A Guide To The Scientific Knowledge Of Things Familiar Part 41

A. As the earth is a _bad conductor_, the burning rays of the sun can penetrate only a few inches below the surface; in consequence of which, the _springs of water are not affected_.

Q. _Why is it COOL under a SHADY tree in a hot summer's day?_

A. 1st--Because the overhanging foliage _screens off the rays of the sun_:

2ndly--As the rays of the sun are warded off, _the air_ (beneath the tree) _is not heated by the reflection of the earth_: and

3rdly--The leaves of trees, being _non-conductors_, allow no heat to penetrate through them.

Q. _Why do the LAPLANDERS wear SKINS, with the FUR INWARDS?_

A. The _dry skin_ prevents the _wind from penetrating to their body_; and as the _fur_ contains a _quant.i.ty of air_ between its hairs (which soon _becomes heated by the body_) the Laplander is clad in _a case of hot air, impervious to the cold and wind_.

Q. _Why does a LINEN s.h.i.+RT feel COLDER than a COTTON ONE?_

A. _Linen is a much better conductor_ than cotton; and, therefore, (as soon as it touches the body) _it more rapidly draws away the heat_, and produces a sensation of cold.

Q. _Why is the FACE COOLED by wiping the temples with a fine CAMBRIC HANDKERCHIEF?_

A. The fine fibres of the cambric have a _strong capillary attraction for moisture_; and are _excellent conductors of heat_: thus the moisture and heat are _both abstracted from the face_, and a sensation of coolness is produced.

"Capillary attraction," i. e. _the attraction of a thread or hair_. The wick of a candle is wet with grease, because the melted tallow runs up the cotton from capillary attraction.

Q. _Why would not a COTTON handkerchief do as well?_

A. The coa.r.s.e fibres of cotton have much less capillary attraction, and are _nothing like such good conductors_ as linen: and, therefore, wiping the face with a _cotton handkerchief_, increases the sensation of warmth.

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.