Water, and even ice, at all temperatures, when not confined within impermeable walls, continually give off vapor, the surface particles assuming the gaseous state with a rapidity determined by the temperature of the mass and the nature and density of the superincumbent atmosphere. When confined, this gasification goes on without regard to the character or density of the atmosphere present until the vapor produced, by gradual accumulation, acquires the maximum density and pressure attainable at that temperature; then the formation of vapor ceases. The minimum temperature at which the substance can exist as vapor under a given pressure, and the maximum at which the water can retain its liquid form under that pressure, are the same. This temperature is called the temperature of saturation under the given pressure. When the process just described is carried on in a vessel open to the atmosphere, the issuing vapor mingles with the molecules of that atmosphere as rapidly as formed, and separates only at the surface, until the boiling point is reached, at which temperature the pressure of the vapor becomes equal to that of the atmosphere; the formation of vapor (heat being supplied in sufficient quantity) becomes rapid, and takes place within the mass as well as at the surface; ebullition or boiling begins, the atmosphere is forced aside, and the ascending steam passes off en masse. (See Boiling Point.) The temperature of the boiling point varies with the tension of the atmosphere.

Its mean temperature in open air at the sea level is 212° F., 100° on the centigrade scale, 80° on the Reaumur scale, and 673.2° on the absolute scale. The temperature of both water and steam in a steam boiler is the boiling point due to the pressure of steam carried. A table of such temperatures and pressures is given below. Superheated steam is that which has a temperature higher than that of saturation at the same pressure. If equal quantities of heat be supplied in equal times, an interval will elapse after the temperature has risen to the boiling point before the water will have become vaporized, which interval will be about 51/3 times that required to heat the liquid from the freezing to the boiling point. Careful experiment has shown that, in the transition from the liquid to the gaseous condition, 51/3 times as much heat is required as to heat the same weight of water from 32° to 212°. The exact ratio is as 180 to 966.6; it being necessary to supply 180 units of heat to each pound of water to raise it in temperature from the freezing to the boiling point, and 966'6 British thermal units to change it into steam.

As no rise of temperature was perceived during this last change of state, this heat was called by Dr. Black latent heat, which name is still retained, although it is now well known that it is this heat which performs the work of vaporization. The quantity of heat required to change water at the boiling point to steam at the same temperature varies with the pressure. Under atmospheric pressure, Dr. Black and James Watt found its amount approximately, and Regnault, who discovered its variation with chancre of pressure, determined it with great accuracy for a wide range of temperatures and pressures. At 212° it is 966.6 British thermal units per pound. At any other temperature it is 1091.7 - 0.695 (T°-32°)-0.00000013 (T - 39.l°)3 (Ran-kine), or nearly 1113.94-0-695 T. The total amount of heat required to raise one pound of water from any given temperature to the temperature of evaporation, and to evaporate it at the latter temperature, or the total heat of evaporation, is often called the total heat of steam.

This varies at different temperatures, and is equal to 1091.7 + 0.305 (T-32°)-c2(T2 - 32°), or 1081.94 + 0.305 T, from 32°. It is nearly 1113.94 + 0.305 T where the initial temperature is hypothetically 0°. In these expressions, T is the temperature of vaporization, c2 the mean specific heat of water between the freezing point and the temperature of the feed water, and T2 the latter temperature. Beckoning from 212°, the values of latent and total heat become l=966.6 - 0'695(T - 212°), and with a given temperature t of feed water, h'= 1178.6 - t + 0.305(T - 212°), the total heat in the latter case being measured from the initial temperature of the feed water i to that of the steam forming at T° F. For the centigrade scale, these values become l=606.5 - 0'695 T°, and h"= 606.5 - t + 0.305 (T° - 100°). The total heat of steam, expressed in foot pounds of energy, is H=835,000 + 235.5T. A pound of good coal, used under a good steam boiler, will evaporate 8½ lbs. of water at a temperature of 320° F., and a pressure of 75 lbs. per square inch above the atmosphere, the temperature of the water when entering the boiler being 40°. Here the total heat per pound of water is (1178.6-40) + 0.305(320 - 212)= 1171*54; the heat per pound of fuel is 1171.54 X 8.5 = 9958.1; and the equivalent evaporation from and at 212° is 9958.1÷ 996.6 = 9-999 lbs. of water per pound of coal.

The specific heat of steam under constant pressure is 0.480. At constant volume it is 0.346; i. e., the quantity of heat per pound required to raise the temperature of steam, where its expansion is just sufficient to keep its pressure constant, is 0.480 British thermal units; and, when confined within an unchanging space, its pressure rising with its increase of temperature, the heat required per degree is 0.346 units. The thermal unit is the quantity of heat required to raise the temperature of one pound of water one degree at the temperature of maximum density. The value at other temperatures is practically the same. - Steam, when perfectly free from particles of water, is dry, invisible, and in its physical properties similar to other gases. Its specific gravity is 0.622. In changing in temperature one degree under constant pressure, it absorbs heat equal to 85'77 foot pounds of work. The work of the evaporation of a cubic inch of water at 212° is nearly equal to that of raising a ton one foot. Its coefficient of expansion becomes equal to that of perfect gases at about 18° above the temperature due to its pressure, according to Fair-bairn and Tate. Steam expanding while doing work, as in the steam cylinder of an engine, becomes partially condensed.

