Hero's Fountain. once reaching the bottom, because the pressure of water then above it will alone produce the condensation of the air required to make it descend. The water required to make the apparatus just float, may be introduced by heating the bulb and suddenly immersing it in cold water. 410. The famous fountain of Hero, by which water is made to spout far above its source, depends upon the elasticity of compressed air. The vessel, d (fig. 111), is first filled with water, while b and a contain only air. On pouring water into a, the water of d darts upwards through the jet-pipe, e, to a height proportioned to the height of a above b. The reason is, that the water from a descends by the tube to b, and compresses the air in c; which compression conveyed along the other tube from to d, acts on the water in the vessel, d, and causes it to jet upwards. As the pressure is produced by the column of water, a b, the jet is proportioned to the length of that column. This kind of fountain may have its parts concealed under a variety of forms, as exemplified in the second figure (fig. 112), and may thus become a pleasing ornament among flowers in a drawing-room or on a dining-table. It may be made of a size to play for an hour or more, and it will always recommence on the water being shifted from the low to the high reservoir. The water which jets from the vessel, d, when caused to fall into the vessel, a, feeds the compressing column, a b. A useful table-lamp, appearing a simple column, has been constructed on the principle of a Hero's fountain. Pressure conveyed equally in all directions." 12 Fig. 111. Fig. 112. 245 246 Modern or Kinetic Theory of Gaseous Pressure. escape from the pressure is equal in all directions, as is The pressure in one direction must be balanced by an equal pressure in the opposite, in order that a gas may remain in quiescence, otherwise there will always be a rush from where there is more pressure to where there is less. The actions of the common fire-bellows, and of the animal chest in breathing, blowing, sucking, &c., furnish instances of this kind. The suddenness with which pressure on one part of a confined gas is communicated through the whole, is strikingly seen in the simultaneous outburst of all the gas-lights over an extensive building, or even in a long street, at any instant when the force supplying the gas is augmented. Before proceeding farther, it may be as well to give an idea of "The modern or kinetic explanation of gaseous pressure.” 412. The elasticity, pressure, outward spring,. or tendency to indefinite expansion, which is the fundamental quality of a gas, is a consequence of an incessant commotion among its particles. We must picture to our minds the molecules of a gas as moving in all directions, constantly impinging against each other, and thus producing pressure on the sides of an enclosing vessel. In a subsequent part of this section, under the head of Diffusion, we shall refer more specially to the facts on which this theory is based. At present we shall merely give a general idea of this kinetic theory of gases. If the least quantity of any gas be introduced into a vessel, it will rapidly permeate the whole space enclosed. A little of the vapour of a perfume very soon fills the whole of a room, and makes its presence known to the olfactory organs. If any two gases, such as hydrogen and chlorine, the latter of which is thirty-six times as heavy as the former, be put into the same bottle or jar, they will be found after a short time to have each completely permeated the containing space, just as if the other had not been present at all. The violence with which the gaseous molecules will beat against the confining walls will be greater, the less the space through which they are allowed to fly. For, if we assume that the elasticity of the particles is perfect, and that their actual velocity therefore remains Boyle's or Mariotte's Law. 247 unaltered by any alteration of the volume of the gas, the number of their impacts against the envelope will obviously be multiplied exactly as this volume is reduced. In other words, the intensity of the molecular impact will increase exactly as the volume diminishes, so long as the temperature remains the same. We have thus a simple explanation of the fundamental law of gaseous pressure, which is known as Boyle's or Mariotte's law, viz. :— The pressure of any quantity of gas increases or decreases at exactly the same rate as its volume decreases or increases, the temperature of the gas being supposed to remain unaltered. This might be readily verified by the following experiment :— Fit into a glass tube, A B (fig. 113), one square-inch in section, an air-tight piston sliding smoothly, and carrying a scale pan on the upper end of its rod, so that it may be pressed with known weights. Let us suppose that the piston and its rod and pan, P, are without weight, and let us consider the quantity of air, A B, in closed between the plug and the end of the tube. This quantity of air is confined within this space by the pressure of the external air, which, by experiments to be described a few pages hence, is about 15lbs. on the square inch; and but for this pressure it would expand indefinitely. We find then that, if an additional pressure of 15lbs is applied to the confined air, the piston sinks from A to C, half down, that is, to the bottom; and if other 15lbs. be laid on P, the piston will farther sink to D, such that D B is one-third of a B, and so on; thus the space within which the air is confined is inversely as the confining pressure. B Fig. 113. 