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 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.

—Facial expression in mouth breathing, and breathing through the nose.

Reasons for Breathing through the Nose (Fig. 83).—(1) The many blood vessels in the mucuous membrane lining the nasal passages so heat the air that it does not irritate the bronchial tubes. (2) The hairs in the nostrils strain the air and catch dust; the cilia of the nasal passages also do this. (3) A mouth-breather often swallows food before chewing it sufficiently, because he cannot hold his breath longer. (4) The nasal mucous membrane of an habitual mouth-breather dries and shrinks and obstructs the circulation, bringing on catarrh of the nose. (5) Mouth breathing causes an unpleasant expression of countenance (see Fig. 84). (6) The