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.