STEAM (AS. steam, Fris. stoame, Dutch stoma, steam; of unknown etymology). Water in a gaseous condition. It is a dry, colorless gas with a specific gravity of 0.625 as compared with air at the same pressure. The white cloud of vapor which rises from boiling water and which is commonly called steam is a vapor composed of minute particles of water suspended in the air and formed by the condensation of the steam coaling in contact with the cool air. When water is subjected to the action of heat, it is converted into steam. Though a change takes place in Op physical condition of the substance, the chemical composition of the steam is in no way different from that of the water from which it was gen erated. if heat be applied to the bottom of a vessel containing water, the air contained in the water will first appear as little bubbles which rise to the surface. Then the water in imme diate contact with the heated portion of the ves sel will be converted into steam, which will form as bubbles on the bottom of the vessel, and these will rise through the liquid, but at the commencement of the operation they will be at once condensed by the cold upper layers of water. Finally, however, the water becomes heated through when the bubbles rise to the surface and the steam exudes upon the air, and we have the phenomenon known as boiling. As the steam passes into the air, it is cooled and collects into small particles of water which are visible as a white cloud above the surface, and the phenome non is called condensation. if all the particles of this white cloud were collected into one mass, there would be a volume of water equal to the volume of water in the original vessel which had been converted into the steam forming the cloud. Boiling occurs only when the water in the vessel has reached a certain temperature. This tem perature varies with the pressure. At the atmos pheric pressure of 14.7 pounds at mean sea level, it is 212° F., but it would be somewhat less on the top of a high mountain and somewhat great er at the bottom of a deep mine. See livPso METER ; HIPSOMETRY.
The boiling temperature or boiling point of water thus varies with the pressure upon it. At
a pressure of 5 pounds per square inch, it is as tow as 162.3° F., and at a pressure of 100 pounds per square inch, it is as high as 327.58° F. Con densation takes place at any temperature lower than the boiling temperature.
To explain more fully the action of heat in the formation of steam, reference will be made to the accompanying diagrams. In Fig. 1, let the cyl inder contain one pound of water at 32° F., and let the pressure of the atmosphere be represented by the weighted piston. Then if heat be applied to the bottom of the cylinder, the temperature of the water will rise higher and higher until it reaches 212° F.; the piston will up to this point remain stationary except for the small expansion of the water. On continuing the heat, the water shows no further rise in temperature, but steam begins to form and to force the piston upward as shown by Fig. 2, and this continues until the last drop of water is converted into steam and we have the condition illustrated by Fig. 3.
Before proceeding further, we must note first that no steam began to form until the water reached a temperature of 212° F., hence this is evidently the lowest temperature at which steam will form under normal atmospheric pressure. Second, we must note that in the condition il lustrated by Fig. 3 we have one pound of steam occupying the least possible volume at atmos pheric pressure. In actual figures this volume is 26.36 cubic feet. Steam in this condition is known as saturated steam. If now we continue to heat the steam in the cylinder Fig. 3, its tem perature will rise above that of saturated steam, and the piston will move upward, and we will have superheated steam. If now we take the cylinder Fig. 3 and plunge it into a vessel of cold water. as shown by Fig. 4, the heat will be taken away from the steam and it will con dense to water. When this water has cooled to 32° F., the whole heat taken away is exactly equal to the whole heat added during the opera tions illustrated by Figs. 1, 2, and 3.