Opacity of Tourmaline Crystals. 113 



a result which amounted to the discovery of a new pheno- 

 menon, " the Convection of Heat by Electricity" — namely, that 

 in a pyroelectric crystal whose temperature is rising, heat- 

 flows more easily with the electricity than it does against the 

 electricity. We were later indebted to Mr. G. F. Fitzgerald, 

 of Trinity College, Dublin, for the suggestion that an analo- 

 gous phenomenon might occur, namely that there might be a 

 unilateral electric convection whilst the electromotive force 

 producing the flow was rising or falling*. 



3. Whilst we were pursuing these investigations, and before 

 we abandoned our original hypothesis, the suggestion was 

 made to me by Dr. Lodge that if tourmaline possessed uni- 

 lateral conductivity along the axis, this might afford a pos- 

 sible explanation of the greater opacity observed in coloured 

 tourmalines for one set of rays. For if, as on our original 

 hypothesis, the elasticity in one sense along the axis were dif- 

 ferent from the elasticity in the opposite sense along the axis, 

 vibrations taking place along the axis would be stopped, and 

 the only rays transmitted would be those vibrating at right 

 angles to the axis. Unfortunately for this suggestion, the ray 

 whose vibrations are executed in a plane at right angles to the 

 axis is the ordinary ray, which is the one to which the tour- 

 maline is opaque ; whilst the ray which it transmits is the 

 extraordinary ray, which, being polarized in a plane at right 

 angles to the axis, is propagated by vibrations (according to 

 Fresnel and Stokes) executed in a principal plane of section. 

 I could not, therefore, agree with this suggestion as to the 

 cause of opacity in tourmaline crystals, though the suggestion 

 that the opacity was involved in the electrical properties of 

 the crystal appeared an extremely likely one. 



4. A theory of the opacity of tourmaline crystals, how- 

 ever, has recently occurred to me whilst considering the 

 general relations of electricity and light ; and I now beg to 

 offer the following explanation, based upon Clerk Maxwell's 

 electromagnetic theory of light. 



According to Maxwell's theory, light is an electromagnetic 

 disturbance propagated in the same medium which transmits 

 other electromagnetic actions, the periodic vibrations being 

 propagated in a wave through media possessing " electric elas- 



* We have not yet put this suggestion to experimental proof. In any 

 case the experiment would be difficult, for the reasons alleged in our 

 paper of 1878. Moreover the question would still be complicated by 

 phenomena of heating ; for, as we pointed out in 1878, the phenomena of 

 pyroelectricity must be reversible, and the application of an external elec- 

 tromotive force to a crystal must produce in it a thermal change just the 

 reverse of that which would itself give rise to the electromotive force in 

 question. 



Phil. Mag. S. 5. Vol. 12. No. 73. Aug. 1881. I 



