2927. Exercise warms, invigorates, and purifies the body; clothing preserves the warmth the body generates; fire imparts warmth externally.
2928. Therefore, to obtain and preserve warmth, exercise and clothing are preferable to fire.
2929. Fire consumes the oxygen of the air, and produces noxious gases.
2930. Therefore, the air is less pure in the presence of candles, gas, or coal fire, than otherwise; and the deterioration should be repaired by increased ventilation.
2931. The skin is a highly-organized membrane, full of minute pores, cells, blood-vessels, and nerves; it imbibes moisture or throws it off, according to the state of the atmosphere and the temperature of the body. It also "breathes," as do the lungs (though less actively). All the internal organs sympathize with the skin.