ratios of the temperature-excesses at intervals of three inches along the 
long bars increase at higher temperatures in iron much faster than in copper. 
In fact, the inferior copper almost realises LAMBERT’s result. 

