476 Intelligence and Miscellaneous Articles. 



externally to a high temperature (Salet, Traite sur Spectro- 

 scopie, p. 174) *. For if the continuous light is watched at 

 the highest temperature attainable with this apparatus, it is 

 seen not to fade equall) r in all parts of the spectrum, — a stage 

 is reached (see fig. 3, b) when, in addition to the D emission- 

 line, a green line appears in the place of a conspicuous absorp- 

 tion-line, and a faint glow remains in the blue corresponding 

 to the absorption-bands in that region (figs. 3-4, b). 



XLVII. Intelligence and Miscellaneous Articles. 



ON THE ROTATIONAL COEFFICIENTS OF THERMAL CONDUCTIVITY 

 IN CRYSTALS. BY M. CHARLES SORET. 



The method used has been pointed out in a previous uote 

 {Archives de Geneve, 1893, xxix. p. 355). 



Heating by the method of Jannettaz a point on a face not 

 perpendicular to the axis of rotation in a crystal in which the 

 coefficients in question are not zero, isotherms should be obtained 

 which are not symmetrical in reference to that diameter which 

 coincides with the projection of the axis of rotation. 



The crystals were mounted and centred at the end of a vertical 

 axis provided with screw motions for setting the face horizontal. 

 A small platinum sphere heated by an electrical current is applied 

 against this face at a point A on the prolongation of the vertical axis. 



Isotherms were obtained in the usual way, taking care to turn 

 the crystal steadily and regularly during the heating so that this 

 was symmetrical about the point A. This point, the intersection 

 of the face investigated and the axis of rotation, was thus without 

 any possible error in the centre of heating. 



After obtaining the isotherm it was examined, without touching 

 the adjustment, by means of an eyepiece with a micrometric scale. 

 The crystal being first placed with its rotational axis in the plane 

 of incidence, a rotation of 180° about A should modify the points 

 of intersection of the micrometer and the isotherm, if this was 

 symmetrical as regards the centre of heating. 



~No\v no appreciable disymmetry could be observed any more 

 than by the other methods previously employed. Small variations, 

 evidently accidental, alter the direction from one observation to 

 another on the same crystal, amounting to -^ to Jg- of the diameter 

 of the curves. Irregularities of the same order were observed on 

 isotropic plates and on faces of crystals, in which no constant 

 deformation of the isotherms could be foreseen. 



The researches were made on crystals of dolomite of Binn (face 

 of the prism), and of Traverselle (face of the rhombohedron) : on 

 crystals of erythrite and on apatite of the Pfrtschthal (face of the 

 prism). The existence of coefficients of rotation in the crystals 

 appears therefore more and more improbable. — From the Archives 

 de Geneve, communicated by the Author. 



* If iodine is heated in a hard glass tube in a furnace until the glass 

 begins to fuse, the colour of the glowing vapour changes from yellow to 

 pale greenish white. This probably indicates the change to a discontinuous 

 emission. 



