~Atmosphere and air.~ The term _atmosphere_ is applied to the gaseous envelope surrounding the earth. The term _air_ is generally applied to a limited portion of this envelope, although the two words are often used interchangeably. Many references have already been made to the composition and properties of the atmosphere. These statements must now be collected and discussed somewhat more in detail. ~Air formerly regarded as an element.~ Like water, air was at first regarded as elementary in character. Near the close of the eighteenth century Scheele, Priestley, and Lavoisier showed by their experiments that it is a mixture of at least two gases,--those which we now call oxygen and nitrogen. By burning substances in an inclosed volume of air and noting the contraction in volume due to the removal of the oxygen, they were able to determine with some accuracy the relative volumes of oxygen and nitrogen present in the air. ~The constituents of the atmosphere.~ The constituents of the atmosphere may be divided into two general groups: those which are essential to life and those which are not essential. 1. _Constituents essential to life._ In addition to oxygen and nitrogen at least two other substances, namely, carbon dioxide and water vapor, must be present in the atmosphere in order that life may exist. The former of these is a gaseous compound of carbon and oxygen having the formula CO_{2}. Its properties will be discussed in detail in the chapter on the compounds of carbon. Its presence in the air may be shown by causing the air to bubble through a solution of calcium hydroxide (Ca(OH)_{2}), commonly called lime water. The carbon dioxide combines with the calcium hydroxide in accordance with the following equation: Ca(OH)_{2} + CO_{2} = CaCO_{3} + H_{2}O. The resulting calcium carbonate (CaCO_{3}) is insoluble in water and separates in the form of a white powder, which causes the solution to appear milky. The presence of water vapor is readily shown by its condensation on cold objects as well as by the fact that a bit of calcium chloride when exposed to the air becomes moist, and may even dissolve in the water absorbed from the air. 2. _Constituents not essential to life._ In addition to the essential constituents, the air contains small percentages of various other gases, the presence of which so far as is known is not essential to life. This list includes the rare elements, argon, helium, neon, krypton, and xenon; also hydrogen, ammonia, hydrogen dioxide, and probably ozone. Certain minute forms of life (germs) are also present, the decay of organic matter being due to their presence.