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In an isotropic, homogeneous material the conduction of heat depends on a single "constant" of the material, the thermal conductivity; this is actually a function of temperature, pressure, and other variables. The quantity of heat, dQ, conducted in unit time across an element of surface dS is given by  
d Q = K d T d n d S ,
where K is the conductivity, and dT/dn the thermal gradient in the direction of the normal to dS. The flow of heat in crystals other than those of the cubic system depends on either two or three principal conductivities; the theory of conduction in crystals is given in Kristallphysik by W. Voigt and also Crystal physics by W. A. Wooster.

Two cgs units of heat are in common use, and thermal conductivity is commonly given in either of two units. Units of cal/cm sec ° C are used here; to convert to watts/cm ° C, the values given in the tables should be multiplied by 4.184. A unit sometimes encountered in engineering work is the British thermal unit per square foot per hour for a temperature gradient of 1 °F per inch; this unit equals 3.446 × 10−4 cal/cm sec ° C.

Rocks are not homogeneous, and their constituent crystals predominantly are of low symmetry. The fundamental relationship given above is not valid for infinitesimal elements in this case. If no large-scale departures from isotropy exist, however, a single conductivity, determined from a sufficiently large sample, suffices to give the relationship between average heat flow and average thermal gradient. Such

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