STEAM. Steam is water in the gaseous form (see HEAT) When dry, it is invisible and transparent, like air, and is not to be confounded with vapor, which is steam' returned to the state of water, and thus become visible—water-dust, as it were. As steam has become the most important of all motive powers, the properties on which its action depends deserve careful consideration. The development of steam is, naturally enough, connected popularly with a high temperature, but the two things do not necessarily go together. Water (or snow, or ice) gives off vapor or steam at um, temperature—a low temperature not preventing the formation of steam, but only decreasing its density. The only limit to this evaporation is when the air surrounding the water (or snow, etc.) is already saturated with vapor of the density which the water can give off at the existing temperature. Thus water at 32° Fain% will give off vapor of a pressure equal to 0.095 lb. per sq.in.; but if the air above it is already saturated with vapor of that density, the tendency of the particles of water to fly apart is exactly,balaneed by the pressure of the vapor on its surface, and no more evaporation takes place. It is a remarkable fact, that while no atmospheric pressure can prevent the water or ice pass ing into vapor, the previous presence in the air of vapor of the required dengity (even when so small as in the instance just given) entirely stops it.
At 32' Fahr., as we have already said, the vapor in the space a would exert a pres sure equal only to 0.085 lb. per sq.inch. If the temperature were raised to 80'. more vapor would rise until its pressure became about 0.5 lb. per sq.in.; at 102' the pressure would be 1 lb.; at 162', 5 lbs. ; at 193', 10 lbs. ; and so on, until at 212° Fahr. the pressure would be 14.7 lbs., or exactly equal to that of the atmosphere. When this point has been reached, it is evident that the piston will he in equilibria, the pressure beneath it being exactly equal to that above. At each intermediate point the downward pressure on pp is equal to the pressure of the atmosphere, minus the pressure of the steam below the piston. So far as the piston is concerned, the conditions area therefore the same as if the vacuum had been impaired by the introduction of a cert8l4 quantity of air below pp; but there is this difference between the two cases: if the space a had been occupied by ratified air, then, by forcing the piston down, and compressing it into less space, its density would increase until its pressure became equal to or greater than that of the external air. With steam, however, if the piston were depressed, and
if the temperature of the steam were preserved the same, instead of its pressure being increased, a portion of it would be liquefied, and the remainder would have the same pressure as before.
It is at 212' that water in an open vessel begins to boil; that is, the vapor rises rapidly and in volumes, being able to displace the atmosphere (see BOILING). In this state it is usually called steam; but there is no essential difference between steam at 212° and steam at 60°. The steam rising from boiling water in an open vessel is of the same temperature as the water—viz., 212°; but notwithstanding this, it eoutains a great deal more heat. This heat is employed in (to use popular language) forcing asunder the mole cules of the steam, and thus causing it to occupy, so much greater a bulk as steam than as water. It does not make itself known by the thermometer (for which reason it is called latent heat), but its existence and amount are known by other means, fe4 which sce the article HEAT.
It is important to note, before going further, that, in speaking of the pressure of steam, we have given it in pounds per sq.in. above a perfiict vacuum, or in what are called absolute pressures. These must be carefully distinguished from pressures (as often given) in pounds above above atmospheric pressure. According to the method we adopt, which is the more scientific one, steam of 14.7 lbs., or one atmosphere, exactly balances the pres sure of the air, and can therefore do no work against it; while, if the other nomenclature had been adopted, steam of 14.7 lbs. above atmospheric pressure would have been really steam of two atmospheres pressure. In reading on this subject the student should always make sure whether the pressures spoken of are measured above an absolute zero, or only above the atmospheric pressure, as much confusion is sure to result from any mistake on this point.