Fire

Abstract

We should start by considering the nature of this element carefully, because it is rather different from the other three. In the case of Earth, Water, and Air (or—as we might put it in other terms—solid, liquid, and gas), these are all types of stuff or substance. They all have mass, the physical state of which is determined by conditions of pressure and temperature. It is possible for a particular chemical element or compound to be known in all three states: the substance given by the chemical formula H₂O, which we generally think of as water (in its liquid state) is also commonly encountered as a solid (ice) or a gas (steam). We normally only encounter a substance such as iron in its solid state unless we are involved in smelting, when its liquid state appears. Iron as a gas is unknown on our planet but undoubtedly occurs in the Sun and other stars.