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 with comfort, ventilation may be secured by having the doors and windows open, thus allowing the fresh air to circulate freely through the house.  In stormy and cold weather, however, some other means of ventilation must be supplied.  If open fires or grates are used for heating purposes, good ventilation exists, for under such circumstances, the foul and impure air is drawn out of the rooms through the chimneys, and the fresh air enters through the cracks of the doors and windows.

Where open fireplaces are not used, several plans of ventilation may be used, as they all operate on the same principle.  Two openings should be in the room, one of them near the floor, through which the fresh air may enter, the other higher up, and connected with a shaft or chimney, which producing a draft, may serve to free the room from impure air.  The size of these openings may be regulated according to the size of the room.