116 Prof. S. P. Thompson on the 



Wiedemann*, both of whom examined the conducting-power 

 of the surfaces of crystals. Their methods and results are 

 examined in detail in the concluding part of this paper. They 

 each found the conductivity of tourmaline to be greater along 

 the axis than across the axis : I shall have occasion for show- 

 ing that this conclusion is fallacious. 



7. Magnetic Properties. — Plucker examined the magnetic 

 deportment of tourmaline. The following account of what was 

 observed is given by Tyndallf : — " A plate of the crystal which 

 had been prepared for the purposes of polarization, 12 milli- 

 metres long, 9 wide, and 3 thick, was suspended by a silk 

 fibre between the poles of an electromagnet. On sending a 

 current round the latter, the plate, which was magnetic, set 

 itself as an ordinary magnetic substance would do, with its 

 longest dimension from pole to pole. The optic axis of the 



crystal, thus suspended, was vertical On hanging the 



crystal, however, with its optic axis horizontal, when the mag- 

 net was excited, the plate stood no longer as a magnetic sub- 

 stance, but as a diamagnetic ; its longest dimension being at 

 right angles to the line joining the poles. The optic axis of 

 the crystal was found to coincide with its length; and the 

 peculiar deportment was considered as a proof that the optic 

 axis was repelled." From all which the logical inference is, 

 that the coefficient of magnetic induction is less along the axis 

 than in a direction at right angles to it. It is curious that 

 Professor Tyndall states % just the opposite to this — that the 

 maximum magnetic induction is, in coloured crystals at all 

 events, in a direction parallel to the axis. 



8. Thermal Conductivity. — De Senarmont examined the 

 thermal conductivity of tourmaline by his well-known method. 

 The conductivity is greater across the axis than along it, the 

 ratio between the major and minor axes of the isothermal 

 curves being (according to my own observations) about '6:2; 

 and these being proportional to the square roots of the con- 

 ductivities, we have 



3 : 2 = v /Z- 2 : V%, 

 or 



conductivity atony axis ; conductivity across axis =4:9. 



The unilateral conductivity observed in my experiments during 

 rise or fall of temperature need not be further noticed here. 



9. Radiating -power. — Kirchhoff§ and Stewart || have shown 



* Ann. de Chim. et de Phys. t. xxix. (1850), p. 229 ; and Pogg. Ann. 

 lxxvi. p. 404 and lxxvii. p. 534. 



t 'Diamagnetism/ p. 2. \ Op. cit. p. 71. 



§ Pogg, Ann, cix. I860, p. 275. || Proc. Roy. Soc. x. p. 503. 



