ON THE condhctiah: poayees oe vaeious .substances. 
839 
large block, and that immediately surrounding the inner block, are tilled witli cotton- 
wool to prevent as much as possible the lateral transference of heat, both from the inner 
block to the outer one, and from the vertical sides of the latter into surrounding space. 
Also, from the upper edges of the outer block (or rather of the wooden frame surround- 
ing it) two or three wooden floors with intervals between them, were constructed of loose 
boards, extending several feet on every side (sections of which are represented in fig. 6 at 
P and P'), and intended to prevent as far as possible the influence of radiation from the 
heated stove below, and the objects immediately surrounding it. The upper surfaces, 
both of the outer and inner blocks (fig. 7), are scooped out, leaving a projecting 
rim round them edges, so as to form shallow vessels for the reception of mercury; or 
the same object is attained by fixing round the edges a thin, narrow rim of iron. These 
spaces, in experimentmg, are filled with mercury of just sufficient depth to cover the 
small spherical bulb of a thermometer. In fig. 6, A, B, A, represent the section of the 
outer and inner blocks, with the interspace between them. One thermometer (T,) 
passes down this interspace into the mercury below the blocks ; while gives the tem- 
perature of the upper mercury on the block B, the conductive power of which is to be 
determined ; and T 2 gives that of the mercury which covers the surface of the outer 
block. This latter mercury has sometimes been dispensed with, in which case the 
temperatui’e of the uncovered surface of the outer block has been determined approxi- 
mately by placing a thermometer in a small hole formed in the surface, and containing 
mercury. 
This arrangement being completed, the fire is placed in the stove, and the lower mer- 
cui*y is heated to the required temperature, as indicated by Tj. The temperature much 
the most frequently used has been that of boiling-water, in which case the vessel V is 
kept nearly full, the exhaustion caused by the continual generation of steam being com- 
pensated by a supply through a tube arranged for the purpose. This temperature is 
preferable on account of the facihty with which it can be preserved for any length of 
time. For higher temperatui’es, which were rarely used, and for which water cannot 
be employed, the vessel V is left empty. In all cases when the lower mercury has 
attained its required temperature, it must be kept as steadily as possible at that tempe- 
rature, till the thermometer, Tg, which gives the temperature of the upper mercury on 
B, becomes stationary. The temperatures indicated by T, and T 2 are those denoted by 
and in the formula (5), (art. I). The remaining temperature in that formula (r) 
is obtained by wa-ving a thermometer above and Avithin a short distance of the blocks, 
the instrument being carefully guarded against the influence of direct radiation from 
suiTounding objects. 
27. To secure entire accuracy by this mode of experimenting, it is manifest that the 
horizontal extent of the mass experimented on ought to be so large, compared with its 
thickness, that no appreciable diminution of the temperature (^ 2 ) of the central portion 
of the upper surface should be produced by the lateral transference of the transmitted 
heat, the whole of which is supposed in our mathematical formula to be transmitted 
5 E 2 
