Sometimes this absorption of gases takes place only at high temperatures. Thus silver in a state of fusion is able to absorb 22 times its volume of oxygen gas. When the metal is allowed to cool this gas is given off, and if the cooling takes place suddenly a crust is formed on the surface, and the phenomenon known as the 'spitting of silver' is exhibited. Sometimes during this operation miniature cones and lava-streams are formed on the surface of the cooling mass, which present a striking resemblance to those formed on a grand scale upon the surface of the globe. Similar phenomena are exhibited by several other metals and by the oxide of lead. The researches of Troost and others have shown that molten iron and steel possess the property of absorbing considerable quantities of oxygen, hydrogen, carbonic acid, and carbonic oxide, and that these gases are given off in the operation known as 'seething,' when either the pressure or the temperature is diminished. Hochstetter has shown that in the process of extracting sulphur from the residues obtained during the manufacture of soda, some very interesting phenomena are manifested. The molten sulphur is exposed to a temperature of 262° Fahrenheit, and a pressure of two or three atmospheres, in the presence of steam; under these circumstances it is found that the sulphur absorbs a considerable quantity of water, which is given off again with great violence from the mass as it undergoes solidification. The hardened crust which forms on the surface of the molten sulphur is agitated and fissured, miniature cones and lava-streams being formed upon it, which have a striking resemblance to the grander phenomena of the same kind exhibited upon the crust of the globe. The observations which we have described prove conclusively that many liquids and solids in a molten condition have the power of absorbing many times their volume of certain gases, and that this action is aided by heat and pressure. That the molten materials which issue from volcanic vents have absorbed enormous quantities of steam and other gases, we have the most undisputable evidence. The volume of such gases given off during volcanic outbursts, and while the lava-streams are flowing and consolidating, is enormous, and can only be accounted for by supposing that the masses of fluid rock have absorbed many times their volume of the gases. But we have another not less convincing proof of the same fact in the circumstance that volcanic materials which have consolidated under great pressure--such as granites, gabbros, porphyries, &c.--exhibit in their crystals innumerable cavities containing similar gases in a liquefied state.