PRESSURE OF FLUIDS.

9

square inch of the bottom, the sides, and the top, of its inner surface, with a force amounting to a weight of fifteen pounds; this pressure being quite independent of the weight of the air. Hence the lateral, oblique, and upright pressures of air, arising from its elasticity, are equal in amount to the downward pressure of the entire weight of the atmosphere. 2. This elastic property of air varies with the increase and decrease of the capacity of the containing vessel, and in an inverse ratio. If one cubic foot of air be introduced into a vessel of two cubic feet in capacity, the amount of pressure in every direction is halved; if the cubic foot of air be compressed into a vessel of the capacity of half a cubic foot, the amount of pressure will be doubled. 3. The elastic property of air may be readily illustrated by means of the common barometer. This instrument is merely a tube containing mercury, having its open end inverted into a small reservoir containing some of the same metal. The pressure of the atmosphere upon the mercury in the reservoir supports, under ordinary circumstances, a column of 30 inches of mercury in the tube. The space above the mercury in the tube is a vacuum. If a portion of air be admitted into this space, it presses by its elasticity upon the mercury, which continues to descend in the tube until its pressure, together with the weight of the mercurial column remaining in the tube, be supported by the weight of the atmosphere. By pursuing this experiment, the elastic force of a small portion of air confined in a tube, is balanced against the entire weight of the whole atmosphere. The elastic force of the air may thus be exactly ascertained: for every two inches of mercury which are expelled from the tube, the amount of the pressure of the enclosed air is one pound, or very nearly so, the weight of two cubic inches of mercury being exactly 15'68 oz., or 0.98 lb. 4. The elasticity of air increases with its temperature; the more heat, therefore, which is applied to any air or gas confined in a vessel, the greater will be the pressure on every part of its interior surface.

10

ELASTIC FORCE OF STEAM.

8. Elastic Force of Steam.-Steam is a vapour consisting of particles of water, which, though of no higher temperature than the water from which it is formed, occupies a space about 1700 times greater than they occupied when in the liquid state; hence a cubic inch of water becomes nearly a cubic foot of steam, at 212 degrees of the thermometer, and under the common pressure of the atmosphere; and the elastic force of the steam is equal to the pressure of the atmosphere under which it is formed. But when steam is confined, and thus subjected to additional pressure, it rises in temperature and acquires great elastic force; it is then called high-pressure steam. The greater the pressure, the more elevated is the temperature required to produce vapour, the denser is the vapour produced, and the greater its elasticity. At 212o, the barometer standing at 30 inches, steam has sufficient elasticity to overcome the atmospheric pressure, and rise against it. The elasticity of steam still in contact with water, increases in a greater ratio than the temperature at which it is produced: tnus, at 212o it is equal to one atmosphere ; at 250 ̊5, to two atmospheres; at 293′7, to four; at 341 78, to eight; at 398-48, to sixteen; at 435·56, to twenty-four. To this rapidly increasing elasticity are owing the frequent explosions of vessels not provided with safety valves.—The elastic force of steam, when heated under pressure, may be illustrated by the annexed figure, which represents a stout copper vessel, of a globular form, called a Papin's Digester. It contains some mercury m, and some water w; and a long glass tubett, open at both ends, is fastened

a

Fig. 2.

into it, the lower extremity dipping into the mercury, and the upper part being furnished with a scale a, divided into inches. There are two other openings in the vessel; into one of these

CONDENSATION OF STEAM.

11

a stopcock b is screwed, and into the other a thermometer 7, the bulb of which is within the vessel. Heat is applied, the stopcock being open, until the water boils. On closing the stopcock, and continuing the heat, the temperature within rises above 212o, as indicated by the thermometer. Steam continues to be formed, and, becoming denser, forces the mercury up in the tube t t to a height proportional to the elastic force of the steam. The weight of the atmosphere being equal to a column of mercury of 30 inches, this pressure has been overcome by the steam at 212o, before the mercury began to rise in the tube t t. For every thirty inches, therefore, which the mercury rises in this tube, the steam is said to have the elastic force of another atmosphere. Thus, if the mercury rise 30 inches, the elastic force of the steam is that of two atmospheres; if it rise 45 inches, it is that of two atmospheres and a half; if 60 inches, of three atmospheres; and so on. The force of steam varies, however, when generated from fresh, and from salt, water. According to Watt, at 180°, the force of pure water is estimated at 14.73 inches of mercury, that of salt water at 10.85; at 212°, fresh water has a force of 2956, salt water only of 22.74 inches.