When expanding without doing work it superheats, the difference of total heats at the temperatures of the extremes of pressure becoming observable as sensible heat in the production of this superheating. The elastic force of saturated steam being dependent only upon its temperature, the relation may be expressed by*a mathematical formula. Many such formulas have been proposed, none of which are exact. The simplest is Tredgold's, t=175 A-75, in which t is the temperature F. and A the number of atmospheres of pressure. This is correct, within two degrees, from one up to above 25 atmospheres of pressure, and is much more nearly accurate at the extremes of that range. In Southern's formula, which has been much used by engineers, P= + 0.1, in which P is the pressure in inches of mercury. These formulas are now seldom employed, as every work upon this subject, contains a table of pressures, temperatures, and volumes. Where great accuracy is required, and no table is at hand, Rankine's formulas, log. P= and may be used. In these formulas, P is the pressure, t absolute temperature (461.2+T° F.), and A, B, and 0 are constants : A=8.259; log. B=3'436; log. C=5-599; 15/20 =0-00344; B2/4C2=0-00001184. The pressure increases with the temperature at a rate which itself also rapidly increases with rise of temperature. The relative volumes of steam and water can be calculated by Pole's formulas: V= ; p= ; and still more accurately by those of Fairbairn and Tate: V= ; P= .

The relative volume or density of steam under varying pressure can be computed by the use of Rankine's formula, in which V and P are the volumes in cubic feet, and the pressure reckoned above a vacuum, in pounds per square inch, of one pound of steam at the given pressure, and V is the volume (26.36 cubic feet) of one pound of steam at P', the atmospheric pressure. A cubic inch of water makes about a cubic foot of dry steam. Steam expanding in the cylinder of a steam engine does not follow the law of expansion of permanent gases, nor does the variation of the ratio of pressure to volume follow any law which has yet been exactly expressed mathematically. Rankine considers that pressure varies inversely as the 10/9 power of the volume, where the steam neither gains nor loses heat, and as the reciprocal of the 17/16 power where kept dry by a steam jacket. More exactly, PaV - 1.0646, and log. V=2.516.0'939 log. P. In the following table constant multipliers are given, the product of which into the initial pressure will give the mean or the terminal pressure for the grade of expansion selected:

Mean And Terminal Pressures (Salter)

A mixture of steam and other gas has a tension which is equal to the sum of the tensions of the two components. Thus, if a cubic foot of air at atmospheric pressure be enclosed in a vessel of that capacity, and if a cubic foot of steam of the same tension be introduced with it, the pressure upon the walls of the vessel will be two atmospheres, the temperature of both gases being the same. Steam formed from sea water is liberated at a higher temperature than when formed from pure water. The boiling point of water is raised about 0'04° F. for each increment of 1 per cent, of its own weight of salt. Sea water, containing 1/32 of its weight of salt, boils at 213.2° under atmospheric pressure. The maximum proportion of salt permitted in marine steam boilers is usually 2/32, the boiling point being raised 2.4° F. Steam, as worked in the steam engine, if not dried by superheaters, is wet; i, e., it carries in suspension fine particles of water. The amount of water so suspended has been found by Prof. Thurston to be from 0.03 to 0.20 of the weight of the mixture. Ten per cent, is a usual proportion with good boilers.

The amount was determined by condensing in a calorimeter a determinable weight of the mixture, by the use of a known weight of water, and noting the rise in temperature of the latter. Knowing the temperature due to the steam pressure, the weights of steam and water can be determined. The principal advantage of superheating is an increase of economy due to the thorough expulsion of water from the vapor, and consequent reduction of loss by condensation and revaporization in the steam engine cylinder. A less degree of improvement is due to the simple increase of temperature, and to the consequent widening of the range of temperature within which it is worked. The most elaborate and most accurate experimental determination of the coincident temperatures, pressures, and volumes of saturated steam were made by Regnault, at the expense of the French government, and under the auspices of the academv of sciences, and published in the Memolres de l'academie for 1847. The following table gives a summary of the properties of steam based upon Regnault's determinations. Pressures are given in pounds per square inch above a vacuum, and in inches of mercury measuring from the same point. Volumes are relative to water at its greatest density.

Weights are given in pounds, and specific gravity is referred to air as unity at a temperature of 32° F. The distribution of heat in each pound of steam evaporated at 212° F. is given as follows:

It is evident that the total latent heat of steam cannot be taken as in any way the measure of the energy or work in, or that can practically be obtained from, the steam. Much the larger part of such heat is expended in merely overcoming the cohesion of the liquid; and at all temperatures but a small fraction of the latent beat can be made available in performing work. Of the total, seven tenths is lost through the existence of natural conditions over which man can probably never expect to obtain control, two tenths through imperfections of mechanism, and but one tenth is utilized in even good engines.

Properties Of Saturated Steam

See King, "Lessons and Practical Notes on Steam," etc. (New York, 1860; 19th ed., 1873); Fairbairn, "Useful Information for Engineers" (3 series, London, 1864-'6); Salter, "Economy in the Use of Steam " (London, 1874); Perry, "An Elementary Treatise on Steam" (London, 1874); Relation des experiences de M. V. Regnault (Paris); and Porter, " Steam Engine Indicator," containing a valuable steam table (New York, 1875).