=Atmospheric Pressure Variable.= Atmospheric pressure diminishes and is constantly variable, according to the height above the sea-level. If we ascend into the air 5000 feet, it is perfectly evident that there are 5000 feet less of atmosphere pressing upon us than at the point from which we started. This diminution of pressure is often measured by the temperature at which water boils at different heights. =Composition.= An average composition of the atmosphere has been previously stated. Besides nitrogen and oxygen, it always contains water in the form of vapor, and carbonic acid. The amount of aqueous vapor in the air changes according to the temperature; the amount of carbonic acid is also constantly variable. Air usually contains, in addition to these, traces of ammonia, organic matter which includes micro-organisms, ozone, salts of sodium, and other mineral matters in minute and variable quantities. =Air in Motion.= The atmosphere is almost always in motion. We feel it in the gentle breeze and the more forcible wind. If it moves at a slower rate than two and one half feet a second this motion is not noticeable. Motion in the air is caused by the unequal heating of portions of it. If from any cause the atmosphere over a certain region becomes warm, it will expand (all bodies expand with heat), become lighter, and its tendency will be to move in the direction of least resistance,--that is, upward; so we say heated air rises. Currents of cooler air will immediately flow in to take its place, and thus we have a breeze, a wind, or a gale, according to the velocity and force with which the currents move. It is upon a knowledge of these movements that the theory of ventilation is based. It is because of the constant motion of air-currents that out of doors, except in densely populated cities, air remains constantly pure. When poisonous gases and other impurities accumulate, winds scatter them far and wide until they are so diluted as to be harmless; or under some conditions they unite with other things and form new and simple substances of a harmless nature, while under others, if they are compounds, they may be decomposed or washed down to the surface of the earth again. =Impurities.= The chief chemical product of fires and of that slower combustion breathing is carbonic acid. Plants during the day, and under the influence of sunlight, take it up from the air for food, use the carbon for their growth, freeing the oxygen which man and the lower animals need. Thus is the balance most beautifully maintained.