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XII. On the Thermal Conductivities of Crystals and, other Bad Conductors. 
By Charles H. Lees, M.Sc. ( Late Bishop Berkeley Fellow at the Owens College, 
Manchester.) 
Communicated by Professor A rthur Schuster, F.B.S. 
Received January 22,—Read February 4, 1892. 
Introduction. 
Our knowledge of the Thermal Conductivities of Crystals is derived mainly from the 
experiments of de Senarmont, von Lang, and Jannetaz/'' who, using the wax 
melting or analogous methods, have determined the ratios of what may be called the 
“ principal conductivities ” and the positions of the axes of conductivity within a 
number of crystals belonging to the simple systems. According to their experi¬ 
ments the isothermal surfaces about a heated point in a crystal are, in general, 
ellipsoids, having their axes parallel to the optical axes. In the case of a uniaxal 
crystal, this ellipsoid becomes a spheroid of revolution about the axis, and is, as a 
rule, oblate or prolate according as the wave-surface for the extraordinary ray is 
oblate or prolate. Although this rule has a number of exceptions, it is sufficiently 
general to render it probable that there may be some relation between the passage of 
light and of heat through a crystal. The recent determinations of the refractive 
indices of metals by Kundt have shown that they stand in the same order as con¬ 
ductors of heat, and as to the velocity of propagation of light through them, and this 
fact brings again into prominence the old determinations with respect to crystals. That 
the comparison which Kundt has made for the metals cannot be carried to other 
bodies is at once seen from the fact that the index of refraction of iron differs little 
from those of glass and several commoner crystals, the conductivities of which are 
shown to be very small compared to that of iron. A comparison may, however, be 
possible among transparent bodies themselves, and the following experiments were 
made with the object of furnishing data for this comparison, the results given by 
previous observers differing greatly from each other. They have, however, been 
extended to embrace non-transparent bodies commonly in use in a physical laboratory, 
and about the conductivity of which we have had a very meagre or absolutely no 
knowledge. 
* Senarmont, ‘ Ann. de China, et de Phys.,’ (3), vols. 21, 22, and 23 (1848) ; von Lang, ‘ Pogg. Ann.,’ 
vol. 135 (1868) ; Jannetaz, ‘Ann. de China, et de Pliys.,’ (4), vol. 29 (1873), 
MDCCCXCIL—A. 3 Q 
27.8.92 