413. In perfect accordance with this kinetic doctrine of gases is the modern or vibratory theory of heat. When a bladder, partially filled with air, and closed at the neck, is put near the fire, the confined air swells up till the bladder becomes quite tight, or even bursts. The heating of the enclosed air is but the increasing of the energy of the molecular agitation within, which, in opposition to a constant and equal resistance (viz. the pressure of the atmosphere without), will manifest itself as an expansion of the enclosed air. It is found that the rate of expansion bears a definite and uniform relation to the rise of temperature in the gas. This is known as the law of Charles, and is as follows :— Air or any other gaseous fluid expands, against the pressure of the atmosphere or any constant pressure, by about the 1-491st part of it! 248 The Weight of the Atmosphere. volume for every additional rise of temperature through 1oF.; so that, by raising the temperature from the freezing to the boiling point of water, its volume would be increased by nearly one-third; or three cubic inches of air would swell up to occupy a little over four cubic inches. "The pressure or weight of the Atmosphere." 414. If a piece of bladder-skin or a pane of glass be lying at the bottom of a cistern holding water, the bladder or the glass exhibits no sign of being pressed upon, although it bears on its upper side the whole weight of the water directly above it; the reason being, that the water beneath the bladder resists just as strongly as the water above it presses. But if the bladder be tied closely over the mouth of a glass filled with water, and placed at the bottom of the cistern, and if, by means of a syringe or pump, the water can be extracted from within the glass, the bladder itself has to bear the whole pressure of the water above it, and will be torn or burst, Now this experiment may be closely copied in relation to our atmosphere or sea of air. If an open glass have its mouth covered over with bladder, no external pressure will be apparent, because there is a resistance of the air within, just equal to the pressure of the air on the outside :-but if air be then extracted from under the covering by means of an air-pump, the bladder is seen sinking down from the weight of the air over it, and at last bursting inwards with a loud report. By placing a circular piece of wood under the bladder-skin, for it to rest on, and a steel spring of known force to support the wood, we might ascertain very nearly the weight and pressure of the air over it. This mode, however, of ascertaining the weight of the atmosphere, is not that commonly used, but is described here as a readily conceived illustration of the present subject. The estimate is made much more elegantly and completely by means of the barometer, to be described farther on. The pressure of the atmosphere is well exhibited by placing the hand on the mouth of a glass so as to cover it closely, and then extracting the air from the vessel the weight of the atmosphere holds the hand down upon the mouth of the glass with a force which soon becomes painful. The pressure may be rendered visible by the following simple experiment :-Fill a short wide jar with carbonic acid gas and pour in enough water to cover the bottom from half an inch to an inch. Add quickly and without agitation, one or two sticks of caustic potash, and immediately cover the mouth of the jar with a thin sheet of india-rubber. This should be firmly tied The Pressure of the Atmosphere. 249 round the neck of the jar. Now agitate the vessel. As the potash is dissolved, the carbonic acid is removed, and a vacuum is thus produced in the vessel. The pressure of the atmosphere forces the india-rubber downwards into the jar, converting it into a deep cup, and sometimes causing it to burst. The most perfect vacuum may be produced by filling a space with pure carbonic acid gas, and subsequently removing this gas by potash. As should follow, from the pressure of fifteen pounds per inch at the surface of the earth being due altogether to the weight of the superincumbent atmosphere, we find that when a person rises from the earth, as in ascending a hill, and leaves part of the atmosphere beneath him, the pressure diminishes. After the explanation of fluid pressure given under hydrostatics, namely, as acting equally in all directions, it is almost superfluous to remark, that the downward weight of the atmosphere is such a pressure. The bladder-skin which closes the mouth of the vessel described above, is as readily burst if turned sideways as if held directly upwards. Every body or substance, therefore, on the surface of the earth, dead or living, solid or fluid, is compressed with this force. In general, the pressure on one side of a body is just balanced by the equal pressure on the other, so that no sensible effcct follows; and it is on this account that people remained so long in ignorance of the fact. "Atmospheric pressure on solids." 415. Because the atmospheric pressure acts equally on the whole surface of any body immersed in the air, if that pressure be in any way prevented from acting on one side, while it continues to act on the other, the one-sided pressure becomes immediately manifest. This is simply but strikingly illustrated by pressing two good bottlecorks together, end to end, so as to expel the air from between them, and tying over the joining a short piece of caoutchouc tube. If one cork be then seized and raised, the other cork will accompany it, as if strongly glued to it, and, if the touching surface has an area of an inch square, will lift a weight of fifteen pounds attached below, Broader barrel-corks so connected may lift more than fifty pounds. The explanation is, that the upper cork keeps off the atmospheric pressure from the upper surface of the lower cork, while that pressure (of fifteen pounds per square inch) continues on the under surface, supporting that cork and the appended weight. The same result is produced if, instead of using the caoutchouc tube to exclude the air, a length of glass tube be taken, into which the two corks, |