( 93 ) 



and h and k the coeflicients of external and internal conductivity. 



§ 4. 1 have endeavoured to determine the (juotient of the chief 

 conductivities by the method proposed by W. Voigt. 



As is wellknown, this method is based on the measurement of 

 the angle, formed by the two isotherms at the line of demarcation 

 between two little plates which have been joined to an artificial 

 twin, when the heat current proceeds along the line of demarcation. 

 If X^ and ).^ are the two chief conductivities of a plate of bismuth 

 cut parallel to the crystallographic main axis, and if the angle which 

 tlie two main directions form with the line of demarcation equals 

 45°, then according to a former formulae: 



§ 5. The bismuth used was kindly funushed to me by Dr. F. L. 

 Perrot, to whom I again wish to express my hearty thanks. 



The prism investigated by me is the one which Dr. Perrot in 

 his publications'') indicates with M, and for which, according to 



Senarmont's method, he found for — the value 1,390. The prism 



given to Dr. van Everdingen yielded in the same manner for 



- the value 1,408. 



Two plates were cut parallel to the crystallographic axis, in two 

 directions forming an angle of 90^ and these were joined to twin 

 plates with <p = 45°. 



It soon appeared that in this case the Voigt method ^) was attended 

 by special difficulties which, as Prof. Voigt informed me, is generally 

 the case with metals. First of all, it is difficult to find a coherent 

 coating of elaidic acid -\- wax; generally the fused mixture on the 

 polished surface forms droplets instead of congealing to an even 

 layer. Secondly, the isotherms are generally curved and their form 

 presents all kinds of irregularities, which are most likely caused 

 by the great specific conducti\ity of the metals, in connection with 

 the peculiarity just mentioned. On the advice of Prof. Voigt I 

 first covered the metallic surface with a very thin coating of 

 varnish ; this dissolves in the fused acid, and causes in many cases 

 a better cohesion, but even this j>lau did not yield xevy good results. 



1) These Proceedings. (190G). March p. 797. 



2) p. 4, note 10. 



3) Voigt, Göltinger Nachr. (189Ü). Hcit 3, p. 1 — 16; ibid. (1897j. Heft 2. 1—5 



