For an example we may turn to Water, which is a solid in the condition of ice; a liquid in the condition of water; and steam in the condition of vapor. Quicksilver is a metal which is in a liquid condition in our ordinary temperature, but which becomes a solid when subjected to a very low degree of temperature, and may be transformed into a gas, under a high degree of heat. Air is a vapor in our ordinary temperature, but has been transformed into "liquid air" under tremendous pressure, which produced a very low degree of temperature, and, theoretically, it may be transformed into a solid under a sufficiently low degree of temperature, although so far, Science has not been able to produce a degree of cold sufficient to "freeze" the liquid air. It is all a matter of "freeze," "melt," and "evaporate," in all forms of Substance--and any substance, at least theoretically, is capable of being subjected to any of the three conditions just named, and being manifested in the respective conditions, of Solid, Liquid, and Aeriform. This may actually be accomplished with the majority of substances at this time, although in some instances we are not able to produce a sufficiently high temperature to "melt and evaporate" certain solid substances, on the one hand, or a sufficiently low degree of temperature to "liquify" or "freeze solid" certain vapors. But the intense heat of the centre of the earth is able to melt rocks, and show them as liquid lava flowing from volcanoes, and Science teaches that the solid Substance of the Earth, and other planets, suns, etc., existed in the shape of a vapor at one time, and would again take on that condition in case of a collision with another great body, which convert motion into intense heat that would first melt, and then vaporize every solid particle of which the earth is composed. If the sun's heat were completely to die out, the cold would be so intense that the air around the earth, and all the gases and vapors, would be frozen to solids. In physics the term "gas" is generally applied to a substance that is aeriform in our ordinary temperature, but which may be liquefied in a low temperature; the term "vapor" being generally applied to the aeriform condition of substances that are solid or liquid in our ordinary temperatures, but which may be "evaporated" by heat, and thus transformed into an aeriform condition, resuming their original form upon cooling. These terms, however, are technical, and practically there is no difference between a gas and a vapor.