rcentages of oxygen and nitrogen that the question has arisen, whether
these two elements are not combined in the air, forming a definite
chemical compound. That the two are not combined but are simply mixed
together can be shown in a number of ways, among which are the
following.

1. When air dissolves in water it has been found that the ratio of
oxygen to nitrogen in the dissolved air is no longer 21: 78, but more
nearly 35: 65. If it were a chemical compound, the ratio of oxygen to
nitrogen would not be changed by solution in water.

2. A chemical compound in the form of a liquid has a definite boiling
point. Water, for example, boils at 100 deg.. Moreover the steam which is
thus formed has the same composition as the water. The boiling point of
liquid air, on the other hand, gradually rises as the liquid boils, the
nitrogen escaping first followed by the oxygen. If the two were
combined, they would pass off together in the ratio in which they are
found in the air.

~Why the air has a constant composition.~ If air is a mixture and changes
are constantly taking place which tend to modify its composition, how,
then, do we account for the constancy of composition which the analyses
reveal? This is explained by several facts. (1) The changes which are
caused by the processes of combustion and respiration, on the one hand,
and the action of plants, on the other, tend to equalize each other. (2)
The winds keep the air in constant motion and so prevent local changes.
(3) The volume of the air is so vast and the changes which occur are so
small compared with the total amount of air that they cannot be readily
detected. (4) Finally it must be noted that only air collected in the
open fields shows this constancy in composition. The air in a poorly
ventilated room occupied by a number of people rapidly changes in
composition.

~The properties of the air.~ Inasmuch as air is composed principally of a
mixture of oxygen and nitrogen, which elements have already been
discussed, its properties may be inferred largely from those of the two
gases. One liter weighs 1.2923 g. It is thus 14.38 times as heavy as
hydrogen. At the sea level it exerts an average pressure sufficient to
sustain a column of mercury 760 mm. in height. This is taken as the
standard pressure in determining the volumes of gases as well as the
boiling points of liquids. Water may be made to boil at any temperature
between 0 deg. and considerably above 100 deg. b