The New Student's Reference Work/Steam

Steam is the vapor of water. Water gives off vapor without being heated, but this process is usually called evaporation, and the word "steam" is limited to the vapor which arises from boiling water. Steam is lighter than air, and occupies much more space than water; a cubic inch of water will make nearly a cubic foot of steam. Water boils at 212° F. and makes steam, which has the same temperature as the water, but really contains more heat, so that the steam from a boiling teakettle will burn more severely than the boiling water. This extra heat is called latent heat, because it does not affect the thermometer. The boiling temperature of 212° F. is practically constant in vessels which have a free outlet for the steam; but in closed vessels, like boilers, the boiling-point may be raised, and much pressure is produced at the same time. The pressure increases faster than the temperature; for instance, the pressure at the boiling-point is one atmosphere, but it becomes two atmospheres at 249°, four at 301° and twenty at 444°. When the engineer opens a valve to let off steam, the rushing noise it makes shows its power. Steam is used to produce motion, as in the locomotive and other machines; in heating houses; and in other operations where heat is required.