In an isotropic homogeneous material the conduction of heat depends upon a single “constant” of the material, known as the thermal conductivity; this “constant” is 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 the fundamental relation, $d Q = − K d T d n d S$ where K is the conductivity and dT/dn the gradient of the temperature in the direction of the normal to the surface element dS. Two “ c.g.s.” units of heat are in common use; the calorie (gram-calorie) and the joule; 1 calorie equals 4.18₅ joules. Using the centimeter, second, and Centigrade temperature scale, the corresponding units for thermal conductivity are the cal.·sec.⁻¹cm.⁻¹deg.⁻¹ and the watt.·cm.⁻¹deg.⁻¹ (1 watt equals 1 joule/sec.).