A. Because the heat (which should be transmitted) produces evaporation, and flies off in the vapour.
Q. Why does a poker (resting on the fender) feel so much colder than the hearth-rug, which is further off the fire?
A. The poker (being an excellent conductor) draws heat from the hand much more quickly than the rug, which is a bad conductor: and, therefore, (though both are equally warm) the poker seems to be much colder.
Q. Why are hot bricks (wrapped in cloth) employed in cold weather to keep the feet warm?
A. Bricks are bad conductors of heat, and cloth or flannel still worse: therefore a hot brick (wrapped in flannel) will retain its heat a very long time.
Q. Why is a tin pan (filled with hot water) employed as a foot warmer?
A. Because polished tin (being a bad radiator of heat) keeps hot a very long time; and warms the feet resting upon it.
Q. What is meant by being a “bad radiator of heat?”
A. To radiate heat is to throw off heat by rays, as the sun; a polished tin pan does not throw off the heat of boiling water from its surface, but keeps it in.