PNEUMATICS.

Pneumatics treats of the mechanical properties and effects of air and similar fluids; these are called elastic fluids and gases, or aëriform fluids.

Hydro-pneumatics.This is a compound word formed from two Greek words signifying water and air; in its primary meaning it conveys the idea ofthe combined action of water and air or gas.

Fig. 330.

Note.—Fig.330is one of the simplest forms of an air pump. The description accompanying Fig.341properly applies to this one.

Air is the respirable fluid which surrounds the earth and forms its atmosphere.It is inodorous, invisible, insipid, colorless, elastic, possessed of gravity, easily moved, rarefied and condensed, essential to respiration and combustion, and is the medium of sound. It is composed by volume of 20.7 parts of oxygen and 79.3 of nitrogen, by weight, of 23 of oxygen and 77 of nitrogen. These gases are not chemically united, but are mixed mechanically. Air contains also1⁄2000of carbon dioxide, some aqueous vapor, about one per cent. of argon, and small varying amounts of ammonia, nitric acid, ozone, and organic matter. The specific gravity of the air at 32° F is to that of water as 1 to 773, and 100 cubic inches of air at mean temperature and pressure weighs 301⁄2grains.

Aëriform fluids are those which have the form of air.Many of them are invisible, or nearly so, and all of them perform very important operations in the material world. But, notwithstanding that they are in most instances imperceptible to our sight,they are really material, and possess all the essential properties of matter. They possess, also, in an eminent degree,all the properties which have been ascribed to liquids in general, besides others by which they are distinguished from liquids.

Elastic fluids are divided into two classes, namely, 1, permanent gases and, 2, vapors.The gases cannot be easily converted into the liquid state by any known process of art;* but the vapors are readily reduced to the liquid form either by pressure or diminution of temperature. There is, however, no essential difference between the mechanical properties of both classes of fluids.

As the air which we breathe, and which surrounds us, is the most familiar of all this class of bodies, it is generally selected as the subject of Pneumatics. But it must be premised that the same laws, properties and effects, which belong to air, belong in common, also, to all aëriform fluids or gaseous bodies.

There are two principal properties of air, namely, gravity and elasticity.These are called the principal properties of this class of bodies, because they are the means by which their presence and mechanical agency are especially exhibited.

Although the aëriform fluids all have weight, they appear to possessno cohesive attraction.

The pressure of the atmosphere caused by its weight is exerted on all substances, internally and externally, and it is a necessary consequence of its fluidity. When the external pressure is artificially removed from any part, it is immediately felt by the reaction of the internal air.

Heat insinuates itself between the particles of bodies and forces them asunder, in opposition to the attraction of cohesion and of gravity. It therefore exerts its power against both the attraction of gravitation and the attraction of cohesion. But, as the attraction of cohesion does not exist in aëriform fluids,the expansive power of heat upon them has nothing to contend with but gravity. Hence, any increase of temperature, expands an elastic fluid prodigiously, and a diminution of heat condenses it.

*Note.—Carbonic acid gas forms an exception to this assertion. Water also is the union of oxygen and hydrogen gas.

A column of air, having a base an inch square, and reaching to the top of the atmosphere, weighs about fifteen pounds.This pressure, like the pressure of liquids, is exerted equally in all directions.

The elasticity of airand other aëriform fluids is that property by which they are increased or diminished in extension, according as they are compressed. This property exists in a much greater degree in air and other similar fluids than in any other substance. In fact, it has no known limit, for, when the pressure is removed from any portion of air, it immediately expands to such a degree that the smallest quantity will diffuse itself over an indefinitely large space. And, on the contrary, when the pressure is increased, it will be compressed into indefinitely small dimensions.

The elasticity or pressure of air and all gases is in directproportion to their density; or, what is the same thing, inversely proportional to the space which the fluid occupies. This law, which was discovered by Mariotte, is called “Mariotte’s Law.” This law may perhaps be better expressed in the following language; namely, the density of an elastic fluid is in direct proportion to the pressure which it sustains.

Air becomes a mechanical agent by means of its weight, its elasticity, its inertia and its fluidity.

The fluidity of air invests it, as it invests all other liquids, with the power of transmitting pressure; fluidity is a necessary consequence of the independent gravitation of the particles of a fluid. It may, therefore, be included among the effects of weight.

The inertia of air is exhibited in the resistance which it opposes to motion, which has already been noticed under the head of Mechanics. This is clearly seen in its effects upon falling bodies, as will be exemplified in the experiments with the air-pump.

