Now at temperatures like that everything we know on the surface of the earth would melt. Something else would happen to it besides that. Those of our readers who have ever seen experiments at the Royal Institution in London by Sir James Dewar or Sir William Crookes will know that if metals are made hot enough they will not only melt but will boil away into vapour as water boils into steam. And perhaps we need tell no one that air, if it be subjected to a low enough temperature, can be made a solid like ice. In fact, everything in nature, whether we generally know it as a solid (like iron), or a liquid (like water), or a gas (like air), can be made to assume either of the two other forms. Thus the solid iron can be turned into a liquid or a gas, and the liquid water can be turned into a gas by boiling, or into ice by freezing. The gaseous air can be turned into a liquid by lowering its temperature to some 300° F. or more below the point at which water turns into ice; while if we lower the temperature to about 390° F. below freezing, it will turn into a solid. At a temperature of about 490° F. below freezing everything in nature, whether gaseous or liquid, would become a solid, and that temperature, which is the lowest that can possibly exist, is called Absolute Zero. But just as every gas becomes a solid at that temperature, so there are temperatures at which every solid becomes a gas. Gold, for instance, begins to be a liquid at about 1900° F., and if we heat it to 2000° it will become a gas.