(95 ) 



position of the isotherms it also follows that P.r > h so that — ^1,35. 



In a quartz-plate obtaine<l from Prof. Voigt I found s = 30^°, 



therefore — z=l,/5. In a plate of Antimonite from Ski kok u in Japan 



'■a 



cut parallel to the plane |010}, — was found to be even much larger 



than 1,74, which value is deduced from the experiments of Senar.mont 



and Jannettaz as they find for the quotient of the demi ellipsoidal 



axes 1.32. 



For Apatite they find similarly 1,08, for quartz 1,73, whilst 



TucHSCHMiDT determined the heat-conductivity of the latter mineral 



according to Weber's method in absolute degree. His experiments 



h 

 give the value 1,646 tor the quotient — . 



The deviations are always such that if P-, ^-i*., the values of the 



quotient — turn out to be larger when Voigt's method is employed 



instead that of de Senarmoxt. The method employed here is, however, 

 so sound in principle, and is so much less liable to experimental 

 errors, that it certainly deserves the preference over the other processes. 

 Finally, a sample of Haematite from Elba was examined as to 

 its conducting power. A plate cut parallel to the c-axis was found 

 not to be homogeneous and to contain gas-bubbles. I repeatedly 

 measured the angles £ of a beautifully polished preparation of Prof. 

 Voigt, and found fairly constantly 10è°, whilst the position of the 

 isotherms showed that ).a was again larger than ).c. 



For the Haematite we thus obtain the value: — = 1,202. The 



;.. 



\alue found by Backstköm and ANCiSTUoM for their mineral with the 

 aid of Christiansen's method was 1,12. In this case the deviation 

 also occurs in the above sense. 



From the experiments communicated we find for the quotient 



y-n : X, in both crvstal phases, if bv this is meant ( ) : ( - | the 

 values : 



Wwh Bismuth: - = 1,128. 



With Hunnatite: - = 1,480. 

 y-c 



In this mv measurements of — are combined with the best value 



