722 PROFESSOR TAIT ON THERMAL AND ELECTRIC CONDUCTIVITY. 
working, after having pushed to the extreme admissible limit every part of the 
process. 
Before giving the results, it may be well to detail with some care the parti- 
culars in which my apparatus and modes of experimenting differ from those 
employed by ForsEs. 
§ 6. With regard to the bars employed—The iron bar experimented on 
was that last made for ForseEs’ experiments. My chief object in employing 
this bar was, of course, to ascertain how nearly I could reproduce Forsss’ results; 
with the view of obtaining, as far as I had the means of doing so, a check upon 
my own work. A couple of copper bars were procured for me, at the instance 
of Mr WitLoucusy SmirTu, from a firm largely engaged in furnishing copper cores 
for submarine cables. These were of the same dimensions as ForBEs’ iron bar 
but, while one (Crown) was made of copper of the highest electric conductivity, 
the other (C) was made of copper of the worst conductivity. The only difference 
in construction between these copper bars (as well as the other bars which I 
employed) and Forses’ iron bar, consisted in the necessary protection of the 
metal from the mercury which was employed to surround the bulbs of the 
thermometers when inserted in the holes. For this purpose it was necessary 
that the holes should be lined with iron; and, therefore, little cups like the 
heads of arrows are sunk into the copper, lead, and German silver bars. The 
thickness of the iron shell is so small that it is not sufficient to influence in the 
slightest measureable degree the progress of the heat along the bar. The 
copper was in the hard state. I propose, at some future time, when some of 
the desiderata after-mentioned are supplied, to have these bars annealed and 
repeat the measurement of their conductivity. 
Along with the copper bars just described, I received some specimens of 
wire for electric testing. These were said to be made of the same materials. My 
experience of them has not been satisfactory, as different specimens from the 
same material show considerable differences in electric conductivity. I there- 
fore defer the consideration of the electric conductivity of these materials till I 
have time to test for this purpose the long bars themselves. 
The German silver bars, long and short, were cut from an exceedingly fine 
casting, procured for me by the late Mr Brecker. Its transverse section is of 
exactly the same dimensions as the others. The bars of lead were cast by 
Messrs MILNE, and are in all respects like the others, save that the bar for 
the statical experiments is not so long. It required special additional supports 
to prevent flexure. 
The bar of gas-coke upon which some experiments have been made, was 
sawn from a block of coke obtained from Mr Youne of the Dalkeith gas-works. 
The bar is exactly of the same transverse section as the other bars employed, 
but though only a few inches in length, it was found sufficient. Even with the 

