cryophorus

thumb|300px|right|Wollaston's diagram of a cryophorus. When the empty ball on the right is immersed in a freezing mixture of snow and salt, the water in the ball on the left freezes in a few minutes.

A cryophorus is a glass container containing liquid water and water vapor. It is used in physics courses to demonstrate rapid freezing by evaporation. A typical cryophorus has a bulb at one end connected to a tube of the same material at the other end. When the liquid water is manipulated into the bulbed end, and the other end is submerged into a freezing mixture (such as liquid nitrogen), the gas pressure drops as it is cooled. The liquid water begins to evaporate, producing more water vapor. Evaporation causes the water to cool rapidly to its freezing point and it solidifies suddenly.