The principle of hot-water heating is shown by the following simple experiment.  Two flasks and two tubes are arranged as in Figure 15, the upper flask containing a colored liquid and the lower flask clear water.  If heat is applied to B, one can see at the end of a few seconds the downward circulation of the colored liquid and the upward circulation of the clear water.  If we represent a boiler by B, a radiator by the coiled tube, and a safety tank by C, we shall have a very fair illustration of the principle of a hot-water heating system.  The hot water in the radiators cools and, in cooling, gives up its heat to the rooms and thus warms them.

[Illustration:  FIG. 14.—­Hot-water heating.]

In hot-water heating systems, fresh air is not brought to the rooms, for the radiators are closed pipes containing hot water.  It is largely for this reason that thoughtful people are careful to raise windows at intervals.  Some systems of hot-water heating secure ventilation by confining the radiators to the basement, to which cold air from outside is constantly admitted in such a way that it circulates over the radiators and becomes strongly heated.  This warm fresh air then passes through ordinary flues to the rooms above.

[Illustration:  FIG. 15.—­The principle of hot-water heating.]

In Figure 16, a radiator is shown in a boxlike structure in the cellar.  Fresh air from outside enters a flue at the right, passes the radiator, where it is warmed, and then makes its way to the room through a flue at the left.  The warm air which thus enters the room is thoroughly fresh.  The actual labor involved in furnace heating and in hot-water heating is practically the same, since coal must be fed to the fire, and ashes must be removed; but the hot-water system has the advantage of economy and cleanliness.

[Illustration:  FIG. 16.—­Fresh air from outside circulates over the radiators and then rises into the rooms to be heated.]

11.  Fresh Air.  Fresh air is essential to normal healthy living, and 2000 cubic feet of air per hour is desirable for each individual.  If a gentle breeze is blowing, a barely perceptible opening of a window will give the needed amount, even if there are no additional drafts of fresh air into the room through cracks.  Most houses are so loosely constructed that fresh air enters imperceptibly in many ways, and whether we will or no, we receive some fresh air.  The supply is, however, never sufficient in itself and should not be depended upon alone.  At night, or at any other time when gas lights are required, the need for ventilation increases, because every gas light in a room uses up the same amount of air as four people.

[Illustration:  FIG. 17.—­The air which goes to the schoolrooms is warmed by passage over the radiators.]

In the preceding Section, we learned that many houses heated by hot water are supplied with fresh-air pipes which admit fresh air into separate rooms or into suites of rooms.  In some cases the amount which enters is so great that the air in a room is changed three or four times an hour.  The constant inflow of cold air and exit of warm air necessitates larger radiators and more hot water and hence more coal to heat the larger quantity of water, but the additional expense is more than compensated by the gain in health.