Opacity of Tourmaline Crystals. 129 



itself a possible explanation of electric absorption ; and if 

 electric absorption were possible in the medium, the variations 

 of vector-potential would no longer be propagated without loss 

 through the medium, any more than they can be through a 

 medium possessing true conductivity. And if, by reason of 

 structural differences in different directions (differences of 

 fibre, lamination, cleavage, and the like), the degree of electric 

 absorption were different in different directions, then such a 

 medium might possess also a directed opacity. But such an 

 opacity would be limited by the limitations of electric absorp- 

 tion in general, and would probably be quite distinguishable 

 from the opacity arising from conduction in the medium. 

 Were electric absorption possible, we could no longer omit 

 functions of ty, the (scalar) electric potential, from the general 

 equations of electromagnetic disturbances assumed in § 10 

 and in the succeeding development of the theory. 



27. The main argument for rejecting the supposition that 

 electric absorption can account for opacity is derived from the 

 consideration that such a medium, after having absorbed 

 charges of opposed signs at different surfaces of its heteroge- 

 neous structure, would, when left to itself, slowly recover; 

 and would give rise to phenomena of recovery, variations of 

 vector-potential taking place in the medium in inverse order 

 after the lapse of time. Whether this action may furnish a 

 possible explanation of the phenomena of phosphorescence 

 (Nachleuchtung), is a suggestion which may be worth future 

 consideration. Whether the optical properties of tourma- 

 line are to be regarded as dependent on its conductivity or on 

 any other physical property, the physical property on which 

 they depend is certainly a directed property; and its action is 

 such that the crystal possesses what Maxwell called " electric 

 elasticity" in a longitudinal and not in a transverse direction. 

 Now, as those bodies which possess "electric elasticity" are 

 observed for the most part to phosphoresce under the impact 

 of electrified molecules in the negative glow produced in very 

 attenuated vacua such as Crookes has obtained, whilst those 

 bodies which do not possess this property do not phosphoresce 

 under the same circumstances, I venture to predict that, if a 

 slice of tourmaline cut parallel to the axis be found to phos- 

 phoresce at all when exposed to the negative discharge, its 

 phosphorescence will be found to consist chiefly of rays 

 polarized in a plane at right angles to the axis — or, in other 

 words, that it will emit the same kind of rays as it transmits. 

 Whether this prediction be verified or not, however, does not 

 affect the validity of the theory of opacity of tourmaline crys- 

 tals here advanced. 



June 4, 1881. 

 Phil. Mag. B. 5. Vol. 12. No. 73. Aug. 1881. K 



