26 REPORT— 187G. 



aspect. Some crystalline rocks and minerals of simple composition (and of 

 the cubic system of crystallization) were selected, and, like quartz, they 

 proved to have heat-conducting properties in a high degree. The thermal con- 

 ductivity of iron-pyrites, resembling apparently that of the metals, could not, 

 from its high value, be accurately determined by the method of experiment 

 pursued ; and it is accordingly omitted (as undetermined) from the list. The 

 diathermancy of rock-salt for the heat radiated and absorbed by the oiled 

 surfaces between which the tiial plate of it was placed will not account for 

 more than Jjj part of the heat which the plate actually transmitted ; and 

 the high position of this substance in the list is consequently due to a really 

 high conductivity which rock-salt possesses (about 113), greater than that 

 of quartz (85-88), and even of fluorspar (92|), the substance found to rank 

 nest to it in high conducting-power. A specimen of galena nearly pure, but 

 enclosing a few fragments of quartz, presented the highest thermal conduc- 

 tivity (TO^) next to that of quartz. A plate of soft, white, opaque calcite, 

 perfectly but irregularly crystallized (forming vein-stuff in Clifton sandstone), 

 agrees exactly in its thermal conductivity (-iG'T) with various kinds of marble 

 (46-49) which were tried last year. On the other hand, a similar specimen 

 of heavy spar (barium sulphate) from a mine of that substance npar Exeter 

 presents, in spite of its great density, a remarkably low thermal conductivity 

 (17-18 in two experiments), not very far removed from that of English 

 alabaster (gypsum, or calcium sulphate, 23-4). English plate-glass (20-4), it 

 may also be remarked, has a low thermal conductivity, differing not very 

 greatly from those of the two substances last named. Finally, the lightest 

 species of rocks examined in the course of these experiments, pumicestone 

 and Newcastle house-coal, have also the lowest conductivities (5-5 and 5-7) 

 hitherto presenting themselves in these investigations. 



As, with the exception of rock-salt, clay-slate and elvan, house-coal and 

 pumicestone, no new thermal resistances of great importance, in a geological 

 point of view, are added in the present list to those already exhibited in the 

 diagram of these Reports (vol. for 1875, p. 59), a new graphic representation 

 of the resistances now found is here deemed unnecessary — the values of the 

 absolute resistances furnished in this Table enabling them to be added without 

 'difficulty in that diagram, where they may thus be exhibited in the same 

 normal scale with the earlier determinations. 



The applications to questions of underground temperatures which these 

 observations suggest have not yet engaged the Committee's attention suffi- 

 ciently to enable them to arrive at definite conclusions certain enough to 

 entitle them to be noticed in this Eeport. Examples of very reliable mea- 

 surements of imderground temperatures, such as have recently been obtained 

 in the tunnels of Mont Cenis and of St. Gothard, and in the deep vertical 

 boring at Sperenberg, near Berlin (the last of which, although extremely 

 deep, passes almost entirely through rock-salt), are ill-adapted to test distinctly 

 the relative values of the thermal conductivities of different species of rocks — 

 the former two from the irregular surface-configurations, and the last from 

 the absence of any change of the strata through which these borings pass. 

 In view also of the many disturbing conditions that affect both the local rate 

 of change and the actual observations and measurements of underground 

 tcmperatui'es in other borings more suitably adapted to exhibit clearly the 

 differences of thermal resistance in geological formations, which the Com- 

 mittee is endeavoiuing to distinguish and to recognize in actual cases, it 

 would be premature, in the present stage of the investigation, to deal more 

 particularly with results derived immediately from these and from similar 

 comparisons, the degree of dependence to 1 e placed on which cannot very 



