( 90) 



various metals ^). The agreeuient between theory and observation is 

 in most cases quite satisfactory, only here and there, as in the case 

 of bismuth ^), the difference is more considerable. From their meas- 

 urements for silver at 18°, the value 47X^0^ may be deduced in 



aT 

 C.G.S. units, for the expression — . (Compare Lorentz, loco cit. 



e 



p. 505); according to Drude's formula: 38 X 10^ 



^2. I hope, shortly, to furnish an experimental contribution 

 towards these theories by means of a series of determinations of an 

 analogous character, but more in particular with crystallised con- 

 ductors, and in the different directions of those crystal -phases. 



If we take the most common case in which may be traced three 

 mutual perpendicular, thermic and electric main directions in such 

 crystals, the propounded theories render it fairly probable for all 

 such conducting crystals that: 



— z= — r= — , and therefore also: ^- : Xy\ ),, = a^ •■ Oi, : Gg. 



Ox Gy Gz 



In conducting crystals, the directions of a greater electric con- 

 ductivity should, therefore, not only be those of a greater thermic 

 conductivity, but, theoretically, the quotient of the electric main- 

 conducti\'ities should be numerically e€[ual to that of the thermic 

 main-conductivities. 



Up to the present but little is known of such data. The best 

 investigated case is that of a slightly titaniferous Haemitate of 



M W. Jaeger und Diesselhorst, Bed. Silz. Ber. (1899). 719 etc. Gomp. Reinganum, 

 Ann. Phys. (1900) 2, 398. 



2) With Al, Cu, Ag, Ni, Zn, the value of - at 18' varies between 636x10** 



and 699X10«; with Gd, Pb, Sn, Pt, Pd between 706X10« and 754x10^; with 



Fe between 802 and 882 X 10^ therefore already more. With bismuth - at 



18° = 962 X 10*- Whilst in the case of the other metals mentioned the values of 



- at 100" and at 18^ are in the average proportion of 1,3:1, with humxith the 

 <l 



proportion is only 1.12. In their experiments, Jaeger and Diesselhorst employed 

 hltle rods, and bearing in mind the great tendency of bismuth to crystallise, their 

 results with this metal cannot be taken as quite decisive, as the values of the 

 electric and thermic conductivity power in the chief directions of crystallised bis- 

 muth dilfer very considerably. 



