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extent. In this position it will be interesting to regard con- 
ductivity in regard to crystalline substances; for in these the 
unequal condition of heat has been well shown by De Sénarmont, 
Rontgen and other scientists—the experiment is simple enough. 
A thin layer of wax having been spread over a plate of the 
crystal, afterwards heat is applied toa single point. The wax 
then melts in a circular or elliptic area, according as the rate 
of conduction is uniform or not. The law deduced from this 
discovery of De Sénarmont would be for a heated crystal of 
quartz thus stated. If the crystal be of the 3rd system 
(hexagonal), then the isothermal surfaces of such a crystal 
heated internally at a point, would be spheroids—the axis being 
in the direction of the axis of symmetry. Hence it follows 
that crystals of quartz conduct heat with equal facility in all 
directions perpendicular to the axis, but with different facility 
parallel to the axis. The law of conduction deduced for the 
felspars Orthoclase and Albite, would be that for the crystallo- 
graphic system characterised by having three unequal axes— 
the isotherms are ellipsoids with 3 unequal axes; so that 
Orthoclase and Albite conduct heat differently in all three 
perpendicular directions. 
For the conductivity of other solids, a committee consisting 
of Professors Herschel: and Lebour and Mr J. F. Dunn, 
appointed by the British Association to determine the thermal 
conductivities of certain rocks, have obtained results from which 
the following selection may be found useful for comparison :— 
Substance Conductivity _k_ | ie eon me pe: 
in C.G.S. Units c ity of unit volume 
Granites, various, about ... 00510 to -0092 0100 to 0120 
Marblesand Limestones 00470 0085 to -0095 
Red Serpentine (Cornwall) ‘00441 to -00560 0065 
Caenstone (Building) ... °00433 0089 
Fire-brick ... ees ... 00174 0053 
Red brick (fine) ... ... (00147 0044 
Quartz and Quartzites ... ‘0080 to ‘0092 ‘0175 to :0190 
(Units and physical constants by Professor Everett, F.R.S., &c., ed. 2, 
p. 111.) 
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