146. _Which are the better conductors of heat, fluids or solids?_
Generally speaking, _solids_, especially those of them that are dense
in their substance.
147. _Why are dense substances the best conductors of heat?_
Because the heat more readily travels from particle to particle until
it pervades the mass.
148. _Why are fluids bad conductors of heat?_
Because of the want of _density_ in their bodies; and because a portion
of the imbibed heat always passes off from fluids by _evaporation_.
[Verse: "He casteth forth his ice like morsels: who can stand before
his word,"--PSALM CXLVII.]
149. _Why are woollen fabrics bad conductors of heat?_
Because there is a considerable amount of _air_ occupying the spaces of
the texture.
150. _Is air a good or a bad conductor?_
Air is a _bad conductor_, and it chiefly transmits heat, as water does,
by _convection_.
151. _Is water a good or a bad conductor?_
Water is an indifferent conductor, but it is a _better conductor than
air_.
152. _Why, when we place our hands in water, which may be of the same
temperature as the air, does the water feel some degrees colder?_
Because water, _being a better conductor than air_, takes up the warmth
of the hand _more rapidly_.
153. _Why, when we take our hands out of water do they feel warmer?_
Because the air does not abstract the heat of the hand so rapidly as
the water did, and the change in the degree of rapidity with which the
heat is abstracted _produces a sensation of increased warmth_.
154. _Why do we see blocks of ice wrapped in thick flannel in summer
time?_
Because the flannel, being a non-conductor, prevents the _external
heat_ from _dissolving the ice_.
Flannel wrapped around a _warm_ body _keeps in its heat_; and
wrapped around a _cold_ body, prevents heat from _passing into it_.
155. _How do we know that air is not a good conductor of heat?_
Because, _in still air_, heat would travel to a given point much more
rapidly, and in greater intensity, through even an indifferent _solid
conductor_, than it would through the _air_.
156. _How do we know that water is not a good conductor of heat?_
Because in a deep vessel containing _ice_, and with heat applied at the
top, some portion of the water may be made to boil _before the ice,
which lies a little under the surface, is melted_.
[Verse: "As snow in summer, and as rain in harvest; so honour is not
seemly for a fool."--PROV. XXVI.]
157. _Why would you apply the heat at the top, in this experiment?_
Because in heating water it _expands and rises_. The boiling of water
is caused by the heated water _ascending from the bottom_, and the
colder water descending to occupy its place. If the heat were not
applied at the top, it would be distributed quickly by _convection_,
but not by _conduction_.
158. _Why are bottles of hot water, used as feet-warmers, wrapped in
flannel?_
Because the flannel, _being a bad conductor_, allows the heat to _pass
only gently_ from the bottle, and preserves the warmth for a _much
longer time_.
159. _Why are hot rolls sent out by the bakers, wrapped up in flannel?_
Because the flannel, _being a bad conductor_, does not _carry off
rapidly the heat of the rolls_.
160. _Why is it said that snow keeps the earth warm?_
Because snow is a _bad conductor_, and prevents the frosty air from
_depriving the earth of its warmth_.
161. _Why are snow huts which the Esquimaux build found to be warm?_
Because snow, _being a bad conductor_, keeps in _the internal heat of
the dwelling_, and prevents the _cold outer air from taking away its
warmth_.
162. _Why is snow, being composed of congealed water (and water being a
better conductor than air), so good a non-conductor?_
Because in the process of congealation it is frozen into crystalline
forms, which, being collected into a mass, form a woolly body, thus
proving the truthfulness of the Bible simile, which says, God "giveth
snow like wool."
[Verse: "He giveth snow like wool: he scattereth the hoar frost like
ashes."--PSALM CXLVII.]
[Illustration: CRYSTALS OF SNOW, AS SEEN THROUGH A MICROSCOPE. FIG. 1.]
163. _Why does it frequently feel warmer after a frost has set in?_
Because, in the act of congealation a great deal of heat is given out,
and _taken up by the air_, and thus _the severity of the cold is in
some degree moderated_.
164. _Why is it frequently colder when a thaw takes place?_
Because, in the process of thawing, a certain amount of heat is
_withdrawn from the air_, and enters the thawed ice.
