420 Screntific Intelligence. 
tions to be embedded in them, and no wider than to admit the 
wires and junctions. Thus, when the two pieces of rock are 
pressed together, and when heat is so applied that the flux lines 
are parallel to the faces of the two parts, we had the same result, 
so far as the thermal conduction is concerned, as if we had taken 
a single slab of the same size as the two together, with long fine 
perforations to receive the electric junctions. The compound 
slab was placed with the perforations horizontal, and their plane 
vertical. Its lower side, when thus placed, was immersed under 
a bath of tin, kept melted byalamp below it. Its upper side was 
flooded over with mercury in our later experiments (pars. 6, 7, 8), 
as in Hopkins’s experiments on the thermal conductivity of rock. 
Heat was carried off from the mercury by a measured quantity of 
cold water poured upon it once a minute, allowed to remain till 
the end of a minute, and then drawn off and immediately replaced 
by another equal quantity of cold water. The chief difficulty in 
respect to steadiness of temperature was the keeping of the gas 
lamp below the bath of melted tin uniform. If more experiments 
are to be made on the same plan, whether rocks or metals, or 
other solids, it will, no doubt, be advisable to use an automatic- 
ally regulated gas flame, keeping the temperature of the hot bath 
in which the lower face of the slab or column is immersed at as 
nearly constant a temperature as possible, and to arrange for a 
perfectly steady flow of cold water to carry away heat from the 
upper surface of the mercury resting on the upper side of the slab 
or column. It will also be advisable to avoid the complication of 
having the slab or column in two parts, when the material and the 
dimensions of the solid allow fine perforations to be bored through 
it, instead of grooves, which we found more readily made with the 
appliances available to us. 
(4.) Our first experiments were made on a slate slab, 25 cm. 
square and 5 cm. thick, in two halves, pressed together, each 25 
em. by 12°5, and 5 cm. thick. One of these parts cracked with a 
loud noise in an early experiment, with the Jower face of the com- 
posite square resting on an iron plate heated by a powerful gas 
burner, and the upper face kept cool by ice in a metal vessel rest- 
ing upon it. The experiment indicated, very decidedly, less con- 
ductivity in the hotter part below the middle than in the cooler 
part above the middle of the composite square slab.- We sup- 
posed this might possibly be due to the crack, which we found to 
be horizontal and below the middle, and to be complete across 
the whole area of 124 cm. by 5, across which the heat was con- 
ducted in that part of the composite slab; and to give rise to 
palpably imperfect fitting together of the solid above and below 
it. We therefore repeated the experiment with the composite 
slab turned upside down, so as to bring the crack in one half of 
it now to be above the middle, instead of below the middle, as at 
first. We still found for the composite slab less conductivity in 
the hot part below the middle than in the cool part above the 
middle. We inferred that, in respect to thermal conduction 