9. Condensation of Steam.-It has been stated, that during the formation of steam from water at 212o, and under the ordinary atmospheric pressure, 1000 degrees of heat are absorbed, which serve merely to change the form of the body; and that, as a consequence of this, one cubic inch of water expands into 1700 cubic inches of steam. We should, therefore, expect that, upon withdrawing this heat, the form of the body would again be changed, or, in other words, that the steam would be condensed into water. And such is the case. When any body which is colder than steam is brought into contact with steam, the latter gives out its latent heat, till the temperature of the cold body becomes the same as that of the steam, or till the whole quantity of steam is condensed to a degree of elasticity corresponding to the tem

12

PRODUCTION OF A VACUUM.

perature to which the cold body is raised by the heat of the steam. In order to produce effective and rapid condensation, several circumstances are required, viz., a large quantity, a great degree of coldness, and a rapid conduction of heat, of the cold body employed for condensation. The greater the quantity of the cold body, the less will its temperature be raised; and the colder it is, the more will the elastic force be reduced; hence, in order to reduce the elastic force of steam as low as possible, the quantity and coldness of the cooling body should be as great as possible. The most effective cooling body for condensation, is water. Its operation may be illustrated as follows. If the cubic foot of steam, formed from the cubic inch of water at 212°, as already described, be received into a vessel which just contains it, and 5 cubic inches of water at 32° be injected into the vessel, the steam will immediately communicate its latent heat to the cold water, and will itself return to the liquid form. The vessel will then be found to contain 6 cubic inches of water at 212°; of these, 5 have been raised from 32° to 212°, by the latent heat of the steam, and the remaining inch retains the temperature which it had when in the form of steam. These results agree with those above described, as taking place in the production of steam from water (p. 5). In order to convert a given quantity of water at 212° into steam, it required 5 times as much heat as was required to raise the same quantity of water from the freezing to the boiling point. Reversely, during the reduction of steam to water, the former parts with as much heat as is sufficient to raise 5 cubic inches of water from 32° to 212°; that is, 5 times 180°; that is, 990°, or the latent heat of steam.

10. Production of a Vacuum.-The condensation of steam, just described, is obviously attended by the important result of producing a vacuum. A cubic foot of steam contains 1728 cubic inches; when this quantity of steam is condensed, one inch of water is found in the containing vessel,

PRODUCTION OF MECHANICAL FORCE.

13

while 1727 inches of it remain unoccupied; in other words, the vessel, with the exception of one cubic inch of water, presents a vacuum. This may be illustrated by a simple experiment. If a little water or ether be put into a glass tube, open at one end, and blown into a bulb at the other, and the bulb be held over the flame of a candle, the liquid will boil, and the steam issue copiously from the tube. If the tube be now inverted, and its open end be plunged under cold water, the steam in the tube will be condensed, and the water will be forced up suddenly into the tube, and fill the bulb. It is a law in physics, that, when a vacuum is produced, the surrounding bodies have a tendency to rush into it with a certain force. The production of a vacuum becomes, therefore, a source of considerable mechanical power.

11. Mechanical Force produced by the preceding operations. The application of the foregoing principles, as a moving power in the Steam Engine, may be illustrated by a little instrument contrived by Wollaston. It consists of a glass tube, which is enlarged at one end into a bulb, and is open at the other. A piston p is fitted to this tube, so as to move up and down with ease, but at the same time to be air-tight. Some water is put into the bulb, and heated; steam is formed, and the piston is raised to the top of the cylinder. In this case, the elastic force of the steam is the moving power, and this force is proportionably greater, as the piston is more loaded, and the steam more confined. If the bulb be now plunged into cold water, the steam

Fig. 3.

in the cylinder is condensed, and a vacuum is produced below the piston, which is now forced down to the bottom of the cylinder by the pressure of the atmosphere. In this