165. _What benefit results from these provisions of Nature?_
They moderate both the _severity of frosts_, and _the rapidity of
thaws_, which, in changeable climates, would be seriously detrimental
to _life_, and to _vegetation_.
166. _Why are furs and woollens worn in the winter?_
Because, being non-conductors, they prevent the warmth of the body from
being _taken up by the cold air_.
167. _Why are the skins of animals usually covered with fur, hair,
wool, or feathers?_
Because their coverings, being _non-conductors of heat_, preserve the
warmth of the bodies of the animals.
[Verse: "He sendeth out his word, and melteth them: he causeth his
wind to blow, and the waters to flow."--PSALM CXLVII.]
168. _How is the greater warmth of animals provided for in the winter?_
It is observed that, as winter approaches, there comes a short woolly
or downy growth, which, _adding to the non-conducting property of their
coats_, confines their animal warmth.
In small birds during winter, let the external colour of the
feathers be what it may, there will be found a kind of _black_
down next their bodies. Black is the _warmest colour_, and
the purpose here is to _keep in the heat_, arising from the
respiration of the animal.
169. _How is warmth provided for in animals that have no such coats?_
They are furnished with a layer of _fat_, which lies underneath the
skin. Fat consists chiefly of _carbon_, and is a _non-conductor_.
170. _Why are summer breezes said to be cool?_
Because, as they pass over the heated surface of the body, they bear
away a part of its heat.
171. _Why is a still summer air said to be sultry?_
Because, being heated by the sun's rays, _and being a bad conductor_,
it does not relieve the body by _carrying off its heat_.
172. _Why does fanning the face make it feel cooler?_
Because, by inducing currents of air to pass over the face, a part of
the excessive heat is taken up _and carried away_.
173. _Why does perspiration cool the body?_
Because it takes up a part of the heat, and, evaporating, _carries it
into the air_.
174. _Why does blowing upon hot tea cool it?_
Because it directs currents of air over the surface of the tea, and
these currents take up a part of the heat _and bear it away_.
175. _Why does air in motion feel cooler than air that is still?_
Because each wave of air _carries away a certain portion of heat_ and
being followed by another portion of air, _a further amount of heat is
borne away_.
[Verse: "Though I walk in the valley of the shadow of death I will
fear no evil, for thou art with me."--PSALM XXIII.]
176. _Is the atmosphere ever as hot as the human body?_
Not in this country. On the hottest day it is 10 or 12 deg. _cooler
than the temperature of our bodies_.
177. _What is the highest degree of artificial heat which man has been
known to bear?_
A man may be surrounded with air raised to the temperature of 300 deg.
(the boiling point being 212), and yet not have the heat of his body
raised more than two or three degrees above its natural temperature of
from 97 deg. to 100 deg.
178. _Why may man endure this degree of heat for a short time without
injury?_
Because the skin, and the vessels of fat that lie underneath it, are
bad conductors of heat.
And because perspiration passing from the skin and evaporating, would
_bear the heat away_ as fast as it was received.
Because, also, the vital principle (life) exercises a mysterious
influence in the preservation of living bodies from physical influences.
179. _Is the air ever hot enough, in any part of the world, to destroy
life?_
Yes. The hot winds of the Arabian deserts, which are called _simooms_,
scatter death and desolation in their track, withering trees and
shrubs, and burying them under waves of hot sand. When camels see the
approach of a simoom they rush to the nearest tree or bush, or to some
projecting rock, where they place their heads in an opposite direction
to that from which the wind blows, and endeavour to escape its terrible
violence. The traveller throws himself on the ground on the lee side of
the camel, and screens his head from the fiery blast within the folds
of his robe. But frequently both man and beast _fall a prey to the
terrible simoom_.
180. _Why are these hot winds so terrible in their effects?_
Because, being in motion, they search their way to every part of the
body, and passing over it _leave some portion of their heat behind_,
which is again followed by _additional heat from every fresh blast of
wind_.
[Verse: "The fear of the Lord is the beginning of knowledge: but
fools despise wisdom and instruction."--PROVERBS I.]