Occlusion is the absorption of gases by metals, the phenomenon being specially remarkable in the case of platinum and palladium. A piece of red-hot platinum wire will dissolve about four times its volume of hydrogen, while warm palladium will hold as much as 643 volumes of that gas. If an electric current be sent through some slightly acidified water, and palladium used as the terminal at which the hydrogen would be evolved, the metal will occlude that gas until it is quite saturated with it, and will also swell in the process. Nickel under similar conditions will also absorb hydrogen. Meteoric iron is often found to contain a considerable volume of mixed gases, among which hydrogen is always found, it being often accompanied by nitrogen and carbon-monoxide.