SECTION III.-PNEUMATICS, OR THE LAWS OF GASEOUS

PRESSURE.

ANALYSIS OF THE SECTION.

in gases, of which air is taken as the type, the particles are mutually repellent, and tend to occupy an indefinitely large space, if unconfined; this property, together with extreme lightness and compressibility, explains the difference between liquids and gases. Yet the agreements between the two classes of fluids are more numerous than might at first sight appear: among the chief of these are, equal transmission of pressure in all directions, and downward pressure through gravity or weight. The latter was first exhibited by the invention of the Torricellian Tube, or the BAROMETER, one of the most interesting and important inventions in science, on account of its many practical applications, such as indicating proximate weatherchanges, the heights of mountains, &c. Many otherwise mysterious phenomena find a simple explanation in the pressure of the atmosphere; the actions of the sucking-pump and of the syphon are illustrations. The air, like the more palpable liquids, buoys up bodies immersed in it with a force equal to the weight of fluid which they displace, and such bodies will float or sink according as this weight is greater or less than their own weight; on this is based the theory of ballooning, of ventilation, of winds, &c.; a lighter gas will thus be buoyed up by a heavier one, as oil or spirits float on water. But the nature of gases, as explained by the modern kinetic doctrine, does not allow the permanent separation of them by gravity; for a mutual interpenetration or diffusion invariably takes place, with a rapidity depending on their relative densities.

395. Pneumatics has for its subject, as explained in the beginning of Section I. (Hydrostatics), the properties of that class of fluids called gases, which are distinguished from liquids by their great rarity or lightness, and by their extreme compressibility and elasticity. The term is derived from a Greek word, Pneuma (ñveνμa), signifying air or breath, because common air is the most accessible for study, and is representative of gaseous substances generally; just as the term hydrostatics comes from the Greek word "dwp, meaning water, that being the most common, and the type, of all liquids. Gases, like liquids and solids, differ in their special

236

Primitive notions on the nature of Air.

or chemical qualities, but these do not interfere with the mechanical conditions common to them all. The laws of " equal pressure in all directions," of "pressure varying with the depth,” of “liquid level,” &c., may be proved experimentally with the most convenient liquid, water, but they are found to be equally true of all other liquids. So the laws which hold in the case of common air will be equally binding in the case of all other gases under similar external circum

stances.

396. While the ancients had that vague notion of air, which made them apply to it almost indifferently the names of air, ether, spirit, breath, life, they never dreamt of making experiments upon it, with a view to prove its identity with grosser matter. And one of the most interesting parts of the history of man's progress in knowledge, is that which tells how the light gradually dawned upon this subject. Galileo was the first to conclude that air made a definite pressure upon things at the surface of the earth-as in forcing water into the exhausted barrel of a common pump; Torricelli and Pascal proved that this was caused by its weight, and even attempted to estimate the height of the aërial ocean; Priestley, Black, Lavoisier, and others discovered that air or gas was of different kinds—that, for instance, one kind, called oxygen, could unite with a metal, so as to increase its bulk and weight, and to produce a compound of totally new qualities; and they at last analysed the atmosphere itself, and exhibited it as a mixture of two distinct substances. The nature of gases has now been so thoroughly investigated, that they can be measured, manufactured, and operated upon just as readily as the more palpable liquids and solids.

397. The suspicion being once excited, that air is as much a material fluid as water, only much less dense by reason of a greater separation and repulsion of the particles, it is easy to confirm the analogy by reference to familiar facts. Thus,—as a leathern bag when opened out under the surface of water becomes full, and, if its mouth be then tied, cannot afterwards be pressed together: so a bladder, opened out in air and then closed, remains bulky and resisting, and forms what is called an air-pillow. The motion of a flat board is resisted in water: the motion of a fan is resisted in air. Masses of wood, sand, and pebbles, are rolled along or floated by currents of water: chaff, feathers, and even rooted trees, are swept away by currents of air. There are mills driven by water; and so there are mills driven by the wind. Oil set free under the surface of water, or placed there in a bladder, is buoyed up to the surface :

The Lightness and Elasticity of Air.

237 hot air or hydrogen gas placed in a balloon, is buoyed up in the air. A fish moves itself by its fins and tail in water: a bird moves and directs itself by its wings and tail in the air,—and as on emptying the water from a vessel in which a fish swims, the creature falls to the bottom, gasps a few moments, and dies; so, on exhausting the air from a vessel in which birds or butterflies are enclosed, their flapping wings are powerless to support them, and if the experiment be continued, they soon die.

Lightness of Air.

