and of some general descriptions. In explaining the
circulation of the blood, for instance, it is most natural for us to
follow the course which the blood takes in circulating through the body.
In explaining the manufacture of articles we naturally begin with the
material as it comes to the factory, and trace the process of manufacture
in order through its successive stages.

In other kinds of exposition a coherent arrangement is somewhat difficult.
We should not, however, fail to pay attention to it. A clear understanding
of the subject, on the part of the listener, depends largely upon the
proper arrangement of topics. As you study examples of expositions of some
length, you will notice that there are topics which naturally belong
together. These topics form groups, and the groups are treated separately.
If the expositions are good ones, the related facts will not only be
united into groups, but the groups will also be so arranged and the
transition from one group to another be so naturally made that it will
cause no confusion.

In brief explanations of but one paragraph there should be but one group
of facts. Even these facts need to be so arranged as to make the whole
idea clear. The writer may have a clear understanding of the whole idea,
but in order to give the reader the same clear understanding, certain
facts must be presented before others are. In order to make an explanation
clear, the facts must be so arranged that those which are necessary to the
understanding of others shall come first.

Examine the following expositions as to the grouping of related facts and
the arrangement of those groups:--

Fresh, pure air at all times is essential to bodily comfort and good
health. Air may become impure from many causes. Poisonous gases may be
mixed with it; sewer gas is especially to be guarded against; coal gas
which is used for illuminating purposes is very poisonous and dangerous if
inhaled; the air arising from decaying substances, foul cellars, or
stagnant pools, is impure and unhealthy, and breeds diseases; the foul and
poisonous air which has been expelled from the lungs, if breathed again,
will cause many distressing symptoms. Ventilation has for its object the
removal of impure air and the supplying of fresh, wholesome air in its
place. Proper ventilation should be secured in all rooms and buildings,
and its importance cannot be overestimated.

In the summer time and in climates which permit of it wit