Chapter 66

CHEMISTRY OF THE ATMOSPHERE.

130. Constituents.--The four chief constituents of the atmosphere are N, O, H2O, CO2, in the order of their abundance. What experiments show the presence of N, O, and CO2 in the air? Set a pitcher of ice water in a warm room, and the moisture that collects on the outside is deposited from the air. This shows the presence of H2O. Rain, clouds, fog, and dew prove the same. H2SO4 and CaCl2, on exposure to air, take up water. Experiment 18 shows that there is not far from four times as much N as O by volume in air. Hence if the atmosphere were a compound of N and O, and the proportion of four to one were exact, its symbol would be N4O.

131. Air not a Compound.--The following facts show that air is not a compound, but rather a mixture of these gases.

1. The proportion of N and O in the air, though it does not vary much, is not always exactly the same. This could not be true if it were a compound. Why?

2. If N4O were dissolved in water, the N would be four times the O in volume; but when air is dissolved, less than twice as much N as O is taken up.

3. No heat or condensation takes place when four measures of N are brought in contact with one of O. It cannot then be N4O, for the vapor density of N4O would be 36--i.e. (14 x 4 + 16) / 2; but that of air is 14 1/2 nearly --i.e. (14 x 4 + 16) / 5. Analysis shows about 79 parts of N to 21 parts of O by volume in air.

132. Water.--The volume of H2O, watery vapor, in the atmosphere is very variable. Warm air will hold more than cold, and at any temperature air may be near saturation, i.e. having all it will hold at that temperature, or it may have little. But some is always present; though the hot desert winds of North Africa are not more than 1/15 saturated. A cubic meter of air at 25 degrees, when saturated, contains more than 22 g. of water.

133. Carbon Dioxide.--Carbon dioxide does not make up more than three or four parts in ten thousand of the air; but, in the whole of the atmosphere, this gives a very large aggregate. Why does not CO2 form a layer below the O and N?

134. Other Ingredients.--Other substances are found in the air in minute portions, e.g. NH3 constitutes nearly one-millionth. Air is also impregnated with living and dead germs, dust particles, unburned carbon, etc., but these for the most part are confined to the portion near the earth's surface. In pestilential regions the germs of disease are said sometimes to contaminate the air for miles around.