840 
ME. W. HOPKIIS’S’S EXPEEIiyrEXTAL EESEAECHES 
vertically. It would have been almost impossible, however, to obtain slabs of any con- 
siderable extent from many of the harder and more intractable rocks on which I have 
experimented. But the fact is, that no great accuracy is requhed in experiments of 
this nature ; for if the conductive power of any piece of rock were determined with the 
ultimate degree of accuracy, it would only tell us apjjroximately, and not accurately, the 
conductive power of another piece of rock of the same kind, however close might be theii' 
resemblance in mineral structure ; so that it must be always necessary, when the con- 
ductive power of any proposed substance is required uith great accuracy, to determine 
that power by direct experiments on the substance itself. I have endeavomed, however, 
to take every precaution necessary to secure all the accuracy required vdth reference to 
my more immediate objects in these researches. 
In order that the error resulting from the observations of temperatui’es may be as 
small as possible, it is manifestly desirable that and t^ — t^ should both be as large 
as possible, and not very different in magnitude. But this condition, if the conductive 
power and the temperatures used were sufficiently great, would requhe a thickness of 
the block which might be undesirable in consequence of its limited extent of horizontal 
surface. The thickness which I have more generally used for mineral substances, is 
about 2 inches, and about 1 inch for substances, such as wax and spermaceti, of lower 
conductive powers, while the outer block has been about 15 inches square, and of very 
nearly the same thickness as the blocks experimented on, and covered vrith mercmy in 
the same manner. Having several of these outer blocks made of substances of different 
conductive powers, I have always been able to select one for each experiment in which 
the conductivity was nearly the same as that of any proposed substance, taking care, 
however, that it should never be greater ; for in such case the temperature of the smface 
of the outer block, as indicated by Tg, fig. 6, would be higher than the proper tempe- 
rature of the surface of B, and would, therefore, tend to raise also the observed tempe- 
ratmre of B, as given by Tg, above the proper temperature, and thus to give the con- 
ductive power of B too great. I have been anxious that, on the contrary, the error 
should always be in defect, and therefore have always selected such a block for each 
experiment that the temperature given by Tg should generally be somewhat less than 
that given by Tg. A small error has, doubtless, been thus superinduced, especially in 
reference to substances of the highest conductive powers, but I am satisfied that it has 
in no case been of any importance as regards the conclusions which I have drawn from 
these results. 
28. In all cases in which it has been necessary to com- 
pare the conductive powers of two blocks with as much 
accuracy as possible, I have adopted a somewhat different 
method from that above described. Instead of a single 
block (B) placed in the central cylindrical hole of the 
outer block (A), the two blocks to be compared have been placed in two cylindrical holes 
pierced through the outer block, as represented in fig. 8, and such that the two blocks 
Fig. S. 