The great degree of elasticitypossessed by all aëriform fluids, renders them susceptible of compression and expansion to an almost unlimited extent. The repulsion of their particlescauses them to expand, while within certain limits they are easily compressed. This materially affects the state of density and rarety under which they are at times exhibited.

It may here be stated that all the laws and properties of liquids (described under the heads of Hydrostatics and Hydraulics) belong also to aëriform fluids.

The chemical properties of both liquids and fluids belong peculiarly to the science of Chemistry, and are, therefore, not to any extent, considered in this volume.

The air which we breathe is an elastic fluid, surrounding the earth, and extending to an indefinite distance above its surface, and constantly decreasing upwards in density. It has already been stated that the air near the surface of the earth bears the weight of that which is above it.

Being compressed, therefore, by the weight of that above it, it must exist in a condensed form near the surface of the earth, while in the upper regions of the atmosphere, where there is no pressure, it is highly rarefied. This condensation, or pressure, is very similar to that of water at great depths in the sea.

Besides the two principal properties, gravity and elasticity, the operations of which produce most of the phenomena of Pneumatics, it will be recollected that as air, although an invisible is yet a material substance, possessing all the common properties of matter, it possesses also the common property ofimpenetrability.

The Thermometer is an instrument to indicate the temperature of the atmosphere.It is constructed on the principle that heat expands and cold contracts most substances. The thermometer consists of a capillary tube, closed at the top and terminating downwards in a bulb. It is filled with mercury which expands and fills the whole length of the tube or contracts altogether into the bulb, according to the degree of heat or cold to which it is exposed. Any other fluid which is expandedby heat and contracted by cold, may be used instead of mercury.

Note.—The terms “rarefaction” and “condensation,” and “rarefied” and “condensed,” must be clearly understood in this connection. They are applied respectively to the expansion and compression of a body.

As it has been proved by experiment that 100 cubic inches of air weighs 301⁄2grains, it will readily be conceived thatthe whole atmosphere exercises a considerable pressure on the surface of the earth. The existence of this pressure is shown by the following experiments. On one end of a stout glass cylinder, about 10 inches high, and open at both ends, a piece of bladder is tied quite air-tight. The other end, the edge of which is ground and well-greased, is pressed on the plate of the air-pump, Fig.331. As soon as the air in the vessel is rarefied by working the air-pump, the bladder is depressed by the weight of the atmosphere above it, and finally bursts with a loud report caused by the sudden entrance of air.

Fig. 331.

Fig. 332.

Fig. 333.

The preceding experiment only serves to illustrate the downward pressure of the atmosphere. By means of theMagdeburg hemispheres, Figs.332and333, the invention of which is due to Otto von Guericke, burgomaster of Magdeburg, it can be shown that the pressure acts in all directions. This apparatus consists of two hollow brass hemispheres of 4to 41⁄2inches diameter, the edges of which are made to fit tightly, and are well greased. One of the hemispheres is provided with a stop-cock, by which it can be screwed on to the air-pump, and on the other there is a handle. As long as the hemispheres contain air they can be separated without any difficulty, for the external pressure of the atmosphere is counterbalanced by the elastic force of the air in the interior. But when the air in the interior is pumped out by means of an air-pump, the hemispheres cannot be separated without a powerful effort.

The Barometer is an instrument to measure the weight of the atmosphere, and thereby to indicate the variations of the weather, etc. It consists of a long glass tube, about thirty-three inches in length, closed at the upper end, and filled with mercury. The tube is then inverted in a cup or leather bag of mercury, on which the pressure of the atmosphere is exerted. The following experiment, which was first made in 1643, by Torricelli, a pupil of Galileo, gives an exact measure of the weight of the atmosphere.

Fig. 334.

A glass tube is taken, about a yard long and a quarter of an inch internal diameter, Fig.334. It is sealed at one end, and is quite filled with mercury. The aperture, C, being closed by the thumb, the tube is inverted, the open end placed in a small mercury trough, and the thumb removed. The tube being in a vertical position, the column of mercury sinks, and, after oscillating some time, it finally comes to rest at a height, A, which at the level of the sea is about 30 inches above the mercury in the trough.

The mercury is raised in the tube by pressure of the atmosphere on the mercury in the trough. There is no contrary pressure on the mercury in the tube, because it is closed; but, if the end of the tube be opened, the atmosphere will press equally inside and outside the tube, and the mercury will sink to the level of that in the trough. It has been shown that the heights of two columns of liquid in communication with each other are inversely as their densities; and hence it follows that the pressure of the atmosphere is equal to that of a column of mercury the height of which is 30 inches. If, however, the weight of the atmosphere diminishes, the height of the column which it can sustain must also diminish.

