A vapour (British spelling) or vapor (see spelling differences) is a substance in the gas phase at a temperature lower than its critical point. This means that the vapour can be condensed to a liquid or to a solid by increasing its pressure without reducing the temperature.
For example, water has a critical temperature of 374 °C (647 K), which is the highest temperature at which liquid water can exist. In the atmosphere at ordinary temperatures, therefore, gaseous water (known as water vapor) will condense to liquid if its partial pressure is increased sufficiently.
A vapour may co-exist with a liquid (or solid). When this is true, the two phases will be in equilibrium, and the gas partial pressure will equal the equilibrium vapour pressure of the liquid (or solid).
Mineral springs are naturally occurring springs that produce water containing minerals, or other dissolved substances, that alter its taste or give it a purported therapeutic value. Salts, sulfur compounds, and gases are among the substances that can be dissolved in the spring water during its passage underground.
Mineral water obtained from mineral springs has long been an important commercial proposition.
Mineral spas are resorts that have developed around mineral springs, where (often wealthy) patrons would repair to “take the waters” — meaning that they would drink (see hydrotherapy and water cure) or bathe in (see balneotherapy) the mineral water.
Historical mineral springs were often outfitted with elaborate stone-works — including artificial pools, retaining walls, colonnades and roofs — sometimes in the form of fanciful "Greek temples", gazebos or pagodas. Others were entirely enclosed within spring houses.
For many centuries, in Europe, North America and elsewhere, commercial proponents of mineral springs classified them according to the chemical composition of the water produced and according to the medicinal benefits supposedly accruing from each: