181. _What is Radiation?_
The radiation of heat is a _motion of the particles_, in a series of
rays, diverging in every direction from a heated body.
182. _What is this phenomena of Radiation understood to arise from?_
From a strongly repulsive power, possessed by particles of heat, by
which they are excited to recede from each other with great velocity.
183. _What is the greatest source of Radiation?_
The sun, which sends forth rays of _both light and heat_ in all
directions.
184. _When does a body radiate heat?_
When it is surrounded by a medium which is _a bad conductor_.
185. _When we stand before a fire, does the heat reach us by conduction
or by radiation?_
By radiation.
186. _What becomes of the heat that is radiated from one body to
another?_
It is either _absorbed_ by those bodies, or transmitted through
them and passed to other bodies by _conduction_, or diffused by
_convection_, or returned by _reflection_.
187. _How do we know that heat is diffused by radiation?_
If we set a metal plate (or any other body, though metal is best for
the experiment) before the fire, _rays of heat will fall upon it_. If
we turn the plate at a slight angle, and place another object in a
line with it, we shall find that the plate will _reflect the rays it
has received by radiation_, on to the object so placed; but if we place
an object _between the fire and the plate_, we shall find that the
rays of heat _will be intercepted_, and that the latter can no longer
_reflect heat_.
[Verse: "The fear of the Lord is the beginning of wisdom: a good
understanding have all they that do his commandments."--PSALM CXI.]
188. _Does the agitation of the air interfere with the direction of
rays of heat?_
It has been found that the agitation of the air does _not_ affect the
direction of rays of heat.
189. _Why, then, if a current of air passes through a space across
which heat is radiating, does the air become warmer?_
Because it takes up _some portion of the heat_, but it does not alter
the direction of the rays.
This is clearly illustrated by reference to _rays of light_ which
are seen under many circumstances. But they are never bent, moved,
nor in any way affected by the wind.
190. _Why will not a current of air disturb the rays of heat, just as
it would a spider's web, or threads of silk?_
Because heat is an _imponderable_ agent, that is, something which
cannot be acted upon by the ordinary physical agencies. It has _no
weight_, presents no _substantial body_, and is, in these latter
respects, similar to _light and electricity_.
191. _What other sources of radiation of heat are there besides the sun
and the fire?_
The _earth_, and all _minor bodies_, are, in some degree, _radiators of
heat_.
192. _What substances are the best radiators?_
All _rough_ and _dark_ coloured substances and surfaces are the _best
radiators of heat_.
193. _What substances are the worst radiators of heat?_
All _smooth_, _bright_, and _light coloured_ surfaces are _bad
radiators of heat_.
Dr. Stark, of Edinburgh, has proved, by a series of experiments,
the influence which the _colours_ of bodies have upon the
_velocity of radiation_. He surrounded the bulb of a thermometer
successively with equal weights of _black_, _red_, and _white_
wool, and placed it in a glass tube, which was heated to the
temperature of 180 deg. by immersion in hot water. The tube was
then cooled down to 50 deg. by immersion in cold water; the
_black_ cooled in 21 minutes, the _red_ in 26 minutes, and the
_white_ in 27 minutes.
[Verse: "Say unto wisdom, Thou art my sister; and call understanding
thy kinswoman."--PROVERBS VII.]
194. _If you wished to keep water hot for a long time, should you put
it into a bright metal jug, or into a dark earthenware one?_
You should put it into a _bright metal_ jug, because, _being a bad
radiator,_ it would not part readily with the heat of the water.
195. _Why would not the dark earthenware jug keep the water hot as long
as the bright metal one?_
Because the particles of earthenware being rough, and of dark colour,
_they radiate heat freely_, and the water would thereby be quickly
cooled.