
PROFESSOR TAIT ON THERMAL AND ELECTRIC CONDUCTIVITY. 719 
energy of the heat in three successive slices of a solid, of equal thickness, 
were always the lowest possible, consistent with the conditions of the ex- 
periment, Forses’ result would follow, and would give, in fact, an excellent 
instance of dissipation of energy. 
§ 2. The subjects I set myself to inquire into were definitely these— 
(1.) Whether in pure metals there is always a decrease of thermal conductivity 
with a rise of temperature. And for this purpose I chose the metals copper and 
lead, because we can easily and at small expense procure them in large quantity 
and in a state of great purity. 
(2.) Whether different specimens of the same metal may not differ in thermal 
conductivity, at least as widely as they are known to do in electric conductivity ; 
and for this purpose, in consequence of Sir W. THomson’s* remarkable obser- 
vations on the electric conductivity of copper, I selected copper. 
(3.) Whether an alloy, such as is chosen for resistance coils because its 
electric conductivity changes little with change of temperature, does not 
show a similar small change of thermal conductivity ; for this purpose I chose 
the alloy, German silver, which is frequently used for such coils. 
(4.) A fourth question, which I have not yet answered, was whether there 
may not be some conduction-peculiarity in a substance whose specific heat 
varies little with temperature. This was suggested to me by the theoretical 
notions above alluded to, and probably falls with them. For such a purpose 
there can be no doubt that the best substance is platinum, because its specific 
heat is known to alter very little, and Messrs Jonnston and MarTTuey were 
kind enough to offer to provide me with a bar of platinum of the same dimen- 
sions as Forses’ iron bar, at the comparatively small expense of working the 
material into the necessary form and working it down again. The value of 
the material of such a bar, it may be well to mention, would have been about 
£2000. 
§ 3. The results I have hitherto published in the Reports of the British 
Association were, of course, strictly preliminary. For, besides the want of 
scale-errors for my thermometers, another great difficulty felt at the commence- 
ment of the experiments was that of maintaining a nearly constant temperature 
in the source of heat for the statical experiment. At the time I gave those 
provisional reports, I had operated only with temperatures not much higher 
than that of boiling water ; through a range, in fact, barely sufficient to indicate 
with certainty a change of conductivity even in iron. 
§ 4. Shortly after Forses published the full result of his experiments on 
iron, another excellent and novel method, quite distinct in principle from his, 
was described by Anestrom.t Of that method I availed myself, with the help of 
* Proc. R. S., 1857 (June 15). + Pogg. Ann., 1862. Phil. Mag., 1863, i. 
