126. Prof. 8. P. Thompson on tli 



\e 



of the crystal with tinfoil through which a truly circular hole 

 had been punched. The foil was fixed over the crystal with 

 gum or varnish, and was connected to earth. At the centre 

 of the exposed circle a metallic point was fixed in an insula- 

 ting support; and by this means the electricity stored in a 

 Leyden jar could be made to flow across the circular space 

 from the centre to the circumference. With powerful dis- 

 charges from a Leyden battery sparks leapt along the crystal 

 face, leaving a permanent trace along the path of least resist- 

 ance. Later experiments were conducted in a partial vacuum, 

 when the discharge took place quietly in luminous streaks of 

 a pale-violet colour along that diameter of the circle which 

 corresponded to best conductivity along the surface. 



The crystals operated upon by de Senarmont were three 

 black tourmalines from Greenland, in the collection of the 

 Ecole des Mines, having plane mirror-like faces tangential to 

 the lateral edges of the primitive rhombohedron and slightly 

 striated (" stries a peine sensibles "). With circles of three 

 different sizes, de Senarmont found the maximum conduc- 

 tivity to lie in a direction parallel to the axis. 



22. Wiedemann's process consisted in producing an electric 

 " figure "" upon the surface of the crystal, which was dusted 

 over with powdered sulphur, or lycopodium, or red-lead, and 

 then exposed to the discharge of electricity from a finely- 

 pointed conductor close to the surface. The powder was 

 repelled from the point, and heaped itself all round a clear 

 space. In the case of equal conductivities this space was a 

 circle ; in the case of unequal conductivities an ellipse whose 

 major axis indicated the greater conductivity. Wiedemann 

 only examined one specimen of tourmaline. He states that 

 "on the faintly striated prismatic faces of the crystal em- 

 ployed, the larger diameter of the electric figure shows itself 

 parallel to the principal axis." He adds, that in the case of 

 other optically negative crystals (including calc-spar) the elec- 

 tric conductivity is greater along than across the axis. (He 

 notes ratile as an exception.) But he appends, as a general 

 conclusion, that the electricity is propagated in crystals more 

 rapidly in that direction in which the propagation of light is 

 the more rapid ; which dictum exactly contradicts his own 

 observations on calc-spar and on tourmaline, in which the 

 velocity of the extraordinary ray is greater than that of the 

 ordinary ray 



23. Now, if we are to accept these observations on the super- 

 ficial conduction of natural tourmaline crystals with striated 

 faces as establishing the electric conductivity of tourma- 

 line to be a maximum along the axis and a minimum across 



