A Guide To The Scientific Knowledge Of Things Familiar Part 43

Q. _What is meant by REFLECTING HEAT?_

A. To reflect heat, is _to throw it back in rays_ from the surface of the reflecting body, towards the place from whence it came.

Q. _What are the BEST REFLECTORS of heat?_

A. All _bright_ surfaces, and _light colours_.

Q. _Are GOOD ABSORBERS of heat GOOD REFLECTORS also?_

A. No; those things _which absorb heat best, reflect_ heat _worst_; and those _which reflect heat worst, absorb_ it _best_.

Q. _Why are those things which ABSORB HEAT unable to REFLECT it?_

A. Because if any thing _sucks in heat_ like a sponge, it cannot _throw it off_ from its surface; and if any thing _throws off heat_ from its surface, it cannot _drink it in_.

Q. _Why are REFLECTORS always made of LIGHT-COLOURED and highly POLISHED METAL?_

A. Because _light_ coloured and _highly polished metal_ makes the best of all reflectors.

Q. _Why do not PLATE-WARMERS BLISTER and scorch the WOOD behind?_

A. Because the bright tin front _throws the heat of the fire back again_, and will not allow it to penetrate to the wood behind.

Q. _If metal be such an excellent CONDUCTOR of heat, how can it REFLECT heat, or throw it off?_

A. Polished metal is a _conductor of heat_, only when _that heat is communicated by actual contact_; but whenever heat _falls upon bright metal in rays_, it is _reflected back again_, and the metal remains _quite cool_.

Q. _What is meant by "heat falling upon metal IN RAYS," and not "by contact"?_

A. If a piece of tin were thrust _into_ a fire, it would be _in actual contact with the fire_; but if it be _held before a fire_, the heat of the fire _falls upon it in rays_.

Q. _What is the use of the TIN SCREEN or REFLECTOR used in ROASTING?_

A. The tin reflector _throws the heat of the fire back upon the meat_; and, therefore, a.s.sists the _process of roasting_ and helps _to keep the kitchen cool_.

Q. _How does a tin REFLECTOR tend to keep the KITCHEN COOL?_

A. Because it _confines the heat to the hearth_, and prevents it from being dispersed throughout the kitchen.

Q. _Why does a LAMP GLa.s.s DIMINISH the SMOKE of a LAMP?_

A. As _gla.s.s is a reflector_, it reflects the heat of the lamp _back upon the flame_; in consequence of which, _less carbon escapes unconsumed_ (as smoke).

Q. _Why are SHOES HOTTER for being DUSTY?_

A. 1st--Because dust absorbs heat: and

2ndly--As it destroys the _blackness of our shoes_, it prevents them from _throwing off the heat of our feet in rays_.

Q. _Why can we not SEE into the ROAD or STREET, when a CANDLE is lighted in a room?_

A. _Gla.s.s is a reflector_; and, therefore, throws the rays of the candle _back into the room_, and thus prevents our seeing into the road or street.

Q. _Why can persons in the DARK STREET see into a ROOM (lighted by a candle or lamp)?_

A. The pupil of the eye _expands greatly_, when persons are in the dark; and, therefore, when any one in the dark street looks into a light room, _his dilated pupil_ sees every thing distinctly.

Q. _Why does it always FREEZE on the TOP of a MOUNTAIN?_

A. Air is heated _by the reflection of the earth_, and not by the rays of the sun; and, as there is no earth round a mountain-top _to reflect heat_, therefore, it remains intensely cold.

CHAPTER XVI.

4.--RADIATION.

Q. _What is meant by RADIATION?_

A. Radiation means _the emission of rays_: thus the sun radiates both light and heat; that is, it emits _rays of light and heat_ in all directions.

Q. _When is heat RADIATED from one body to another?_

A. When the two bodies are _separated by a non-conducting medium_: thus the sun _radiates_ heat towards the earth, because the _air comes between_ (which is a very bad conductor).

Q. _On WHAT does RADIATION DEPEND?_

A. On the _roughness_ of the radiating surface: thus if metal be _scratched_, its radiating power is increased, because the _heat has more points to escape from_.

Q. _Does a FIRE RADIATE heat?_