ON THE THERMAL CONDUCTIVITIES Or CEKTAIN ROCKS. 



57 



presence of the thin layer of moist luting of linseed-meal (mixed with between 

 three and four times its weight of water) in the new series of experiments 

 makes it probable, on the other hand, that the absolute conductivities pre- 

 sented in the following list are under rather than above, and may occasionally 

 be five, ten, or, in the better-conducting rock-specimens, possibly even twenty 

 per cent, below their real values ; but the present arrangement of the series 

 in a progressive scale of the observed values of the conductivities may bo 

 more certainly regarded as in the main correct. 



Some experiments to compare the resistances of different rock-sections 

 with that of the luting and india-rubber faces between which they were 

 placed, with a view of obtaining relative conducting-powers more expe- 

 ditiously without the use of a thermopile, were also made ; but although this 



Absolute Conductivities and Sesistances of Roclc- Sections. 



Section of rock. 



Absolute conductivities in 

 C.G.S. units. 



Thermopile 



with local 



currents. 



"Rocks between 



velvet faces. 



(1874.) 



Opaque white quartz 



Slate (cut acrcBS the cleavage. 

 Specimen A, Fcstiuiog). 



Calcite (white vein-stuff in 

 mountain-limestone). 



Kenton sandstone (thoroughly 

 wet). 



Do. (dry) 



Grey Aberdeen granite 



Irish fossil marble 



Devonshire red do 



Sicilian white do 



Eccl Cornish serpentine 



Whinst one 



Slate (cut parallel to the cleav- 

 age. Specimen A). 



Do. (do. Specimen A, Fes- 

 tiniog). 



Call on Hill trap-rock (from foot 

 of the Observatory garden, 

 Edinbiu-gh). 



Red brick (thoroughly wet) . . . 



English alabaster 



Plaster of Paris (plate, tho- 

 roughly wet). 



Eed brick (dry) 



Plaster of Paris (plate, dry) . . 



Cannel-coal 



0C04S9 



o-coc.oo 



O-COo.59 

 0-0052.5 

 0005C9§ 

 0-00483 

 000312 § 

 '00392 



0-00620 



0-00412 



000163 

 0-00161 § 



Compensated 



Thermopile, 



moistly luted 



to the rocks 



with pressure. 



(1875.) 



0-008821 I 



000876/ 



OOOGCO 



000596 



0-00594 



000549 

 0-00514 

 0-00488 

 0-00469 

 0-00462 

 0-00399 

 000366 

 000363 



0-00325 



0-00332 



0-00247 

 0-00234 

 0-00160 



000147 

 0-00120 

 0-00065 



Absolute 

 resistances 

 (or 1 -^ con- 

 ductivity). 

 (1875.) 



114 



152 



les 



168 



182 

 195 

 205 

 213 

 216 

 251 

 273 

 275 



308 



301 



405 

 427 

 026 



680 



833 



1538 



Resistances in 

 an ascending 

 symbohc scale. 



(1874*.) 



(B) 



A 



15 



B 



B 







D 



D 



(ro 



(C) 



K 

 K 



(1875+.) 



A 

 A 



A 



A 

 A 

 D 

 B 

 B 

 B 

 B 

 B 



C 



C 



c 

 c 



D 



D 

 E 

 K 



* See Report, 1874, p. 132. t See Diagram, page 59. 



\ Average of two equally good determinations 000879. 



^ The conductivities marked thus were obtained by solid junctions of the thermopile 

 to the rock-faces with plaster of Paris; the remaining numbers of the column were de- 

 rived (in a constant proportion obtained from these) from conductivities observed with 

 the thermopile pressed against the rock-surfaces by velvet faces. 



