Heat transfer is always directed from a higher to a lower temperature. Denser http://en.wikipedia.org/wiki/Density substances are usually better conductors; metals are excellent conductors. The law of heat conduction also know as Fourier's law states that the time rate of heat flow Q through a slab (or a portion of a perfectly insulated wire, as shown in the figure) is proportional http://en.wikipedia.org/wiki/Proportionalto the gradient http://en.wikipedia.org/wiki/Gradientof temperature difference: