Opacity of Tourmaline Crystals. 127 



it, we shall have to admit that, of all substances examined, 

 tourmaline forms the solitary exception to the rule that the 

 thermal conductivity and the electric conductivity of sub- 

 stances go together pari passu*. There is no uncertainty 

 as to the thermal conductivity of tourmaline, its transverse 

 conductivity being about twice as great (see § 8 ante) as its 

 longitudinal conductivity. 



24. Both observers stated that the faces of the crystals were 

 striated. De Senarmont appeared, indeed, to think that natural 

 striation interfered less with the regularity of conduction than 

 the invisible striations of an artificially polished facej. I am 

 inclined to think from my own observations that, on the con- 

 trary, the natural striations of the crystal greatly affect the 

 conditions of surface-conductivity. I have repeated both 

 methods of experiment upon tourmalines with natural and 

 with artificially-cut faces. In the former case, the crystals 

 tried were black striated crystals from Norway. They showed, 

 as did the crystals used by de Senarmont and by Wiedemann, 

 a longitudinal conductivity higher than the transverse con- 

 ductivity. I cleaned one of these crystals, first with hydro- 

 chloric acid, and then by rubbing its surface in boiling paraffin. 

 When cold it was carefully cleaned with a piece of dry leather, 

 and the experiments were repeated. De Senarmont's method 

 now showed a transverse conductivity, while Wiedemann's 

 method gave an indecisive result. 



25. I therefore tried another method, as follows : — A small 

 circular drop of pure olive-oil was deposited upon the middle 

 of the face of the crystal. A pointed needle held in a non- 

 conducting support was fixed centrally above it, and then the 

 knob of a charged Leyden jar was cautiously approached. 

 The oil-drop spread out somewhat irregularly across the sur- 

 face; and at its centre appeared a clear space from which the 

 oil was repelled. This clear space was distinctly elliptical, 

 the major axis of the ellipse being transversely placed across 

 the axis of the crystal. The natural striations, however, ren- 

 dered the form irregular. When the artificially-cut green 

 tourmaline ("C") mentioned in § 19 was employed, the ellip- 

 tical space in the centre of the oil-drop was much more satis- 



* See Tait, Trans. Roy. Soc. Edinb. 1878, where, however, it appears 

 that German silver, while superior in thermal conductivity to lead, is 

 inferior to that metal in electric conductivity, being the single known ex- 

 ception to Forbes's rule. 



t " Quant aux stries et aux asperites naturelles, elles ne semblent pas 

 avoir une influence appreciable tant qu'elles ne sont pas extremeinent pro- 

 noncees. J'ai done autant que possible, pour etre siir des resultats, 

 cherche a operer sur des faces naturelles ou obtenus par le clivage."' Loc. 

 cit. p. 264. 



