( 794 ) 



The method oï Voigt is far more aoourale tlian that of de Senar- 

 mont') or even of Röntgen ^), and, requiring for other purposes to 

 investigate the relative conductibility of heat in crystals, it was 

 obvious that 1 should make use of the method indicated by Voigt. 



For a crystal, for which the rotatory coefficients, found in accord- 

 ance with the theory of G. C. Stokes '), are = 0, Voigt deducts 

 the relations required here by constructing the equations of the flow 

 of heat, conformable to the conditions of limit which are common 

 to the lateral boundaries of both plates; i.e. that along that line the 

 loss of temperature must be the same, and moreover that in a 

 direction normal to that boundary-line the entire flow of heat must 

 be the same in the two contiguous plates. 



Prof. LoKENTZ had the kindness to derive tlie above mentioned 

 relations in an analogous manner and to note down the conditions 

 under which the break in the isothermal lines will reach its maximum. 



If 6 be the break, and the angle, formed in the plates by the 

 two principal directions, is 45°, the proportion of tiie two coeflicients of 



the conduction of heat in those directions, consequently — is found 



as follows : 



If <p differs from 45°, Voigt finds iu that case: 

 (Aj — k^)si?i2<p 

 '■''^ ^ (X,^ )..;)-{?.,-?.,) cos 2<p' 

 which for (/ equal to 45° passes into the formula of Prof. Lorentz 

 by introducing tij — {= f[/i^ according to Voigt's deduction) instead 



of tye. 



Instead of the complicated formulae which are required for the 

 determination of these relations, we here give a simple geometrical 



demonstration, which, besides presenting — in a form which is iinme- 



diately available for logarithmic calculations, possesses at the same 

 time the advantage of beiug easily discernible. 



If, from a given point in the centrum of a crystal, a flow of heat 

 can take place without interruption in all directions, the isothermal 



1) DE SÉNARMONT, Gompt. rencf. 25, 459, 707. (1847). 



2) RöNTGEN, Posg. Ann. 151, 603, (1874). 



^) Stokes, Gambr. and Dublin Math. Journal. 6 215, (1851). 



