A Guide To The Scientific Knowledge Of Things Familiar Part 40

Similar evils would be felt also by all the animal and vegetable world.

Q. _Does not the bad conducting power of air enable persons to judge whether an EGG be NEW or STALE?_

A. Yes; touch your tongue against the sh.e.l.l at the larger end; if it _feels warm_ to the tongue, the _egg is stale_; if _not_, it is new-laid.

Q. _Why will the Sh.e.l.l of a STALE EGG feel WARM to the tongue?_

A. Between the sh.e.l.l and the "white of the egg" _there is a small quant.i.ty of air_, which _expands in a stale egg_, from the _shrinking of the white_.

Q. _Why does the expansion of air (at the end of an egg) make it feel WARM to the tongue?_

A. As air is a very bad conductor, the _more air an egg contains_, the _less heat will be drawn from the tongue_ when it touches the sh.e.l.l.

Q. _Why do ladies FAN themselves in summer, to make their FACES COOL?_

A. The fan _puts the air in motion_, and makes it pa.s.s more _rapidly over their face_; and (as the temperature of the _air is always lower_ than that of the human _face_) each puff of air _carries off some portion of heat_ from the face.

Q. _Does FANNING the air make the AIR itself COOLER?_

A. No; fanning makes the _air hotter and hotter_.

Q. _Why does FANNING the air increase its HEAT?_

A. By causing the air continually to _absorb heat from the human body_ which it pa.s.ses over.

Q. _If fanning makes the AIR HOTTER, how can it make a PERSON feel COOLER?_

A. Fanning makes the _air hotter_, but the _face cooler_; because it keeps _taking the heat out of the face_, and _giving it to the air_.

Q. _Why is BROTH COOLED by BLOWING it?_

A. The breath causes a rapid _change of air_ to pa.s.s over the broth; and (as the air is not so hot as the broth) _it keeps absorbing heat_, and thus makes the broth cooler and cooler.

Q. _Would not the air absorb heat from the broth just as well WITHOUT BLOWING?_

A. No; _air is a very bad conductor_; unless, therefore, _the change be rapid_, the air nearest the surface of the broth _would soon become as hot as the broth itself_.

Q. _But would not the hot air PART with its heat instantly to the CIRc.u.mJACENT air?_

A. No; not instantly. Air is so bad a conductor, _that it parts with its heat very slowly_: unless, therefore, the air be kept in _continual motion_, it would _cool the broth very slowly indeed_.

Q. _Why does WIND generally feel COOL?_

A. Wind is only air in motion; and the more quickly the _air pa.s.ses over our body_, the more rapidly it _absorbs the heat_ therefrom.

Q. _Why does AIR ABSORB heat more QUICKLY by being set in MOTION?_

A. Because every fresh gust of air _absorbs a fresh portion of heat_; and the more rapid the _succession of gusts_, the greater will be the quant.i.ty of air absorbed.

Q. _If the AIR were HOTTER than our body, would the WIND feel COOL?_

A. No; if the air were _hotter than our body_, it would feel _insufferably hot_.

Q. _Why would the AIR feel INTENSELY HOT, if it were WARMER than our BLOOD?_

A. Because then the wind would _add to the heat of_ our body, instead of _diminis.h.i.+ng it_.

Q. _Is the AIR EVER as HOT as the human BODY?_

A. Not in _this_ country: in the hottest summer's day, the air is always 10 or 12 _degrees cooler than the human body_.

Q. _Is the EARTH a GOOD CONDUCTOR of heat?_

A. No; the power of _conducting_ heat depends upon the _continuity of matter_; if the particles of which a thing is composed are not _continuous_, they have very little power to _conduct heat_.

Q. _Why is the earth (BELOW the SURFACE) WARMER in WINTER than the surface itself?_

A. Because the earth is a _bad conductor of heat_; and, therefore (although the ground be frozen) the frost never penetrates _above an inch or two below the surface_.

Q. _Why is the earth (BELOW the SURFACE) COOLER in SUMMER than the surface itself?_

A. Because the earth is a _bad conductor of heat_; and, therefore, (although the surface be scorched with the burning sun) the intense heat cannot penetrate to _the roots_ of the plants and trees.

Q. _Shew the WISDOM of G.o.d in making the EARTH a BAD CONDUCTOR._

A. If the _heat and cold could penetrate the earth_ (as freely as the heat of a fire penetrates iron), the springs would be dried up in summer and frozen in winter, and all vegetation would perish.

Q. _Why is WATER from a SPRING so COOL in SUMMER?_