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CHEMISTRY OF THE ATMOSPHERE.

COMBUSTION.

123. Composition of the Atmosphere. - We live at the bottom of an aerial ocean called the atmosphere, which is some fifty miles in depth. This atmosphere contains free oxygen; as may be shown by inverting a jar of it over a jar of nitric oxide. The gases, on mixing, become cherry-red (33).

Burn a piece of phosphorus in a jar of air over water. The jar is filled with dense white fumes, which are soon absorbed by the water, which rises and partially fills the jar. Introduce now some nitric oxide into the jar while still over the water, and the gas shows no trace of red color. The free oxygen then can be removed from the air by burning phosphorus in it. After all the oxygen has combined with the phosphorus, and the compound formed has been absorbed by the water, the jar is found to be about one-fifth filled with water; showing that about one-fifth of the air is oxygen.

The gas remaining in the jar is found to be almost pure nitrogen.

Besides the oxygen and nitrogen, there is a small quantity of carbonic anhydride (carbonic acid); as may be shown by passing a large volume of air through limewater. There is also watery vapor, which is continually condensed in the form of rain and dew. A trace of ammonia, and of nitric acid, is also found ; and to

these are to be added the exhalations continually rising from the earth. The whole quantity of these minor ingredients, with the exception of watery vapor, does not amount to more than one part in a thousand. The proportions of oxygen and nitrogenoare almost invariable, while those of the other ingredients are continually fluctuating.

It must be borne in mind that the atmosphere is not a chemical compound, but merely a mixture of these dif

“Indeed, we may regard the globe as surrounded by at least three separate atmospheres, - one of oxygen, one of nitrogen, and one of aqueous vapor, — all existing simultaneously in the same space, yet each entirely distinct from the other two, and only very slightly influenced by their presence.”

The following Tables are from Miller:

ferent gases.

Composition of the Atmosphere.
Oxygen

20.61
Nitrogen.

77.95 Carbonic Acid .

.04 Watery Vapor (average)

1.40

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Composition in Tons.
Oxygen

1,233,010 billions of tons.
Nitrogen

3,994,593 Carbonic Acid

5,287 Watery Vapor

54,460

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124. The Products of the Burning of a Candle or Coal-Gas are Carbonic Acid and Water.

When a lighted taper is held to a candle, or to a jet of coal-gas, the latter takes fire. In what does its burning consist?

Invert a bottle for a short time over the candle or gasburner; then remove it, pour in a little lime-water, close the mouth of the bottle with the hand, and shake it. The lime-water becomes milky-white; showing that carbonic acid has been produced by the burning.

If the candle or gas be burnt under a tin funnel connected with a long glass tube (which must be kept cold), moisture will collect on the inside, and, after a short time, will trickle down, and drop from the end of the tube. When potassium is put into water, it burns with a rosecolored flame. If the liquid from the tube be tested with a bit of potassium, it proves to be water, which is therefore another product of the burning.

Fig. 18.

No other substance is produced, except in very minute quantities, by the burning of the candle or the

gas. 125. The Candle, in Burning, removes Oxygen from the Air. --Arrange a candle, so that it can be covered with a bell-jar, over the water-trough; then light it, and cover it with the jar. It soon ceases to burn, and the water rises in the jar. The burning, then, removes something from the air.

Put a lighted candle into air from which the oxygen has been removed, and it is at once extinguished; showing that it cannot burn without oxygen. It must then be oxygen which it removes from the air when it burns.

126. All ordinary Combustion consists in the Combination of the Oxygen of the Air with the burning Substance. - We have seen that carbonic acid, CO2, and water, H,0, are the products of the burning of a candle. Of the three elements in these products, the O, as we have seen, comes from the air; the C and H exist in the candle.

The force, then, which causes the candle to burn, is affinity, and the burning is the combination of the oxygen of the air with the elements of the candle.

Experiments similar to those which we have tried with the candle, will show that any ordinary combustible substance removes oxygen from the air when it burns, and that it cannot burn without oxygen. We conclude, then, that all ordinary combustion is the combination of the oxygen of the air with the burning body. 127. Why a Draft is necessary in Stoves and Fur

- We see now why our stoves and furnaces must have a draft. As fast as the oxygen is taken from the air by the burning fuel, this air must be removed, and a fresh supply must take its place. In other words, a stream of air must be kept constantly flowing over or through the fuel.

We see, also, how the fire can be regulated by means of the draft. If the doors or dampers through which the air is admitted be partially closed, the supply of air will be diminished, and the burning will therefore be retarded.

128. Combustibles and Supporters of Combustion. — Any substance, as coal-gas, which can be made to burn, is called a combustible; while any substance, as air or oxygen, in which it can burn, is called a supporter of combustion. These terms are convenient, though, strictly speaking, the one substance is no more a combustible or a supporter of combustion than the other. Since the

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Fig. 19

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burning of coal-gas consists in the combination of the gas with oxygen, the oxygen in reality burns, as well as the gas; and, on the other hand, the gas is as much a supporter of the combustion as the oxygen. The burning must of course take place where the gases come together. A jet of oxygen would appear to burn in an atmosphere of coal-gas, just as a jet of coal-gas appears to burn in an atmosphere of oxygen.

Fit a cork to one end of a lamp chimney, and let the tip of a gas-burner pass through it, as represented in the figure. Allow the gas to escape for some time, and then light it at the top of the chimney. It will burn quietly, and the chimney will evidently be filled with coal-gas. Fill a gas-bag with oxygen, and fasten to the bag a bent glass tube drawn out into a fine jet. Force the oxygen through the tube in a gentle stream, and introduce the end of the tube through the flame into the chimney. As it passes the flame, the oxygen takes fire, and burns brightly in the coal-gas; the oxygen apparently becoming the combustible body, and the coalgas the supporter of combustion.

In both the flames which we have here, it will be seen that gases are burning where they come together ; - the coal-gas and the oxygen of the air, where they meet at the top of the chimney; the oxygen from the bag and the coal-gas, where they meet at the end of the tube inside the chimney.

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