After steam has been once generated, the temperature remains constant, and the latent heat, not observable by the thermometer, is absorbed. The temperature of steam in contact with the water from which it is generated depends upon the pressure. If the vessel is closed, as in boilers, the pressure becomes greater and raises the boiling point of the water. Steam under pressure and confined has considerable energy due to heat, which is measured, as already noted, by the heat unit, B. T. U.

When the steam is taken directly from the boiler to the engine, it is termed saturated steam and is generated in contact with its water of generation.

When the boiler is overworked, the steam, due to the violent action of its generation, takes with it particles of water. Such steam is called wet steam. Dry steam contains no watery moisture; it may be saturated or supersaturated.

Steam from the boiler, heated to a higher temperature by passing it through a vessel or coils of pipe separated from the boiler, called a superheater, is termed superheated steam. Steam loses heat as quickly as it acquires it, and so every passage conveying superheated steam should be well covered with non-conducting material.