TABLE I
Percentage Distribution of Gases in the Atmosphere
| Height in Kilo- meters. | Gases. | Total Pressure in Millimeters. | ||||||
|---|---|---|---|---|---|---|---|---|
| Argon. | Nitro- gen. | Water Vapor. | Oxygen. | Carbon Dioxide. | Hydro- gen. | He- lium. | ||
150 |
|
|
|
|
| 99.73 | 0.27 | 0.0043 |
140 |
|
|
|
|
| 99.70 | 0.30 | 0.0048 |
130 |
| 0.02 |
|
|
| 99.64 | 0.34 | 0.0054 |
120 |
| 0.10 |
|
|
| 99.52 | 0.38 | 0.0060 |
110 |
| 0.40 |
| 0.02 |
| 99.16 | 0.42 | 0.0067 |
100 |
| 1.63 |
| 0.07 |
| 97.84 | 0.46 | 0.0076 |
90 |
| 6.57 |
| 0.32 |
| 92.62 | 0.49 | 0.0090 |
80 |
| 22.70 |
| 1.38 |
| 75.47 | 0.45 | 0.0123 |
70 | 0.02 | 53.73 |
| 4.0 |
| 41.95 | 0.27 | 0.0248 |
60 | 0.04 | 78.16 |
| 7.32 |
| 14.33 | 0.15 | 0.0810 |
50 | 0.08 | 86.16 |
| 10.01 |
| 3.72 | 0.03 | 0.466 |
40 | 0.16 | 86.51 |
| 12.45 |
| 0.88 |
| 1.65 |
30 | 0.22 | 84.48 |
| 15.10 |
| 0.20 |
| 8.04 |
20 | 0.55 | 81.34 |
| 18.05 | 0.01 | 0.05 |
| 39.6 |
15 | 0.74 | 79.56 |
| 19.66 | 0.02 | 0.02 |
| 88.2 |
11 | 0.94 | 78.02 | 0.01 | 20.99 | 0.03 | 0.01 |
| 168 |
5 | 0.94 | 77.89 | 0.18 | 20.95 | 0.03 | 0.01 |
| 405 |
0 | 0.93 | 77.08 | 1.20 | 20.75 | 0.03 | 0.01 |
| 760 |
Fixing attention first upon the gases other than water, it will be at once observed from the table that these gases show a very uniform mixture in the moist and turbulent region, while farther aloft the lighter of them tend to predominate in relative proportion. This uniformity of composition at the lower levels, which accords with experience, is due to the constant circulation and turmoil in that region. But for this constant agitation, the uniformity of mixture could not last. If the atmosphere were perpetually at rest throughout, or moving only in horizontal flow, each constituent gas would assume the same status and distribution as if the others were absent. Each, therefore, obeying Dalton’s law of diffusion, would form an atmosphere of itself, independent of the others, and unaffected in density by them. Such a condition is assumed for the higher levels. The percentage distribution in the higher levels is calculated from the known elasticity and density of the gases, assumed as resting in perpetual calm at a constant temperature of .55° C. beyond eleven kilometers, or above the highest ascent of man, and, furthermore, as having at the earth’s surface 1.2 per cent moisture and a temperature of 11° C.
But only in the quiescent outersphere can that dynamic gradation be established or perpetuated. Below this lofty region is the sea of water vapor, mingled intimately with the dry air, and churned with it, yet not sharing its uniformity of distribution. Why this rapid diminution of moisture with elevation, as shown in the table? Because throughout the moist region the temperature falls rapidly—about 6° C. per kilometer ascent above the earth—thus chilling and precipitating the vapor, whose pressural resistance to liquefaction diminishes with waning temperature. The explanation is obvious; but why does it not apply as well to the other elements of the atmosphere: why do not the other gases present liquefy with falling temperature as well as the water vapor, which is merely water in the gaseous state? The question cannot be answered very profoundly, but an essential condition of liquefaction of any gas can be stated in learned phraseology, after the preliminary exposition of certain general properties of matter.
We may first set forth those general physical properties, then apply them to answering the above question. Every known substance may exist in either of three states, the solid, liquid or gaseous. For every substance there is a critical temperature above which it can exist only as a gas, and cannot be liquefied by any pressure, but below which a suitable pressure will cause liquefaction. Below its critical temperature a gas is called a vapor, above it a permanent gas. Now in the free atmosphere some of the gases are never below their critical temperatures and, therefore, cannot be liquefied by any pressure, without special cooling; others are sometimes below their critical temperatures and are then capable of liquefaction by sufficient pressure, which however is not always found in free space, but can be supplied by a compression pump; one other gas, that is water vapor, is always below its critical temperature in the free atmosphere, and therefore may always be turned into water by sufficient pressure at its actual atmospheric temperature. Such sufficient pressure in the water vapor actually occurs from time to time in all parts of the atmosphere from the earth’s surface to the highest cirrus region, but more frequently in the nimbus levels, a mile or two above the earth. Thus at all parts of the lower atmosphere liquefaction of aqueous vapor is sometimes observed, either as mist or rain, snow or ice particles, and on the earth as dew or frost. In order to illustrate the above ideas by numerical citation, the accompanying table is given, showing the critical temperature and pressure of the chief gaseous constituents of the atmosphere.
TABLE II
Critical Temperature and Corresponding Pressure of Liquefaction for the Chief Constituent Gases of the Atmosphere.
| Substance | Critical Temperature C. | Critical Pressure Atmospheres. |
|---|---|---|
| Dry Air | −140 | 39 |
| Nitrogen | −146 | 34 |
| Oxygen | −118 | 50 |
| Carbonic Acid | − 31 | 75 |
| Argon | −120 | 51 |
| Hydrogen | −242 | 20 |
| Ammonia | 130 | 115 |
| Water | +365 | 200 |
A glance at this table shows that for the pressures and temperatures prevailing in our atmosphere most of the constituents are permanent gases. The conspicuous exception is water which, when in the gaseous state, always exists as a vapor, and never as a permanent gas, since it never even approaches the critical temperature. Fortunately for all life on earth the aqueous vapor condenses at very ordinary temperatures and pressures, else there would be no rainfall for irrigation and drinking. Fortunately also the other gases do not so precipitate, else the world might be flooded with liquid nitrogen and oxygen, entailing who knows what disastrous consequences.