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CHAP. III.] to one sitting by it; but the heat of a poker held in the hand is conducted along the poker from the end held in the fire to that held in the hand.

Although all substances conduct heat, they do so with different rapidity; those that conduct it quickly are called conductors, while those through which its passage is slow are called non-conductors. For example, metals are good conductors; but wood, ivory, and animal substances, such as wool or hair, are bad conductors, or, as they are generally called, non-conductors. It is owing to this fact that substances, which are really of the same temperature, may appear to the touch to be of quite different degrees of warmth. A bit of wood, a piece of cloth, and a stone, lying side by side, will, according to the facts just mentioned, be of the same temperature, but the cloth will feel warmest, the stone coldest, because the cloth does not rapidly conduct away the heat of the hand, whereas the stone does.

Similarly air, although of the same temperature, as shown by the thermometer, will feel colder or warmer to us according to whether it is in motion or still; because when in motion it removes heat from our bodies quicker than when at rest, since more of it passes over our surface. Wind, as we know, is air in motion, and we feel colder on a windy than on a calm day, although the thermometer may register the same degree. Nevertheless, air is a bad conductor of heat: compare it, for instance, with water, which scalds at 150° Fahr.; whereas a man in a Turkish bath can without