THE best example of a gaseous substance, as well as the most common, is atmospheric air. There are others of great importance, such as oxygen, hydrogen, carbonic acid, and watery vapour or steam; but, so far as their mechanical properties are concerned, air may be taken as the type and representative of them all. It will therefore be proper, in explaining the properties of gases, to speak of air alone.

Now, since air is material, it must have all the essential properties of matter, such as impenetrability and inertia. For the same reason it must also be acted on by gravity, or, in other words, it must have weight. Being a fluid, it possesses the property, common to all fluids,* of transmitting pressure equally in every direction. And, in addition to these, it has its own peculiar properties of compressibility and elasticity, by which it is distinguished from both solids and liquids. It is true that, strictly speaking, neither solids nor liquids are entirely destitute of these last mentioned qualities; but air and the other gases exhibit them in a much higher degree, and even in a somewhat different sense.

That air is impenetrable was shown in a former lesson (page 219). A vessel plunged into water with its mouth. downwards, does not become entirely filled with water, for the air confined in it, however much it may be compressed, must still occupy space, and exclude the water from that space. The same property may be illustrated in various other ways. One example is familiar to every boy. If a paper bag be inflated by blowing into it, and its mouth held tight, a forcible attempt to crush the sides together will cause it to burst with a loud report. A pair of bellows would burst in the same manner, if, after it has been filled * See page 302,

with air, the nosle and valve were closed, and the boards strongly pressed together. These two last examples illustrate both the impenetrability of air, and its power of transmitting pressure in different directions.

The inertia of air was also referred to in a former lesson (page 222) as opposing and retarding the motion of every moving body near the earth's surface. In this resistance we have abundant proof that air, when at rest, needs force to make it move. Nor is it less certain that force is also required to stop it when in motion. In order to be satisfied of this, we have only to think of the windmill driven by the force of the wind, of the ship sailing before the breeze, of the tree bending in the storm, or of the waves lashed into fury by the tempest. Wind, let it be borne in mind, is only air in motion.

It is a more startling assertion, but not the less true, that air, like other kinds of matter, has a certain weight. This can be proved in various ways. Why, for instance, does a balloon rise in the atmosphere? It could not do so, unless it were lighter than its own bulk of the air in which it floats. This was already explained in the case of liquids, and in gases the principle is exactly the same. The balloon ascends, just as a cork would ascend if plunged below the surface of the water, because it displaces a mass of the fluid heavier than itself. Thus we see that air has an appreciable weight. But the same thing can be shown more directly by actually weighing it. For this purpose we require a large glass or copper flask, with a narrow neck, which must be provided with an air-tight stop-cock. By means of an air-pump, such a flask can be emptied of nearly all the air which it contains. When this has been done, let the stop-cock be turned so as to shut out the external air, and let the flask be weighed. Taking care that the weights exactly counterpoise it, let the air be re-admitted by again. turning the stop-cock. Immediately the flask will be seen to become heavier, the difference being the weight of the air which has rushed into it from the surrounding atmosphere. Seeing, then, that air undoubtedly has weight, it

becomes an interesting question, what is the weight of the atmosphere itself in which we are placed. We live, as it were, at the bottom of a great ocean of air, and though that air is light compared with solid or liquid substances, yet its quantity is so great, that the weight of the whole must be enormous. We shall soon find, too, that its pressure on solids immersed in it depends on its weight. We are not without the means of calculating both, but the subject is so important, that it must be taken up in a separate lesson.

Air possesses most of the properties of liquids, except in so far as they are modified by its elasticity. When, for example, a solid is surrounded by air, as we ourselves and all other bodies near the earth's surface are, it is buoyed up by a force equal to the weight of a mass of air of the same bulk as the solid itself. Hence it weighs less than it would do in a vacuum. In most cases, this loss of weight is so insignificant, compared with the weight of the solid, that it may be disregarded. But it is a curious fact, that a pound of cork, or wool, or feathers, weighed in air in the usual way, is actually heavier than a pound of lead. It would be seen to be so, if they were weighed against each other in a vacuum. But when they are immersed in air, the bulkier body displaces more of the fluid, and therefore loses more weight, so that they exactly balance each other.

