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sphere which are too numerous and in too small quantity to take account of. As the sea contains a little of everything that is soluble in water, so the atmosphere contains a little of everything capable of existing in the gaseous form at common temperatures. Ammonia, which is a compound of hydrogen and nitrogen, is present in the atmosphere, and is supposed to be the source of nitrogen in plants; while in crowded cities, and in the neighbourhood of gas-works, smelting furnaces, sewers, stagnant pools, sulphur springs, &c. there is much local contamination of the air from the presence of different gases. Various forms of infection, malaria, and marsh-miasma probably arise from the presence of noxious gases in the air.

4. This invisible compound fluid, the atmosphere, possesses many of the properties of solid matter, and also many that are peculiar to fluids. In a former treatise the chief properties by which solids are distinguished from fluids were pointed out; but it is now necessary to consider these and other properties more fully with reference to the atmosphere.

In common with matter in every state, the air possesses impenetrability. It is obvious to the senses, as far as regards solids and liquids, that no two bodies can occupy the same place at the same time; in order that one body should occupy the place of another, it is obviously necessary that the second should move away or be displaced; but the evidence of the senses fails us in the case of air. If we step into a bath completely full of water, a portion will overflow precisely equal in bulk to that part of the body which is submerged; the same thing takes place in an empty bath as it is called, or a bath full of invisible air, and hence called empty. When a person enters a room, a quantity of air precisely equal to his own bulk

IMPENETRABILITY AND WEIGHT.

is displaced, and escapes by the door, or window, or other opening. But the proof of the impenetrability of air may be made more obvious by the following experiment. Plunge an inverted goblet into a vessel of water, keeping its edge horizontal, and it will be found that, to whatever depth we plunge the goblet, the water will not fill it entirely. The air will be compressed into a smaller space, but not annihilated or displaced. At a depth of 34 feet below the surface of the water, the vessel containing the air will be half filled with water; at 100 feet it will be three quarters filled; at 1000 feet it will be filled to within a thirtieth; but even this small remaining space contains all the air which previously filled the vessel, and in drawing it up again to the surface the air will expand to its original bulk and drive out all the water. In fact, we can only get rid of the air by inclining the vessel, when so much of the air as is below the level of the highest part of its mouth will rise in bubbles through the water and escape; and in this way all the air in the vessel must be decanted or poured up before it will be filled with water.

5. The impenetrability of air is alone sufficient to prove it to be a material body, and, though formerly supposed to be without weight, it is now well known to possess this property in common with all other known states of matter; that is, it obeys the attractive influence of the earth and gravitates towards its centre. The proof that air has weight will be abundantly shown hereafter, when we come to speak of the barometer; but for our present purpose the following experiment will suffice:-A copper flask of the capacity of 100 cubic inches, furnished with a stop-cock, is fixed to one extremity of the arm of a balance, and accurately counterpoised by weights in the opposite scale.

An exhausting syringe (an instrument to be described hereafter, 10) is then screwed upon the neck of the flask with the stop-cock open, and by working the syringe nearly the whole of the air can be pumped out of the flask. On closing the stop-cock, to prevent the admission of the air, and detaching the flask from the syringe, it is again weighed, and is found to have lost about 31 grains; or, in other words, 100 cubic inches of air weigh about 31 grains. On opening the stop-cock, the air will be heard to rush in, and the equilibrium of the balance will be restored as before.

If, instead of screwing the copper flask to an exhausting syringe we screw it to a condensing syringe, we can force or condense a quantity of air into the flask, in addition to what it naturally holds. After a few strokes of the condensing syringe the stop-cock of the flask is closed, to prevent the additional air from rushing out; the flask is then detached, and hung to the arm of the balance. The flask is no longer counterpoised, but will require additional weights in the opposite scalepan to restore it to equilibrium. If we again apply the condensing syringe to the flask, it will be found that every additional stroke of the syringe will require additional weights in the scale-pan to restore equilibrium, on account of the additional quantities of air forced into the flask.

6. Here, then, is a very clear proof that air has weight, and, in common with all heavy matter, air also possesses inertia, that is, it cannot be set in motion without the communication of some force; and, when in motion, it cannot be retarded or brought to rest without the opposition of force. Its inertia (like that of all other bodies) is also exactly proportional to its weight; and, as we have seen the latter to be very small compared with its bulk, a very small amount of

MOMENTUM OF AIR.

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force is sufficient to impart motion to a large bulk of air; it obeys the laws of motion common to ponderable bodies, and its momentum, or amount of force which it is capable of exerting upon bodies opposed to it, is estimated in the same way as for solids, namely, by multiplying its weight by its velocity. The momentum of air may be illustrated by the following experiment:Place three lighted tapers in a row at the distance of three inches apart; if we then direct an unloaded gun towards the centre taper at the distance of ten feet, and set in motion the small volume of air contained in the barrel, by discharging the percussion cap on the nipple, the flame of the centre taper will be blown out, without in the least degree disturbing the other two. Another excellent illustration of the momentum of air, and the facility with which a rotatory movement may be communicated to it, is derived from the phenomena of smoke or steam which render the motions of the air visible. When bubbles of phosphuretted hydrogen burst in a still atmosphere, each one, as it bursts, produces a beautiful ring of smoke, expanding larger and larger as it ascends. The whole circumference of each circle is in a state of rapid rotation, as shown by the arrows in Fig. 2; it being this rotation, in fact, which confines the smoke within its narrow limits, and causes the circles to be so well defined. The same phenomena may often be observed in the first puff from the chimney of a manufactory or of a steam boat, and also from the mouth of a skilful tobacco smoker. In the firing of ordnance on a still day these rings may be seen on a grand scale, and still more perfectly if the

Fig. 2.

mouth of the cannon be greased and no shot employed. In fact, any force acting suddenly upon the air from a centre imparts to it a rotatory motion.

The momentum of air is usefully employed as a mechanical force in imparting motion to windmills and ships; but it occasionally exerts itself with fearful effect in those strong winds or hurricanes, which sometimes occur in the West India Islands, where trees are torn up by the roots, buildings levelled to the ground, and the sea is driven with irresistible fury over the desolated country. Such awful calamities are caused by the momentum of the air being greater than the force by which a tree clasps the earth or a building its foundation.

7. Another consequence of the weight of air is its pressure. We have already seen that 100 cubic inches of air weigh about 31 grains. It is necessary, however, in order to obtain this result, that the experiment be performed at the level of the sea; it is further necessary that at the time of the experiment the barometer should stand at 30 inches and the thermometer at 60°. But, disregarding for the present these two last conditions, let us note the change arising from difference of level only. At the level of the sca the 100 cubic inches of air contained in the flask would weigh say 31 grains. On taking this flask to the top of a mountain 20,000 feet high, the 100 cubic inches will have expanded to 200; so that, if the flask be made of some elastic material, it will have expanded to twice its former size; or, if the copper flask full of air have its stop-cock closed at the level of the sea, and opened at an elevation of 20,000 feet, exactly 100 cubic inches of air will rush out, leaving 100 cubic inches of air behind of half its former density. The reason for this is, that at the height of nearly 31⁄2 miles we have ascended

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