Occlusion

Occlusion, a term applied to the solution of a gas by a melted solid—as of oxygen by melted silver—which gas is given up by the melted material when it solidifies, so that in the case of silver the metal sometimes 'spits' or gives off the gas in bubbles, thereby roughening its otherwise smooth surface. Sometimes the gas is absorbed or 'occluded' (in a wider sense) even though the metal be not fused—e.g. hydrogen gas by cold palladium, carbonic oxide by red-hot cast-iron.