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.