STEAM, in physics, water in its gas eous form. It is a colorless, invisible gas, quite distinct from the visible cloud which issues from a kettle, etc., which is composed of minute drops of water produced by the condensation of the steam as it issues into the colder air.
Under ordinary atmospheric pressure, water boils in an open vessel at a tem perature of 212°, and the steam always has this temperature, no matter how fast or on a screw. The term especially be longs to steam river craft; ocean-going craft being called steamers, steamships, etc. The first steamboat was built the water is made to boil. The heat which is supplied simply suffices to do the work of converting the liquid water at 212° into gaseous steam at 212°, with out raising the temperature of the steam at all. If the temperature of steam at 212° is lowered by only a very small amount, part of the steam is condensed; hence steam at this temperature is termed moist or saturated steam. At high tem peratures and pressures, steam behaves like a perfect gas; but at lower pres sures and at temperatures near the boil ing point of water, its behavior differs markedly from that of perfect gases; and this change of properties has to be taken into account in all calculations connected with the expansion of steam in steam engines. The terms high pres
sure and low pressure are applied to steam without any sharply defined limit between them. If the steam is super heated by passing it through hot pipes, it is converted into dry steam, which, within certain limits, behaves like a per fect gas. If, instead of allowing the steam to escape freely, the water is boiled in a closed vessel, the steam accumulates, and both pressure and temperature rap idly increase, till the former becomes several times greater than that of the atmosphere. If now the steam is allowed to escape, it rapidly expands, and if it escapes into the cylinder of a steam en gine the expansion can be utilized and converted into work. As the steam ex pands, its pressure of course becomes less and less till it is not greater than that of the atmosphere; and at the same time its temperature is reduced, the re duction depending on the rapidity with which expansion takes place.