398. Air, as it exists near the general surface of the earth, is so light that a cubic foot of it weighs only about an ounce and a quarter. The same bulk of water weighs nearly a thousand ounces; in other words, water is above eight hundred times heavier than air. Other gases have their different specific gravities, just as liquids or solids have. Thus steam-that is water in the form of gas or vapour-is little more than half as heavy as the same bulk of air: hydrogen is only one-fourteenth as heavy, and carbonic acid gas, which gives the effervescence to soda-water, brisk ale, and champagne, is so much heavier than air, that it may be poured out of one open vessel into another, almost as a liquid may be, or, more exactly, as water might be poured upon oil.

Elasticity of Air.

399. A small bladder or india-rubber balloon full of air may be squeezed between the hands so as to be much reduced in size, but on being relieved from the pressure it immediately regains its former bulk.

If a glass or metal tube, a b, of uniform bore (fig. 105), be fitted with a moveable air-tight plug or piston, , the air between the piston and the close bottom b, may be compressed to a very small part of its usual bulk; but when allowed, will push the piston back again with the same force as it opposed tɔ the condensation, and will recover the volume which it had before the experiment.

Fig. 105.

α

b

Again, if the plug were at first only an inch from the bottom, enclosing air of the usual density, then on drawing it up to the top, the inch of air beneath it would expand so as to occupy the whole tube of say six inches length, and would have, of course, only a sixth of its original density.

238

Valves: the Air-condenser.

The tube with its piston just described becomes, according to the position of valves, either a forcing syringe for injecting and con densing air in a vessel, or what is called a sucking pump for exhausting or removing air from a vessel; both operations depending on the elasticity of the air.

400. That useful contrivance, a valve, for whatever purpose used, is in principle merely a moveable flap, or little door, a (fig. 106), hinged over an opening, b, which it is made to close by its weight, or other gentle force. Such a flap, it is evident, will allow fluid to pass only in one direction, viz. outwards from the opening, for any fluid tending inwards must shut the flap. The flap of a common bellows is a familiar example. Fig.105. 401. A barrel and piston is a condensing syringe, when, in a passage of communication between the bottom of the syringe and a receiving vessel, there is a flap or valve allowing air to pass towards the receiver but not to return. The piston, therefore, at each stroke forces what the barrel contains of air into the receiver. When the piston is lifted again after the stroke, air re-enters the barrel from the atmosphere, either through a valve in the piston itself, or through a small hole near the top of the barrel. A second, and each succeeding downward stroke sends a like measure of air into the receiver, until the desired quantity is accumulated.

"The Air-pump.”

402. To convert a forcing into an exhausting syringe or pump, commonly called an air-pump, it is necessary only to reverse the position of the valves; then, on the descent of the piston, all the air between it and the bottom of the barrel, instead of entering the vessel or receiver, as in the last case, escapes by a valve in the piston itself towards the atmosphere. On the raising of the piston, a perfect vacuum would be left under it, but that the valve below, in the passage from the receiver, being then opened by the elasticity of the air in the receiver, allows a part of that air to follow the piston. Thus, at each stroke, a quantity of the air, proportioned to the size of the barrel, is removed from the receiver..

In the ordinary air-pump there are usually two cylinders or barrels, in which tightly-fitting pistons are worked by the pinion and rack arrangement shown in fig. 107. The double barrel construction not only quickens the rate of exhaustion, but has the farther advantage that the atmospheric pressure, of fifteen pounds per square inch on the upper surface of either piston, and which for a

single piston would have to be overcome by the worker in lifting. it, is here balanced always by the same pressure on the other piston. Both barrels

communicate with the upright tube, to which the flat smoothly-ground plate, P, is screwed airtight.

The glass bell, or receiver, R, with a smoothground lip, being placed on this plate, forms an air-tight enclosure, from which we can exhaust the air. It will be understood from the figure, (in which to avoid confusion the

P

R

Fig. 107.

framework is not shown,) that while the piston, A, is being raised and B is depressed, the valve in A is closed, while that at the bottom of its cylinder, is opened by the elastic force of the air in the receiver. The positions of the valves in the other cylinder, B, are just the reverse of these, so that the air is prevented from returning to the receiver, and is expelled through the valve in the piston, B. Thus, the air within R gradually gets rarer and rarer, till at last a more or less perfect vacuum (or empty space) is obtained, and we have the means of exhibiting the many interesting phenomena that we now come to review.*

403. The law of elasticity of air or any gas is, that its outward spring, or resistance to compression, increases exactly with its density, or the quantity of it collected in a given space.

It has been ascertained, by experiments to be described presently, that in the atmospheric ocean surrounding the earth there are nearly fifteen pounds of air above every square inch of the surface of the earth. It is found, also, that air is reduced to half its bulk, or becomes of double its ordinary density, by an additional pressure of fifteen pounds on the square inch; to one third of its bulk, or of triple density, by triple pressure, and so forth. On the other * Other means of effecting the same purpose will be described at the end of this section.