Why a vacuum gauge is graduated in inches instead of in pounds is thus explained.Take a tube say 35 inches long, closed at one end, filled with mercury and inverted with its open end in a bowl containing the same liquid.

The atmosphere will exert on the surface of the mercury in the bowl a pressure of about 15 pounds per square inch and this pressure will be transmitted to that in the tube so that the upward pressure inside the tube at the level of the mercury in the bowl will be 15 pounds per square inch.

Below the surface the pressure increases, due to the depth of mercury, but the weight of mercury inside the tube below the level in the bowl counteracts the weight of that outside so that the upward pressure per square inch at the surface line is 15 pounds per square inch inside the tube no matter how much or little it is submerged. In the upper end of the tube the mercury has dropped away, leaving a complete vacuum.

Note.—Moreover it has the advantage over a scientifically graduated gauge, which would be graded at 0 for a perfect vacuum and 15, or more nearly 14.7, for atmospheric pressure, that the inch indication increases as the vacuum is more complete while the absolute pressure decreases. The inch of mercury has also the advantage over the pound as a unit for measuring the degree of vacuum or the difference between the pressure in the condenser and that of the atmosphere thatthere are twice as many inches in a perfect vacuum as there are poundsso that the gauge can be read more closely without fractional units. It is easier to say 23 inches than eleven and a half pounds.

The 15 pounds will force the mercury up into the tube until the column is high enough to balance that pressure. One cubic inch of mercury weighs about half a pound. It would take two cubic inches to weigh a pound and a column two inches high to exert a pressure of one pound per square inch of base, or a column 30 inches high to balance the pressure of 15 pounds.

Fig. 335.

Fig. 336.

If instead of a perfect vacuum there was a pressure of two pounds in the upper end of the tube the column would have to balance a pressure of 15-2 = 13 pounds and would be 26 inches high. As the absolute pressure in the top of the tube gets greater, that is to say, as the difference between that pressure and that of the atmosphere or the so-called vacuum gets less, the column of mercury gets lower, and its height is a measure of the completeness of the vacuum.

Hero’s fountain, which derives its name from its inventor, Hero, who lived at Alexandria, 120B.C., depends on the elasticity of the air. It consists of a brass dish, D, Fig.335, and of two glass globes, M and N. The dish communicates with the lower part of the globe, N, by a long tube, B; and another tube, A, connects the two globes. A third tube passes through the dish, D, to the lower part of the globe, M. This tube having been taken out, the globe, M, is partially filled with water; the tube is then replaced and water is poured into the dish. The water flows through the tube, B, into the lower globe, and expels the air, which is forced into the upper globe; the air, thus compressed, acts upon water, and makes it jet out as represented in the figure. If it were not for the resistance of the atmosphere and friction, the liquid would rise to a height above the water in the dish equal to the difference of the level in the two globes.

The fountain in vacuo, Fig.336, shows an interesting experiment made with the air-pump, and shows the elastic force of the air. It consists of a glass vessel, A, provided at the bottom with a stop-cock, and a tubulure which projects into the interior. Having screwed this apparatus on the air-pump, it is exhausted, and the stop-cock being closed, it is placed in a vessel of water, R. By opening the stop-cock, the atmospheric pressure upon the water in the vessel makes it jet through the tubulure into the interior of the vessel, as shown in the drawing.

Note.—Reference is hereafter very largely made to the mechanical use of air as a moving power, or rather as a means for transferring power, just as it is transferred by a train of wheelwork. Compressed air can be employed in this way with great advantage in mines, tunnels, and other confined situations, where the discharge of steam would be attended with inconvenience. The work is really done in these cases by a steam-engine or other prime mover in compressing the air. In the construction of the Mont Cenis tunnel the air was first compressed by water-power, and then carried through pipes into the heart of the mountain to work the boring machines. This use of compressed air in such situations is also of indirect advantage in serving not only to ventilate the place in which it is worked, but also to cool it; for it must be remembered that air falls in temperature during expansion, and therefore, as its temperature in the machines was only that of the atmosphere, it must, on being discharged from them, fall far below that temperature. This fall is so great that one of the most serious practical difficulties in working machines by compressed air has been found to be the formation of ice in the pipes by the freezing of the moisture in the air, which frequently chokes them entirely up.

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