Opacity of Tourmaline Crystals. 115 



ordinary ray is transmitted, though with some loss by absorp- 

 tion. The transmitted ray is polarized in a plane at right 

 angles to the axis of the crystal ; and its vibrations are there- 

 fore executed (if we assume with Fresnel and Stokes that 

 the vibrations are at right angles to the plane of polarization) 

 in a direction parallel with that of the axis of the crystal. 



We may remark parenthetically, that the almost complete 

 opacity of coloured tourmalines to rays travelling along the 

 axis affords a strong confirmation of the views of Fresnel and 

 Stokes. For let this axis be the #-axis of a system of rectan- 

 gular coordinates, the transverse vibrations to a ray travelling 

 along x must be in planes parallel to the plane (yz) ; and they 

 are suppressed by absorption whether parallel to y or z. Now 

 for a ray travelling along y the vibrations are in planes parallel 

 to the plane fe), and the ^-component is suppressed by absorp- 

 tion, the ^-component only being transmitted; while for a ray 

 travelling along the axis of z the vibrations are in planes 

 parallel to (yv), and, the ?/-component being suppressed by 

 absorption, the ^'-component is alone transmitted. 



6. Electrical Properties. — With the characteristic pyroelec- 

 tric properties of tourmaline we have not much concern here; 

 for the tourmalines which are most highly pyro electric appear 

 to be least opaque. 



The electric conductivity of tourmaline is in general very 

 small. The resistance of a centimetre cube of the crystal 

 at ordinary temperatures is certainly many thousand ohms. 

 Gaugain states that the resistance decreases rapidly if the tem- 

 perature is raised to 400° or 500°, and he has observed* some 

 crystals, on being cooled from this temperature, to retain a 

 high surface-conductivity, which he attributed to their having 

 become hygroscopic — an explanation which seems somewhat 

 doubtful, since he states that washing them in ordinary water 

 rendered them non-conducting as before. Becquerel sought 

 to explain the disappearance of pyroelectric phenomena at 

 temperatures above 150° C. by supposing an increase of con- 

 ductivity to be assumed at that temperature. In the researches 

 of Dr. Lodge and myself in 1878-9 we found no such great 

 decrease in resistance, w T hich at 300° C. was still enormous. 

 We were not able to compare the conductivity in a direction 

 at right angles to the axis with that along the axis. 



The only experiment directed toward the comparison of the 

 electric conductivities in different directions with respect to 

 the axis of the crystal, are those of de Senarmontt and of G. 



* Annates de Chimie et de Physique, 3 e s£rie, t. yii. p. S. 

 t Ibid. t. xxviii. (1850), p. 257. 

 12 



