128 REPORT—1874. 
Report of a Committee, consisting of Prof. A. S. Herscuer, B.A., 
F.R.A.S., and G. A. Lusour, F.G.S., on Experiments to determine 
the Thermal Conductivities of certain Rocks, showing especially the 
Geological Aspects of the Investigation. 
Description and Results of the Experiments, By Prof. A. S. Herscurt. 
Iy the introductory notes on these experiments in the Transactions of the 
Sections, p. 223, in the volume for 1873 of these Reports, the list of rocks 
selected and the manner of experimenting on them were described. With 
the exception that sections of Calton trap-rock, of a great pyramid casing- 
stone (nummulitic limestone), Caenstone (or Normandy building-limestone), 
cannel-coal, chalk, and red brick were added to this list, and that the 
apparatus received some small but very important improvements to make it 
heat-tight, the material of the experiments, as well as the method of making 
them, remained substantially the same as last year. Instead of a conical tin 
vessel with 1 Jb. of water, a cylindrical one holding 23 lbs., with an internal 
agitator and thermometer, was used as the cooler. The opposing surfaces 
of the heater and cooler are faced with velvet, and are each encircled by a 
caoutchoue collar, which projecting a little beyond them clasps the circular 
edge of the rock plate when it is placed between them ; two small slits in 
each collar-edge allow the wires of a thermocouple to be introduced, touching 
the rock-surfaces while the rock is being heated. With the view of traversing 
the plate with the thermopile in different directions, the piece of stout 
palladium wire (about 18 gauge), used as the electromotive element between 
two iron wire terminations of a delicate reflecting galvanometer, was silver- 
soldered to the iron wires at its two ends, all the wires being first rolled thin 
and flat to some distance from the junctions. The scythe or scimitar-blade 
shape generally given to the wire in rolling it thin was advantageous in the 
construction, because instead of uniting the wires continuously in one straight 
length and folding the points of junction upon opposite sides of the rock (thus 
confining their range upon it to a single diameter or to one straight line), 
advantage of the curvature was taken to connect the wires by superposition, 
instead of by prolongation at their junctions, without overlying each other, 
into two flat ogee-arches or merry-thought-like blades, between which the 
rock is held as in a foreeps. The unrolled parts of the wires are bound very 
firmly to a small square picce of wood, which acts as a handle to guide the 
points of the forceps to various parts of the rock-faces, while it keeps them 
securely in their places, and also allows the small elastic pressure of the wires 
to help to clasp the rock gently between the points of the thermoelectric 
pincette without assistance from the velvet covers. After thus inserting a 
rock section in the apparatus, protecting the rock and cooler from below with 
a stout wooden screen, and from loss or gain of heat in other directions by 
a suitably thick case of woollen stuff and a few bandages of similar mate- 
rials, the rate of rise of temperature in the cooler, when agitated, was noted 
by the average number of seconds taken by a delicate thermometer con- 
tained in it to rise 1° F, (one graduation on its stem), as soon as this rate 
of rise was found to have become sensibly constant. About twenty minutes 
were usually occupied in the beginning of an experiment with watching for 
a steady condition of the thermometer-readings ; and ten or twelve minutes 
more were required to ensure it, and to obtain the average rate of their 
increase for the rock specimen under observation. The temperature-differ- 
ence shown by the galyanometer at the same time at first rose rapidly to a 
