ON THE THERMAL CONDUCTIVITIES OF CERTAIN ROCKS. 



61 



Another subject of important applications in questions relating to under- 

 ground temperature ■which has engaged the attention of the Committee, 

 is the different degrees of facility -with which some rocks conduct heat in 

 different directions, recent researches by M. E, Jannettaz haviiig shown 

 that this property is possessed not only (as was long since shown by De 

 Sdnarmont) by certain crystals, but also by other mineral substances, and in a 

 very high degree by rocks having schistose and laminated structures. To verify 

 this important fact, specimens of "Welsh slate were prepared by cutting 

 plates from the same piece across and parallel to the plane of cleavage ; and 

 the rate of conduction of heat through the plates cut in the former manner 

 was found to be nearly twice as rapid as that through plates cut parallel to 

 the cleavage-plane. Thus the resistance of slate (shown in the Table) to 

 transmission of heat along the cleavage-planes is only half as great as that 

 offered to its passage across them. The example of this slate is not an 

 exceptional one among laminated or schistose rocks ; and the extreme ratios 

 of the resistances in the parallel and transverse directions to the planes of 

 cleavage or foliation, which have been studied and measured by M. Jan- 

 nettaz * in a variety of cases, exhibit some much more remarkable pro- 

 portions. 



The method of experiment adopted by M. Jannettaz is that originally 

 employed by De Senarmont, of coating thin sections of crystals with grease or 

 wax, and observing the ratio of the diameters of the oval figures formed on 

 their surfaces by the melted grease round a point which is heated at the 

 centre of the plate. In a series of earlier experiments f on thin plates of 

 crystals, M. Jannettaz had determined the ratios of the diameters for forty 

 or fifty mineralogical species ; and the values of these ratios are included 



+ 



* Bulletin de la Soci^t^ G6ologique de France, tome ii. p. 264 (April-June 1874). 



t Annales de Chimie et de Physique, s6rie 4, tome xxix. (1873). 



J The cases of ratios excepted from the above limitation are those of the following 

 metals or minerals having either two or three priyicipal axes of conduction : — Quartz, 

 1-312:1; amphibole hornblende and tremolite, 1-42 and 1-67:1; selenite, 1-54 : 1 ; 

 metallic antimony (rhombohedral), 1-59 : 1 ; antimonite, 1836 : 1-451 : 1 ; mica, 2-5 or 

 2-4: 2-4 or 2-3: 1. The extreme ratio of good to bad conductivity in mica is 5-76 or 

 6-25 : 1 ; and, as in selenite and in some other crystals possessing very distinct cleavages, 

 the direction of good conduction is along and that of bad conductiou is across the cleavage- 

 plaues. 



