Examine a pan of water over the fire. Note that the heat passes first to the particles of the pan, then to the water nearest to the source of heat. As these particles expand, they become lighter and pass to the surface of the water. This process continues until the whole mass of water reaches a uniform and fixed temperature called the boiling point - 212° F. under ordinary conditions. In the generation of steam under pressure higher than the ordinary air, the boiling point varies, increasing in proportion to the pressure. With a pressure of 16 lbs. to the square inch, water boils at 212.1° F.; with a pressure of 20 lbs. at 228.4°, etc.
After the boiling point has been reached the temperature of the water remains constant, however long the heat is applied to the vessel. The steam bubbles will rise rapidly, the whole mass will be in a state of agitation (ebullition), and the steam vapor will be given off in large quantities. The heat that is absorbed and given off without raising the temperature of the water is called the latent heat of the steam. This latent heat is either lost or dispelled in the air or is given off when the steam is condensed.
When a substance is heated as it passes from the solid to the liquid state, and from the liquid to the gaseous state, a certain amount of heat is expended in molecular work, separating the molecules of the substance without raising the temperature. The heat thus absorbed or lost is spoken of as latent. For example, when a pound of ice is heated its temperature remains the same until the melting point (32° F. or 0° C.) is reached; further application of heat, however intense, will cause no further rise in temperature until the ice has been entirely melted. Experiment shows that 144 B.T.U. are required to convert a pound of ice into water at 32° F. Further application of heat causes a rise in temperature, 180 B.T.U. raising it to the boiling point (212° F.). The rise in temperature ceases until all the pound of water at 212° F. has been converted into steam, which requires 970.4 B.T.U. This is called the latent heat of vaporization of water. When the steam is condensed to water, the same amount of heat is given off.