=Ventilation= presents no difficulties in the summer time or in warm climates. The reason that it is a difficult question in cold weather is because the air furnished must be not only pure, but warm. To keep cold air out often means to keep foul air in. _Heating with hot air_, by which system pure air is passed over a furnace, and fresh air constantly admitted, may be a good method (Figs. 80, 81), but is often a dismal failure because it dries out the air, which in turn dries out the skin. To prevent this, wide vessels of water should be set at the inlets. Dry air is cooling. Why? Dr. Barnes proved that moist air at 65° is as comfortable as dry air at 71°. Air saturated with vapor at 60° will _only be 50 per cent saturated at 80°_. Such air dries out the mucous membrane of eyes, nose, and throat. Heating by _hot water_ circulating in pipes, or _by steam_, gives no means of introducing fresh air, and is likely to cause worse ventilation than any other method. The radiators should stand close to windows or other fresh-air inlet, that the air may be heated as it enters, and the outlet for air should be farthest from the radiators. The same rules apply to heating by _stoves_. An oil stove for heating is an inconceivable iniquity to any but a person densely ignorant of hygiene. Heating by _fireplaces_ (Fig. 82) is the most healthful of all methods, for there is a constant removal of air through the chimney, and this air will be replaced; even if all doors and windows are closed, it will come in through tiny cracks. _Radiant heat_ travels in straight lines from a fireplace and _warms solid objects_, but not the air passed through. Hence an open fire will keep the body warm with the room at a low temperature. Fireplaces, however, do not afford sufficient heat in severe climates. _Stoves_ are not as healthful as fireplaces, for there is not so much air removed through the pipe as through the chimney. _Carbon monoxid_, unlike carbon dioxid, is an _active_ poison causing the blood corpuscles to shrivel. It passes through red-hot iron or a cracked stove or furnace. [Illustration: FIG. 83.--BLACKBOARD SKETCH.] [Illustration: FIG. 84.--Facial expression in mouth breathing, and breathing through the nose.]