Intelligence and Miscellaneous Articles. 339 



or absence of rotational coefficients of thermal conductivity in 

 crystals may be ascertained in a simple manner. 



I wish here to point out briefly the principle of an analogous 

 method which I was led to try some time ago, and which, as I have 

 subsequently learned, is identical with the method proposed by 

 Boltzmann for the investigation of Hall's phenomenon. 



If a point sufficiently distant from the edge of a thin crystallized 

 plate be heated, the isothermal curves obtained by Senarmont's 

 method are ellipses, whatever be the values of the coefficients of 

 rotation. 



If instead of working with a continuous plate the plate is split 

 by a straight saw-cut in the direction of a radius from the centre 

 of heating, the isothermal curves will be scarcely modified, and will 

 shorten on each side of the slit if the coefficients of rotation are 

 null, but will exhibit a break in the same region if the coefficients 

 are not null. In this case the now from the centre tends to 

 follow a spiral line ; there should be an accumulation of heat on one 

 of the edges of the slit and a falling off on the other. 



The experiment may- be made still more simply. Instead of 

 sawing the plate it is sufficient to heat a point of the rectilinear 

 edge. If the coefficients of rotation are not zero the isothermal 

 line must undergo a spiral modification, and the distances at 

 which it cuts the edge of the plate on the right and left of the 

 point heated are not equal. 



This method is not very certain : it is better to cut the plate by 

 a saw-cut in two halves, which are then adjusted in a suitable 

 support in their original position, leaving a slight interval between 

 the two edges of the slit. By heating a point of this the spiral 

 deformation should be separately produced on each piece, and the 

 isothermal will present discontinuities in the opposite direction at 

 the two points where it meets the slit. 



I have tried these various methods on plates of gypsum ; in no 

 case have I observed a discontinuity indicating an appreciable 

 spiroidal deformation of the isotherms. I hope to pursue these 

 experiments. 



What we have said applies to tliin plates perpendicular to the 

 axis of rotation. It will be seen in like manner that on heating 

 by Jannettaz's method a point in a face cut in an unlimited crystal, 

 parallel to the axis of rotation, isotherms should be obtained which 

 are not symmetrical in reference to that diameter which is parallel 

 to this axis. The fact that this deformation has not hitherto been 

 noticed appears to prove that the coefficients of rotation are always 

 zero, or at any rate very small. — Bihliotheque Universelle, No. 4, 

 1893. 



