71. _What is heat?_
Heat is a principle in nature which, like light and electricity, is
best understood by its _effects_. We popularly call that heat, which
raises the temperature of bodies submitted to its influence.
72. _What is caloric?_
Caloric is another term for heat. It is advisable, however, to use the
term _caloric_ when speaking of the _cause_ of heat, and of _heat_ as
the _effect_ of the presence of _caloric_.
[Verse: "While the earth remaineth, seed-time and harvest, and
cold and heat, and summer and winter, and day and night, shall not
cease."--GEN. VIII.]
73. _What is the source of caloric?_
The sun is its chief source. But caloric, in some degree, exists _in
every known substance_.
74. _What are the effects of caloric?_
Heat which, in proportion to its intensity, acts variously upon all
bodies, causing _expansion_, _fusion_, _evaporation_, _decomposition_,
_&c._
75. _Why is caloric called a repulsive agent?_
Because its chief effects are to _expand_, _fuse_, _evaporate_, or
_decompose_ the substances upon which it acts.
76. _What is an attractive agent, in contradistinction to a repulsive
agent?_
Chemical attraction, or affinity, is an attractive agent--as when
bodies seek of their own natures to unite and form some new body.
77. _When is a body said to be hot?_
When it holds so much _caloric_ that it diffuses heat to surrounding
objects.
78. _When is a body said to be cold?_
When it holds less _caloric_ than surrounding objects, and absorbs heat
from them.
79. _How may caloric be excited to develop heat?_
By any means which cause agitation, or produce an active change in the
condition of bodies. Thus friction, percussion, sudden condensation
or expansion, chemical combination, and electrical discharges, all
develope _heat_.
80. _Why do "burning glasses" appear to set fire to combustible
substances?_
Because they gather into one point, or _focus_, several rays of
_caloric_ as they are travelling from the sun, and the accumulation of
caloric developes that intensity of _heat_ which constitutes _fire_.
81. _What is a focus?_
In optics, it is the point or centre at which, or around which,
divergent rays are brought into the closest possible union.
[Verse: "Yet man is born to trouble, as the sparks fly upward.--I
would seek unto God, and unto God would I commit my cause."--JOB V.]
82. _What is fire?_
It is a violent chemical action attending the combustion of the
ingredients of _fuel_ with the _oxygen_ of the air.
83. _What are the properties of fire?_
It imparts heat, which has the effect of expanding both fluids and
solids.
It cannot exist without the presence of combustible materials.
It has a tendency to diffuse itself in every direction.
It cannot exist without oxygen or atmospheric air.
84. _What elements take part in the maintenance of a fire?_
Hydrogen, carbon, and oxygen. Hydrogen and carbon exist in the _fuel_,
and oxygen is supplied by the _air_.
85. _How does the combustion of a fire begin?_
A match made of phosphorous and sulphur (highly inflammable substances)
is drawn over a piece of sand-paper; the _friction_ of the match
induces the presence of _caloric_, which developes _heat_, and ignites
the match, the burning of which is sustained by the _oxygen_ of the
air. The flame is then applied to paper or wood, and the heat of the
flame is sufficient to drive out _hydrogen gas_, which unites with the
_oxygen_ of the air, and burns, imparting greater heat to the _carbon_
of the coals, which assumes the form of carbonic acid gas by union with
_oxygen_, and in a little while all the conditions of _combustion_ are
established.
86. _What are the properties of heat?_
It may exist without _fire_ or _light_.
It is not sensible to _vision_.
It makes an impression upon our _feelings_.
It acts powerfully upon _all bodies_.
It has no _weight_.
It attends, or is connected with, _all the operations of nature_.
It radiates from _all bodies_ in straight lines, and in all
_directions_.
It strikes most powerfully in _direct lines_.
Its rays may be collected into a _focus_, just as the rays of the sun.
It may be _reflected_ from a polished surface.
It is more easily _conducted_ by some substances than by others.
[Verse: "For my days are consumed like smoke, and my bones are
burned as an hearth."--PSALM CII.]
87. _What is animal heat?_
Animal heat is derived from the slow combustion of _carbon_ in the
blood of animals with the _oxygen_ of the air which the animals breathe.
88. _What is latent heat?_
Latent heat (or more properly _latent caloric_) is that which exists,
in some degree, in all _bodies_, though it may be imperceptible to the
_senses_.
89. _Is there latent caloric in ice, snow, water, marble, &c?_
Yes; there is some amount of _caloric_ in all substances.
A blacksmith may hammer a small piece of iron until it becomes
_red hot_. With this he may light a match, and _kindle the fire
of his forge_. The iron has become more dense by the hammering,
and it cannot again be heated to the same degree by similar
means, until it has been exposed _in fire_, to _a red heat_. Is
it not possible that, by hammering, the particles of iron have
been driven closer together, and _the latent heat_ driven out?
No further hammering will force the atoms nearer, and therefore
no further heat can be developed. But when the iron has _again
absorbed caloric_, by being plunged in a fire, it is again charged
with latent heat. Indians produce _sparks_ by rubbing together
_two pieces of wood_. Two pieces of ice may be rubbed together
until sufficient warmth is developed to _melt them both_. The
axles of railway carriages frequently become _red hot_ from
_friction_.
90. _Have vegetables heat?_
Yes; whenever oxygen combines with carbon to form carbonic acid gas,
an extrication of heat takes place, however minute the amount. Such
a combination occurs much more extensively during the germination of
seeds and the impregnation of flowers, than at any other time. In the
germination of barley heaped in rooms, previous to being converted
into malt, it is well known that a _considerable amount of heat is
developed_.
91. _Has any investigation of this subject ever been carefully made?_
Yes. Lamarck, Senebier, and De Candolle, found the flowers of the
_Arum Maculatum_, between three and seven o'clock in the afternoon, as
much as 7 deg. Reaum. warmer than the external air. Schultz found a
difference of 4 deg. to 5 deg. between the heat of the spathe of the
_Canadian pinnatifolium_ and the surrounding air, at six to seven
o'clock p.m. Other observations have established differences of as
much as 30 deg. between the temperature of the spathe of the _Arum
cordifolium_, and that of the surrounding atmosphere.
[Verse: "And there are diversities of operations, but it is the
same God which worketh in all."--CORINTHIANS XII.] 92. _Have plants
sometimes a temperature lower than that of the surrounding air?_
Yes. It has not only been found that under particular circumstances
the heat of certain parts of plants is elevated to a very remarkable
degree, but that, under nearly all circumstances, they have a
temperature different from that of the external air, being _warmer in
winter, and cooler in summer_.