"P ASS your hand rapidly before your face. Do you not feel a breeze? Now, instead of your hand use a good-sized piece of cardboard. The breeze becomes stronger. Try to run while holding an open umbrella behind you. You run with much difficulty and seem to be dragging not a light umbrella but a heavy load which opposes all its resistance to your progress, and soon your strength is exhausted.
"Whence comes that breeze, and what causes that resistance? It is the air that is answerable for both, the air in which we are all submerged like fishes in water. Is it not true that the hand, moved rapidly back and forth in the water, produces currents and eddies, with little waves that ruffle the surface and beat against the banks? Precisely the same thing happens when air takes the place of water. Agitated by hand or cardboard, it is displaced, set in motion, and made to beat in successive waves against everything it encounters. Hence the puff of air that cools the cheek when a fan is used.
"If you undertook to run in the water and at the same time to drag after you a towel or even a handkerchief arranged so as to form a wide-mouthed pocket secured by the four corners, do you not think you would experience a resistance difficult if not impossible to overcome? In similar manner the fabric of the umbrella, arrested by the air, will not let you run fast. The more air you move, the more force you must exert. It is all simple enough. A wide piece of cardboard fans us much better than the hand alone; a large umbrella impedes our course far more than a small one.
"Have you ever noticed the reeds that grow in a running brook? They are kept in constant agitation. The dragon-flies or darning-needles, with great gauze wings and long green or blue bodies, that alight for a moment's rest on the tips of these reeds, have difficulty in maintaining their balance on that unsteady perch. Why are those reeds in continual motion?"
Marie hastened to reply: "The running water is striking against them all the time."
"Yes, that is plain. And the great trees, especially the tall poplars, that sway and bend in deep bows and then straighten themselves up, only to repeat the performance—what is it that moves them in that fashion? A giant's hand striving to uproot them would produce no such swaying to and fro. Obviously it is the air in motion that causes the trees to move, just as the water in motion makes the reeds move. Wind is air in motion, and its force is sufficient to snap the poplar tree, rend branches from the oak, and even overthrow solid walls.
"Invisible though it is, therefore, air is a very real substance; it is tangible matter no less than the water of the brook, the flood of the mighty river, the billows of the ocean. So long as it remains at rest we are unconscious of it; but let it be set in motion in great waves, we feel it as wind and are very sensible of its buffets. Without waiting for the next gale to convince us that air is matter, we can, with a little contrivance, examine at close quarters this substance that the hand cannot grasp or the eye see.
"Let us take a drinking-glass and plunge it into the water. It fills of itself. Now that it is full, let us hold it in any position we wish, but without taking it from the water. Whether its mouth be upward or downward or sidewise, the glass will remain full. As long as it is in the water we cannot empty it, not even by turning it upside down. And that is just what might have been expected; for what disposition could we make of its contents when there is water on every side to take the place of any that might flow out?
"So much then being understood, what is the condition of a drinking-glass as we see it standing on the table among other preparations for dinner? Is it really empty as we say it is before filling it with water or wine? Does it contain nothing, absolutely nothing? If you insist that it is empty, I shall proceed to show you that it is full, full to the very brim. But full of what? Full of air, nothing but air.
"Being immersed in air, the glass has filled itself with air without any help from us, just as it would fill itself with water if it were plunged to the bottom of a well. Moreover, it remains full in any position, even upside down; for if any of the contained air should escape, the surrounding air would immediately take its place. Everything is ordered here just as we have seen in the case of a glass immersed in water.
"Accordingly, when we say of a glass or of a carafe, of a cask or a barrel, of a jug or a pitcher or any vessel whatever, that it is empty, the current expression used by us is not in accord with the exact truth. The vessel designated as empty is in reality full of air and remains full in whatever position it is placed.
"Returning now to the drinking-glass, let us plunge it into the water, holding it in a vertical position but with the opening downward. In vain do we push it deeper and deeper into the liquid as far as the arm can reach; this time the glass does not fill with water. Being already filled with air, as has just been explained, it cannot be filled with anything else until that has been emptied out. The air imprisoned in the glass with no way of escape acts as an obstacle to the entrance of the water. Under this aspect, then, we see once more that air is real matter, capable of resistance and not yielding its place so long as there is no way open for it to go elsewhere.
"Let us release the prisoner. To do this we gently tip the glass sidewise while still holding it immersed. A diaphanous globule shoots up through the water and bursts on reaching the surface. Other globules follow, and still others, as we tip the glass more and more. They look like crystal pearls of incomparable clearness. These transparent globules, these pearls that make the water seem to boil, are nothing but air escaping from the glass in little spurts or bubbles; and thus the air is rendered visible despite its ordinary invisibility. We can distinguish it clearly from the water in the midst of which it makes its ascent. We follow its exit from the glass and note its upward passage in the form of bubbles; but, once arrived at the surface and mixed with the outer air, it escapes the keenest eyesight.
"We have just taken considerable pains to prove the existence of a substance unseen by anybody. Is it, then—this substance that we call air—something of importance? Assuredly it is: air is of the very first importance, since without it neither animal nor plant could exist. To give you an idea of the immense part it plays would require too much science and too much time. Let us confine ourselves to a more cursory treatment of the subject.
"When the fire burns low on the hearth and the glowing sticks of wood are turning dull, emitting smoke without flame and threatening to cease burning altogether, what do we do to revive the fire? We take the bellows and supply a blast of air. With each puff the dull coals turn brighter, the fire regains its vigor, and flames begin again to flicker. If generously fed with air from the bellows, the fireplace once more resumes its radiance.
"If, on the other hand, we wish to prevent a too rapid consumption of fuel, we partly cover the firebrands with a shovelful of ashes. Under this cover, which tends to keep out the air, the fire dies down somewhat. Indeed, it would go out entirely if the layer of ashes were to cover it completely and thus wholly exclude the air.
"When on a cold winter day we gather around the glowing stove to warm our benumbed hands, we hear a subdued murmuring sound that tells us the fire is burning. We say then that the stove snores. This sound is caused by the inrush of air through the door of the ash-pit and its assault upon the mass of glowing firebrands, the heat of which it helps to maintain. The more air admitted, the hotter the stove becomes. If we wish to moderate the heat, we have only to close the door of the ash-pit. Thereupon, as air is then admitted only in small quantities through the joints and seams, the fire will slacken and the stove lose its red glow and turn black. The fire would die out entirely if no air whatever gained access to the mass of burning fuel.
"These examples make sufficiently clear to us that air is indispensable to all combustion; it revives a dying fire; it enters into the consumption of the fuel, producing in the process both heat and light. Without air, no fire on our hearths, for wood and coal and other fuel burn only with the help of air. Without air, no light at night in our homes, for the illuminating flame of the lamp, of the candle, and of various other contrivances for dispelling darkness, goes out as soon as the supply of air fails."