actually... silver is a much better thermal conductor than almost anything (429 W/mK)... water is only 0.6, aluminium is 200 and copper is 380...
what zepfan was saying silver heats very rapidly (takes less energy to increase the atomic kinetic energy), while aluminium heats slowest of any metal... when building a rad, you want the material that will heat as slow as possible, because once the surface gets to a certain heat (based on the air temperature) its starts to lack heat dissipating ability, so if this energy can be stored in the metal, the overall performance increases
high conductivity, high heating metals (Cu, Ag, etc) are used to conduct heat away from components (CPU, GPU, etc)