Niton, or radium emanation, is one of the products of the disintegration of radium. Niton itself disintegrates very rapidly, one-half of any given quantity disappearing in about four days, and one of its products is helium. The amount of niton in the atmosphere is never more than an infinitesimal trace. Thus we are told that the total quantity of this substance present in the atmosphere of the whole earth up to an altitude of one kilometer (0.6 mile) weighs less than nine ounces, and that each cubic centimeter of air contains among its thirty million million million molecules only between one and two molecules of niton, on an average.

Turning, now, to the more abundant constituents of the atmosphere, we find that oxygen and nitrogen differ strikingly from each other in the fact that, while the former has a strong chemical affinity for nearly all other elements, the latter is chemically inert, having little tendency to unite directly with other elements, though by indirect processes, and chiefly through the agency of plants and animals, a large number of nitrogen compounds are produced. Oxides of nitrogen are formed directly from the atmospheric gases by lightning discharges, and these unite with the moisture of the air to form nitric and nitrous acids. A certain amount of ammonia (a compound of nitrogen and hydrogen) may also be formed by lightning from nitrogen and atmospheric water, but most of the ammonia in the air is derived from the decomposition of plant and animal matters. The compounds of nitrogen that occur in the air are washed down by rain in considerable quantities. Analyses of rain water made in different parts of the world show from one to nine pounds of such substances per acre per annum.

Carbon dioxide (more familiarly known as carbonic acid gas) occurs in the atmosphere in the almost constant proportion of three parts in 10,000 by volume. It is a little more abundant in the air of towns than in the open country or over the ocean, and it undergoes slight periodic variations, but the fact that it is not much more variable is rather surprising, considering that it is continually being added to and abstracted from the air by numerous agencies that have no dependence upon one another. It is supplied to the air by volcanoes, mineral springs, the combustion of fuel, the respiration of animals and plants, and the decay of organic matter. The amount supplied annually by the burning of coal alone is estimated to be equivalent to more than one-thousandth of the total volume of the gas present in the atmosphere at any one time. On the other hand, all green plants, in the presence of sunlight, withdraw carbon dioxide from the air, abstract the carbon from it for the use of the plant, and return the oxygen to the atmosphere. Thus it is estimated that an acre of beech forest takes a ton of carbon out of the air annually. A vast amount of atmospheric carbon dioxide enters into chemical combination with certain rocks at the earth’s surface. Lastly, a large quota of this atmospheric gas is absorbed by sea water, and certain authorities have seen in this process a regulator of the total amount in the atmosphere, the hypothesis being that the ocean gives back some of the carbon dioxide whenever this substance becomes deficient in the air.

Water vapor—i. e., water in an invisible gaseous form—is always present in the atmosphere, but its amount is subject to wide fluctuations. An important fact in this connection is that, at any given temperature, the air can hold only a definite amount of this vapor. This maximum amount increases rapidly with temperature. When the air is fully charged with water vapor it is said to be “saturated.” Properly speaking, the temperature limits the amount of the vapor that can occur in a given space, regardless of the presence of the other constituents of air, and in scientific language it is the vapor itself that is said to be saturated, and not the air; but in a popular book about the atmosphere, where much has to be said about atmospheric water vapor, adherence to scientific usage in this matter invariably leads to awkward complications. Speaking, then, in familiar terms—when the air is saturated with water vapor, a fall in temperature causes some of the vapor to condense in visible form, as cloud, fog, rain, dew, snow, hail, etc. As the sole source of these various forms of moisture, and on account of the important part it plays in many atmospheric processes, water vapor is, from a meteorological point of view, the most interesting constituent of the atmosphere.

One more atmospheric gas requires notice here, both on account of the great popular interest attaching to it, and because of recent scientific discoveries concerning it—viz., ozone. This substance may be described, in nontechnical language, as a concentrated form of oxygen. It is one of the most powerful oxidizing agencies known, and has found useful applications in medicine and various industries. Its popular renown, however, is due to the fact that for many years it was regarded as a great natural purifier of the atmosphere. “Life-giving ozone” was reputed to be abundant in the air of forests, mountains, and the seashore. Systematic observations were made of the prevalence of ozone at different places throughout the world, generally by noting the change of color of test-papers exposed to the air. These “ozonometric” observations are now a closed chapter in the history of meteorology, for it has been found that the reactions of so-called ozone papers are due chiefly or entirely to atmospheric substances other than ozone. Moreover, direct examination of the air by more accurate methods—including samples collected with the aid of kites and balloons up to a height of several thousand feet above the earth—shows that the amount of ozone in the whole of the lower atmosphere is exceedingly small—much too small to be of hygienic significance. Whatever ozone is produced from oxygen at such levels by lightning discharges or other possible agencies probably enters promptly into chemical union with oxidizable substances and therefore has only a brief existence.

On the other hand, the spectroscope has brought us evidence that far aloft in the atmosphere, many miles above the earth, ozone is quite abundant. Here it is supposed to be generated by two agencies—the electrical discharges of the aurora and ultra-violet radiations from the sun. The ultra-violet rays that help to produce it are prevented from reaching the earth, and astronomers are thus deprived of much interesting information they might otherwise obtain concerning the spectra of the sun and stars. However, as the present Lord Rayleigh has pointed out, we can console ourselves for this fact by reflecting that if the ozone did not shut off much of the ultra-violet light from the sun, this light would probably ruin our eyesight; or, rather, we should be put to the inconvenience of constantly wearing some sort of protective spectacles in the daytime.

The high-level ozone is further interesting because of exercising a certain control over the temperature of the lower air. It is more transparent for incoming solar radiation than for outgoing earth radiation. Hence, when it is unusually abundant, it should raise the general temperature of the earth. This presumably happens when the condition of the sun is such that an unusual amount of ultra-violet radiation reaches the upper atmosphere, a fact that must be taken into consideration in any attempt to establish a relation between climatic fluctuations and the sun-spot period.

The lowest part of our atmosphere is the densest because it is compressed by the weight of the air above it. Thus it happens that, although the atmosphere is at least several hundred miles in height,