CHAPTER XIII.

COMMUNICATION OF HEAT.

1.--CONDUCTION.

Q. _How is HEAT COMMUNICATED from one body to another?_

A. 1. By Conduction. 2. By Absorption. 3. By Reflection. 4. By Radiation: and 5. By Convection.

Q. _What is meant by CONDUCTION of heat?_

A. Heat communicated from one body to another, _by actual contact_.

Q. _Why does a PIECE of WOOD (blazing at ONE end) NOT FEEL HOT at the OTHER end?_

A. _Wood is a bad conductor of heat_; and, therefore, heat does not traverse freely through it: hence, though one end of a stick be blazing-hot, the other end may be quite cold.

Q. _Why do SOME THINGS feel so much COLDER than others?_

A. Princ.i.p.ally because _they are better conductors_; and, therefore, draw off the heat from our body (which touches them) so much faster.

Q. _What are the BEST CONDUCTORS of HEAT?_

A. _Dense solid bodies_, such as metal and stone.

Q. _Which METALS are the most RAPID CONDUCTORS of HEAT?_

A. _Silver_ is the best conductor, then _copper_, then _gold_ or _tin_, then _iron_, then _zinc_, and then _lead_.

Q. _What are the WORST CONDUCTORS of HEAT?_

A. All _light and porous bodies_, such as hair, fur, wool, charcoal, and so on.

Q. _Why are COOKING VESSELS so often furnished with WOODEN HANDLES?_

A. Wood is _not a good conductor, like metal_; and, therefore, many vessels (which are exposed to the heat of the fire) _have wooden handles, lest they should burn our hands_ when we take hold of them.

Q. _Why is the HANDLE OF A METAL TEA-POT made of WOOD?_

A. As _wood is a bad conductor_, the heat of the boiling water is _not so quickly conveyed to the wooden handle_, nor so quickly _poured into the hand_ by it, as when the handle is made of metal.

Q. _Why would a METAL HANDLE BURN the HAND of the tea-maker?_

A. As metal is an _excellent conductor_, the heat of the boiling water _rushes quickly into the metal handle_, and _into the hand that touches it_.

Q. _How do you know that a METAL HANDLE would be HOTTER than a WOODEN one?_

A. By _touching the metal collar_ into which the wooden handle is fixed: though the _wooden handle is quite cold_, this _metal collar is intensely hot_.

Q. _Why do persons use paper or WOOLLEN KETTLE-HOLDERS to take hold of a kettle with?_

A. Paper and woollen are both very _bad conductors of heat_; and, therefore, the heat of the kettle does _not readily pa.s.s through them to the hand_.

Q. _Does the heat of the boiling kettle NEVER get through the woollen or paper kettle-holder?_

A. Yes; but though the kettle-holder became as hot as the kettle itself, it would never _feel_ so hot.

Q. _Why would not the kettle-holder FEEL so hot as the kettle, when it really is of the same temperature?_

A. Because (being a very _bad_ conductor) _it disposes of its heat so slowly_, that it is _scarcely perceptible_; but metal (being an _excellent_ conductor) disposes of its heat so _quickly_, that the sudden influx is painful.

Q. _Why then does HOT METAL feel so much MORE intensely WARM than HOT WOOL?_

A. Because it gives out a much _greater quant.i.ty of heat in the same s.p.a.ce of time_; and the _influx_ of heat is, therefore, _more perceptible_.

Q. _Why does MONEY in our pocket feel so HOT, when we stand BEFORE a FIRE?_

A. Metal is an _excellent conductor_; and, therefore, becomes rapidly heated. For the same reason it becomes _rapidly cold_, when it comes in contact with a body _colder than itself_.

Q. _Why does a PUMP-HANDLE feel intensely COLD in WINTER?_

A. As metal is an _excellent conductor_, when the hot hand touches the cold pump-handle, the heat pa.s.ses rapidly _from the hand into the iron_; and this rapid loss of heat produces a sensation of intense coldness.

Q. _Is the iron HANDLE of the pump really COLDER than the wooden PUMP itself?_

A. No; every inanimate substance (exposed to the same temperature) possesses the _same degree of heat_.