iews, it is possible that life
might continue to exist in the absence of the atmospheric
nitrogen, yet the conditions of life would be entirely changed.
Moreover, nitrogen is an essential constituent of all animal
and plant life. It was formerly supposed that neither animals
nor plants could assimilate the free nitrogen, but it has been
shown recently that the plants of at least one natural order,
the Leguminosae, to which belong the beans, peas, and clover,
have the power of directly assimilating the free nitrogen from
the atmosphere. This is accomplished through the agency of
groups of bacteria, which form colonies in little tubercles on
the roots of the plants. These bacteria probably assist in the
absorption of nitrogen by changing the free nitrogen into
compounds which can be assimilated by the plant. Fig. 27 shows
the tubercles on the roots of a variety of bean. (3) The
presence of water vapor in the air is necessary to prevent
excessive evaporation from both plants and animals. (4) Carbon
dioxide is an essential plant food.

[Illustration: Fig. 27]

~The quantitative analysis of air.~ A number of different methods have
been devised for the determination of the percentages of the
constituents present in the atmosphere. Among these are the following.

1. _Determination of oxygen._ (1) The oxygen is withdrawn from a
measured volume of air inclosed in a tube, by means of phosphorus.

To make the determination, a graduated tube is filled with
water and inverted in a vessel of water. Air is introduced into
the tube until it is partially filled with the gas. The volume
of the inclosed air is carefully noted and reduced to standard
conditions. A small piece of phosphorus is attached to a wire
and brought within the tube as shown in Fig. 28. After a few
hours the oxygen in the inclosed air will have combined with
the phosphorus, the water rising to take its place. The
phosphorus is removed and the volume is again noted and reduced
to standard conditions. The contraction in the volume of the
air is equal to the volume of oxygen absorbed.

[Illustration: Fig. 28]

(2) The oxygen may also be estimated by passing a measured volume of air
through a tube containing copper heated to a high temperature. The
oxygen in the air combines with the copper to form copper oxide (CuO).
Hence t