Compressibility and elasticity are the distinguishing characteristics of gaseous bodies. The particles of these bodies are not only destitute of all cohesion, but seem to be endowed with a mutual repulsion. Hence, if a little air be inclosed in a cylinder by an air-tight piston moving up and down above it, however far the piston may be raised, the air will spread itself through the whole space which is thus offered to it. Nor is there any known limit to its expansion. is to this property, in virtue of which air fills all the space to which it has access, that we give the name of elasticity. Liquids and solids are said to be elastic, if, after being compressed or dilated by any force, they return to their natural dimensions on the force being removed; but the elasticity of air is always expansive, and always acting in opposition

It

The bulk of any

to some external compressing force. quantity of air depends entirely on the force with which it is compressed. Relieve it of pressure, and it will expand almost without limit; apply sufficient force, and it may be compressed into the smallest space. And the more it is compressed, the greater will be its elasticity, or tendency to expand. If, for example, the compressing force be doubled, the elasticity will be doubled too, while the bulk of the compressed air will be reduced to one-half. Hence we have the two following general laws, which are of vast importance both in nature and art: (1) The elasticity of air is equal to the force which compresses it, and (2) the bulk or volume of any quantity of air is inversely proportional to either of these equals.

WEIGHT AND PRESSURE OF THE ATMOSPHERE-THE BAROMETER.

We have seen that air has always a tendency to expand, and fill every part of space to which it has access. Why then, it may be asked, does the atmosphere remain in its position, swathed round the surface of our globe? Why does it not diffuse itself through the boundless regions of surrounding space? The answer is one which every careful reader should be able to find for himself. It is simply because the atmosphere is subject to the earth's attraction, or, to adopt the usual expression, it is retained in its place by gravity. Gravity is therefore the cause of atmospheric pressure. But here is an important difference to be noticed between the pressures at different heights in this vast aerial ocean. Every particle of air it contains is not only urged downwards by its own weight, but sustains also the weight of all the particles above it. Hence the lowest strata are the most compressed, and therefore also the densest and the most elastic. As we ascend, the pressure and elasticity gradually diminish, and, since a given quantity spreads through a larger and larger space, the density also decreases in the same proportion.

Whateve pressure a particle of air sustains, it transmits undiminished to any solid or liquid surface with which it is in contact. Nor does it matter whether that surface be horizontal, slanting, or vertical; for we know that air, like other fluids, presses equally in all directions. Would it not, then, be interesting to know with what force air at the carth's surface presses upon our bodies? We do not, indeed, feel any pressure at all, yet it is obvious from what has been already explained that such a pressure must exist. A few simple considerations will enable us to ascertain its

amount.

A

B

B

Suppose we take a glass tube A B, open at the end A, FIG. 43. and closed at the end B. Its length must exceed 30 inches. This tube having been filled to the brim with quicksilver, the finger is placed on the open end to prevent spilling, and the tube is then inverted. The open end is now dipped into a basin of quicksilver, and the finger withdrawn. Immediately the quicksilver begins to sink in the tube, and we might be apt to suppose that it would continue to sink till it attained the same level in the tube as in the basin outside. But it does not. It rests in equilibrium when about 28 or 30 inches higher than the level of the quicksilver in the basin. In the accompanying figure, the dark portion represents the quicksilver. Now, the question which naturally presents itself is this, Why does the quicksilver in the tube sink so far, and no farther? What force balances the weight of this column, nearly 30 inches high, of so heavy a liquid, when, according to the principles of hydrostatics, the surface of the liquid ought to be everywhere at the same level? It will easily be seen that above the mercury in the tube there is no air. It is impossible for air to gain access, so that the white space in the figure is necessarily a vacuum. But the surface of the mercury in the basin is in contact with the atmosphere, and