The law governing ‹physical› or ‹heterogeneous equilibrium› applies to all cases where, at a constant temperature, one and the same chemical substance is present in two or more physical conditions, or "phases," in contact and in equilibrium with each other. We have, for instance, the common case of a liquid, say water, in contact with its vapor, or the liquid in contact with its solid phase (ice) and its vapor; or, we may have a gas, say oxygen, in contact with its solution in some solvent like water. We may have a solid, like cane sugar, in contact with its solution. We may also have a substance like bromine, which is soluble both in chloroform and in water, present in both solutions at the same time, the two solutions being in contact but immiscible. These cases represent the most common types of systems to which the law of physical equilibrium may be applied, although the list has by no means been exhausted. ‹The law of physical or heterogeneous equilibrium states that when one and the same chemical compound is present in two physical states or phases›, as expressed in the equation S_{1} ⇄ S_{2}, then ‹when equilibrium is reached›, at a given temperature, ‹the ratio of the concentrations of the substance in the two phases is some constant number›: [S_{1}] : [S_{2}] = «k.» The bracketed symbols denote concentrations. It should be noted that the condition of equilibrium is independent of the total quantity[227] of substance present in either phase or in both phases; that is, provided the ‹ratio› of the ‹concentrations› is maintained constant at a given temperature, the ‹quantity of substance present in both phases or in either phase is variable›. For instance, the condition of equilibrium between water and [p119] water vapor is independent of the quantity of water or of water vapor present: in a closed liter bottle containing water and water vapor, the ratio of the concentrations is maintained, irrespective of the question whether the bottle contains 10 c.c. of water and 990 c.c. of vapor or 990 c.c. of water and 10 c.c. of vapor.