CHAPTER XIV.

2.--ABSORPTION OF HEAT.

Q. _What is the difference between CONDUCTING heat, and ABSORBING heat?_

A. To _conduct_ heat, is to _transmit it from one body to another_ through a conducting medium: to _absorb_ heat, is to _suck it up_, as a sponge sucks up water.

Q. _Give me an example._

A. _Black cloth absorbs_, but does not _conduct heat_: thus, if black cloth be laid in the sun, _it will absorb the rays_ very rapidly; but if _one end of the black cloth_ be made hot, it would not _conduct the heat_ to the _other_ end.

Q. _Are good CONDUCTORS of heat, good ABSORBERS also?_

A. No; every _good conductor of heat_ is a _bad absorber of it_; and _no good absorber of heat_ can be a _good conductor_ also.

Q. _Is IRON a good ABSORBER of heat?_

A. No; _iron is a good conductor_, but a very _bad absorber_ of heat.

Q. _Why do the FENDER and FIRE-IRONS (which lie upon it) remain COLD, although they are before a good fire?_

A. Because the metal fender and fire-irons have very _little capacity for absorbing heat_; although they are soon made hot (by conduction), when placed in _contact_ with the hot fire or stove.

Q. _Why does a KETTLE boil faster, when the bottom and back are COVERED with SOOT?_

A. The _black soot absorbs heat_ very quickly from the fire, and the metal _conducts it to the water_.

Q. _Why will not a NEW KETTLE boil so fast as an OLD one?_

A. Because the _bottom and sides_ of a new kettle are _clean and bright_; but in an _old_ kettle _are covered with soot_.

Q. _Why would the KETTLE be SLOWER BOILING, if the BOTTOM and BACK were CLEAN and bright?_

A. _Bright_ metal does _not absorb heat_, but _reflect it_ (i. e. throw the heat _back_ again); and as the heat is _thrown off from the surface of bright metal_, therefore, a new kettle is longer boiling.

Q. _Why do we wear WHITE LINEN and a BLACK outer DRESS, if we want to be warm?_

A. The _black outer dress_ quickly _absorbs heat from the sun_, and conveys it to the body; and the _white linen_ (being a _bad_ absorbent) abstracts no heat from the warm body.

Q. _Why do persons WEAR WHITE dresses in SUMMER time?_

A. White _throws off the heat of the sun by reflection_, and is, therefore, a very bad absorbent of heat; in consequence of which, it never becomes _so hot from the scorching sun_ as dark colours do.

Q. _Why do NOT persons WEAR WHITE dresses in WINTER time?_

A. _White will not absorb heat_, like black and other dark colours; and, therefore, _white_ dresses are _not so warm as dark ones_.

Q. _What COLOURS are WARMEST for dresses?_

A. For _outside_ garments _black is the warmest_, and then such colours as _approach nearest to black_ (as dark blue and green). _White is the coldest colour_ for external clothing.

Q. _Why are DARK COLOURS (for external wear) so much WARMER than LIGHT ONES?_

A. Because _dark colours absorb heat from the sun_ more abundantly than _light_ ones.

Q. _How can you prove that DARK colours are WARMER than LIGHT ones?_

A. If a piece of _black_ cloth and a piece of _white_ were laid upon snow, in a few hours the _black cloth will have melted the snow beneath_; whereas the _white_ cloth will have produced little or _no effect upon it at all_.

N. B. The darker any colour is, the warmer it is, because it is a better absorbent of heat. The order may be thus arranged:--1. Black (warmest of all).--2. Violet.--3. Indigo.--4. Blue.--5. Green.--6. Red.--8. Yellow: and 9. White (coldest of all).

Q. _Why are BLACK KID GLOVES so HOT in summer time?_

A. 1st--Because the _black absorbs the solar heat_: and

2ndly--The _kid_ will not allow the heat of the hand _to escape through the glove_.

Q. _Why are LISLE THREAD GLOVES so COOL in summer time?_

A. 1st--Because thread _absorbs the perspiration of the hands_: and

2ndly--It _conducts away the heat_ of our hot hands.

Q. _Are Lisle thread gloves ABSORBENTS of heat?_

A. As Lisle thread gloves are generally of a _grey or lilac colour_, they do _not absorb solar heat_.

Q. _Why is a PLATE-WARMER made of UN-PAINTED BRIGHT TIN?_

A. Bright tin reflects (or _throws back_) _the heat_, which issues from the fire in rays; and (by reflecting the heat upon the meat) assists greatly in roasting it.

Q. _Why would not the tin REFLECTOR do as well if it were PAINTED?_

A. If the tin reflector were _painted_, it would be utterly spoiled, because it would then _absorb_ heat, and _not reflect it at all_. A plate-warmer should be kept _very clean, bright, and free from all scratches_.

Q. _Why should a REFLECTOR be kept so very CLEAN and free from SCRATCHES?_

A. If a reflector be _spotted, dull, or scratched_, it will _absorb_ heat, instead of _reflecting_ it; and, therefore, would be of no use whatsoever as a _reflector_.

Q. _Why does HOAR-FROST remain on TOMBSTONES, long after it has melted from the GRASS and GRAVEL-WALKS of a church-yard?_

A. Tomb-stones being _white_, will _not absorb heat_, like the darker grass and gravel; and, therefore, _the white tombstones_ (being so much colder) _retain the hoar-frost_ after it has melted from other things.

Q. _If black absorbs heat, why have those who live in HOT climates BLACK SKINS, and not WHITE skins (which would not absorb heat at all)?_

A. Though the black skin of the negro _absorbs heat_ more plentifully than the _white skin of a European_, yet the _blackness_ prevents the sun from _blistering_ or _scorching it_.

Q. _How is it known that the BLACK colour prevents the sun from either BLISTERING or SCORCHING the skin?_

A. If you put a _white glove_ on _one hand_, and a _black glove_ on _the other_ (when the sun is burning hot), the hand with the _white_ glove will be _scorched_, but _not the other_.

Q. _Which hand will FEEL the HOTTER?_

A. The hand with the _black glove_ will _feel_ the _hotter_, but it will not be _scorched_ by the sun; whereas the hand with the _white glove_ (though much _cooler_) will be _severely scorched_.

Q. _Why does the BLACK skin of a NEGRO NEVER SCORCH or BLISTER with the hot sun?_

A. Because the _black colour absorbs_ the heat,--conveys it _below the surface_ of the skin, and converts it to _sensible heat_ and _perspiration._

Q. _Why does the WHITE EUROPEAN SKIN BLISTER and SCORCH when exposed to the hot sun?_

A. Because the _white will not absorb_ the heat; and, therefore, the hot sun _rests on the surface of the skin_, and scorches it.

Q. _Why has a NEGRO BLACK EYES?_

A. The black colour of a negro's eyes defends them from the strong light of the tropical sun. If a negro's eyes were not _black_, the sun would _scorch them_, and every negro would be blind.

Q. _Why is WATER KEPT COOLER (in summer time) in a BRIGHT TIN POT, than in an EARTHEN one?_

A. Because bright metal will _not absorb_ the heat of the summer sun, like an _earthen_ vessel.

Q. _Why is BOILING water KEPT HOT in a BRIGHT TIN VESSEL longer, than in an earthen one?_

A. Because bright tin will not suffer the heat of the boiling water _to escape in rays_, as an earthen vessel does.