1805.] I 'atiation of Thermal Conductivity of Rocks. 163 



peratures at these points ; and &(T, M), fc(M, B), the mean conduc- 

 tivities between T and M, and between M and B respectively. 



3. The rock experimented on in each case consisted of two equal 

 and similar rectangular pieces, pressed with similar faces together. 

 In one of these faces three straight parallel grooves are cut, just 

 deep enough to allow the thermoelectric wires and junctions to be 

 embedded in them, and no wider than to admit the wires and junctions 

 (see diagram, 8 below). 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 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 by 

 a lamp below it. Its upper side was flooded over with mercury in 

 our later experiments ( 6, 7, 8), as in Hopkins' 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 for rocks 

 or metals, or other solids, it will, no doubt, be advisable to use an 

 automatically 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 the 

 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 cm. by 12'5, 

 and 5 cm. thick. One of these parts cracked with a loud noise in 

 an early experiment, with the lower face of the composite square 

 resting on an iron plate heated by a powerful gas burner, and the 

 * upper face kept cool by ice in a metal vessel resting upon it. The 

 experiment indicated, very decidedly, less conductivity in the hotter 

 part below the middle than in the cooler part above the middle of 

 the composite square slab. We supposed this might possibly be due 

 to the crack, which we found to be horizontal and below the middle, 



