118. _Why does not a piece of wood which is turning at one end, feel
hot at the other end?_
Because wood is _a bad conductor of heat_.
119. _Why is wood a bad conductor of heat?_
Because the arrangement of the particles of which it is composed does
not favour the transmission of _caloric_.
120. _Why do some articles of clothing feel cold, and others warm?_
Because some are bad conductors of heat, _and do not draw off much of
the warmth of our bodies_; while others are _better conductors_, and
_take up a larger portion of our warmth_.
[Verse: "The fining pot is for silver, and the furnace for gold: but
the Lord trieth the hearts."--PROVERBS XVII.]
121. _Which feels the warmer, the conductor or non-conductor?_
The non-conductor, as it does not readily _absorb_ the warmth of our
bodies.
122. _What substances are the best conductors of heat?_
Gold, silver, copper, and most substances of close and hard formation,
&c.
123. _What substances are the worst conductors of heat?_
Fur, eider down, feathers, raw silk, wood, lamp-black, cotton, soot,
charcoal, &c.
124. _Why has the toasting-fork a wooden handle?_
Because wood is not _so good a conductor_ as metal, therefore the wood
prevents the heat from being transmitted _by conduction_ to our hands.
125. _Why has the coffee-pot a wooden handle?_
Because the metal of the coffee-pot would otherwise _conduct the heat
to the hand_; but wood, _being a bad conductor_, prevents it.
126. _Why does hot water in a metal jug feel hotter than in an
earthenware one?_
Because metal, being a good conductor, _readily delivers heat to the
hand_; but _earthenware, being an indifferent conductor_, parts with
the heat slowly.
127. _How can we ascertain that wood prevents the conduction of heat to
the hand?_
By passing the top of the finger along the wooden handle of the
coffee-pot, until it reaches the point where the wood meets the metal.
The wooden handle will be found to be _cool_, but the metal will feel
_very hot_.
128. _Of what use are kettle-holders?_
Being made of _bad conductors_, such as wood, paper, or woollen cloth,
they will not readily _conduct_ the heat from the kettle to the hand.
[Verse: "Wisdom is the principal thing; therefore get wisdom: and
with all thy getting get understanding."--PROVERBS IV.]
129. _Will a kettle-holder, being a bad conductor, sometimes conduct
heat to the hand?_
Yes. But so slowly that the hand will not _feel the inconvenience of
too much heat_.
130. _Why does hot metal feel hotter than heated wool, though they may
both be of the same degree of temperature?_
Because metal gives out heat _more rapidly than wool_, by which it is
made _more perceptible to our feelings_.
131. _Which would become cold first--the metal or the wool?_
The _wool_, because, although the metal conducts heat more rapidly, to
a substance in contact with it, it does not _radiate heat_ as well as a
_black and rough substance_.
132. _Why do iron articles feel intensely cold in winter?_
Because iron is one of the best conductors, and draws off heat from the
hand very rapidly.
133. _What is the cause of the sensation called cold?_
When we feel cold, heat is being _drawn off from our bodies_.
134. _What is the cause of the sensation called heat?_
When we feel hot, our bodies are _absorbing heat_ from external causes.
The condition here implied is that of health, and of ordinary
circumstances. A person in a condition of fever, suffering from
intense heat arising from a diseased state of the blood, could not
be said to be _absorbing heat_. Nor could such a description apply
to a person who, by a very rapid walk, has raised the temperature
of his body considerably above its natural state, by the _internal
combustion_ which has already been described. A person feeling hot
in bed, from excessive clothes, feels hot from the _development of
heat internally_, which is not _conducted away_ with sufficient
rapidity to maintain the natural temperature of the body.
135. _If a person, sitting before a fire-place, without a fire, were
to set one foot upon a rug, and the other upon the stone hearth, which
would feel the colder?_
The foot on the stone, because stone is a good conductor, and would
_conduct the warmth of the foot away from it_.
[Verse: "The earth is the Lord's, and the fulness thereof; the
world, and they that dwell therein."--PSALM XXIV.]
136. _What does the hearth-stone do with the heat that it receives?_
It delivers it to the surrounding air, and to any other bodies with
which it may be _in contact_--and as it parts with heat, _it takes up
more from any body hotter than itself_.
137. _When there is no fire in a room, what is the relative temperature
of the various things in the room?_
They are all of the same temperature.
138. _If all the articles in the room are of the same temperature, why
do some feel colder than others?_
Because they differ in their relative powers of _conduction_. Those
that are the best conductors feel coldest, as they convey away the heat
of the hand most rapidly.
If you lay your hand upon the _woollen table cover_, or upon the
_sleeve of your coat_ or mantle, it will feel _neither warm nor
cold_, under ordinary circumstances. But if you raise your hand
from the table cover, or coat, and lay it on the marble mantel
piece, the mantel-piece will feel _cold_. If now you return
your hand from the mantel-piece to the table cover or coat, _a
sensation of warmth will become distinctly perceptible_. This will
afford a good conception of the relative _powers of conduction of
wool and marble_.
139. _How long does a substance feel cold or hot to the touch?_
Until it has brought the part touching it to the same temperature as
itself.
140. _When do substances feel neither hot nor cold?_
When they are of the same temperature as our bodies.
141. _Why, under these circumstances, do they feel neither hot nor
cold?_
Because they neither take heat from, nor supply it to, the body.
142. _Which would feel the warmer, when the fire was lighted, the
hearth-rug or the hearth-stone?_
The hearth-stone, because it is a _good conductor_, and would not
only _receive heat_ readily, but would _part with it as freely_
(thereby making its heat _perceptible_). But the hearth-rug, _being
a bad conductor_, would part with its heat very slowly, and it would
therefore be _less perceptible_.
[Verse: "Fire and hail; snow and vapour; stormy wind fulfilling his
word."--PSALM CXLVIII.]
143. _Would the hearth-stone feel hotter than the hearth-rug though
both were of the same temperature?_
It would feel _hotter than the hearth-rug_, because it would part with
its heat so rapidly that it would be the _more perceptible_.
144. _But if the hearth-stone and the hearth-rug were both colder than
the hand, which would feel the colder of the two?_
Then the hearth-stone would feel the colder, because, _being a good
conductor_, it would _take heat_ from the hand more freely than the
hearth-rug, which is a _bad conductor_.
145. _Why would the hearth-stone feel comparatively hotter in the one
case, and colder in the other?_
Because, _being a good conductor_, it would conduct heat rapidly _to_
the hand when hot, and take heat rapidly _from_ the hand when cold.