Bodies that transmit radiant heat freely are called diathermanous; those that do not are called athermanous. These terms are to heat, what transparent and opaque are to light. Rock salt is the most diathermanous substance known. Heat that is radiated from a non-luminous source, as from a ball heated below redness, is called obscure heat; while part of that radiated from a luminous source, as from the sun or from a ball heated to redness, is called luminous heat. Heat from a luminous source is generally composed of both luminous and obscure rays.