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Pure air is a mechanical mixture of oxygen and nitrogen. While different authorities give slightly varying values for the proportion of oxygen and nitrogen contained, the generally accepted values are: By volume,  oxygen 20.91 per cent,  nitrogen 79.09 per cent. By weight,  oxygen 23.15 per cent,  nitrogen 76.85 per cent. Air in nature always contains other constituents in varying amounts, such as dust, carbon dioxide, ozone and water vapor. Being perfectly elastic, the density or weight per unit of volume decreases in geometric progression with the altitude. This fact has a direct bearing in the proportioning of furnaces, flues and stacks at high altitudes, as will be shown later in the discussion of these subjects. The atmospheric pressures corresponding to various altitudes are given in Table 12 .

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