LIGHT, HEAT, AND TEMPERATURE
MORE WONDERFUL THAN ANY FICTION ARE THE FACT OF INVISIBLE LIGHT,
AND THE DIFFERENCE BETWEEN HEAT AND TEMPERATURE
The heat that escapes from the earth’s interior is minute in
comparison to that received from the sun, which is the main source
of the earth’s supply. Heat is manifested by the motions of the
molecules of matter, whether solid, liquid, or gaseous. It is
transmitted through space in some mysterious manner, for space is
practically void of an atmosphere. One cannot conceive of motion
taking place in a void, for there is nothing to move. Therefore it is
assumed that interstellar space must be filled with a transmitting
medium; to this the name of ether has been given. Nothing is known of
its structure, but it is believed that it penetrates all bodies and
fills the space between their molecules.
=How Heat and Light Reach the Earth.= The heat of the sun is
some forty-six thousand times as intense as is the heat of the
earth. The violent agitations of the molecules of the sun’s hot
atmosphere impart vibrations to the ether of space, which decrease
in effectiveness inversely as the square of the distance; that is
to say, that if the earth were twice as far from the sun as it is,
the intensity of the solar rays would be one fourth of what they are
now. These vibrations are called solar energy. They pass through
space without perceptibly warming or lighting it. When they encounter
the molecules of the earth’s atmosphere, and the dust and cloud in
suspension in the air, or impinge upon the solid matter of the earth,
they are transmuted back into molecular agitations, and manifest
themselves in a multitude of forms, such as heat, light, chemical
rays, electricity, etc.
=The Difference between Heat and Temperature.= The agitation of
the molecules of a substance set up by the absorption of heat is
indicated by temperature, which gives no measure of the quantity of
heat absorbed, the quantity varying widely for different kinds of
matter. The amount of heat necessary to raise one pound of water
1° F. is the heat unit generally employed in commerce; but in
scientific research the amount necessary to raise one gram of water
1° Centigrade is the unit of heat best adapted to use. It is called
the gram-calorie.
Let us take a glass filled with boiling water. You see the glass and
the water because they reflect to the eye light waves received from
some source,—possibly the sunlight that is diffused by the dust motes
of the air into the room through the window. But the glass and the
water radiate other waves to which the eye is not sensible; these
invisible long heat waves may be felt by the nerves of the hand.
They warm all matter upon which they fall by adding to the agitation
of the molecules of which it is composed; but they do not warm all
matter equally. The waves that reach dark bodies are broken up; that
is to say, absorbed. Their energy is transmuted into sensible heat,
and in the place of the waves we have molecular vibrations in the
matter, which are made manifest by a rise in its temperature. Dark
rough surfaces more completely absorb the waves and therefore rise
to a higher temperature than the same surfaces when smooth. When the
waves encounter bright and highly polished surfaces the effect is
quite different; then most of them are reflected away and therefore
warm the matter but little. These reflected waves are not broken up,
but on the contrary start off in some new direction, possibly falling
upon and warming some matter more receptive to their influence. The
higher the polish the more completely are the waves reflected.
=Difference between Light Waves, Heat Waves, and Sound Waves.= The
light and the heat waves of the ether are infinitesimal ripples as
compared to the backward and forward pulsations that constitute
the sound waves of the air. Within a space of one inch there are
sixty-six thousand of the violet waves of light, which are the
shortest etheric vibrations to which the human eye responds, and over
thirty thousand of the red waves, the longest that affect the eye;
while the sound waves of the air vary from about one foot for the
shrill notes of the human voice to four feet for the middle C of the
pianoforte. A shrill whistle produces waves of about one half inch.
There are twenty-two thousand of certain heat waves to the inch, and
these, like some of the light waves of the ether, are invisible.
There is also a vast difference between the velocity of vibration
of the air waves and those of the ether. The human ear is sensitive
to sound waves of somewhere between twenty-nine per second to
thirty-eight thousand per second; while the eye responds to light
waves of from five hundred million to one billion per second. Some
ears are better adjusted to the low vibrations and some to the high,
and the ears of no one hear any but a small part of the melody of a
great symphony. Tyndall could hear the sharp chirp of thousands of
insects that were inaudible to his guide as the two climbed the Alps,
but the guide’s ears responded to the long, slow waves that came from
the dull tread of the donkey’s hoofs farther up the mountain, which
waves the scientist was unable to hear. Likewise some eyes are able
to penetrate far into the violet, or the red, or both, and some are
unable to distinguish between certain colors.
=Chemical Rays of Light.= The chemical or photographic rays
have still shorter waves than the violet. They produce special
physiological effects in vegetable and animal tissues, and, acting
upon particular kinds of matter, they cause fluorescence, which
is the property possessed by some bodies of giving off, when
illuminated, light of a color different from their own and from that
of the light that illuminates them. These chemical rays are sometimes
called ultra-violet rays.
=Invisible Light.= From a reading of the immediately preceding
paragraphs one may be prepared for the startling statement that there
is such a thing as invisible light. Vibrations of the ether that
move slower than those that give to the eye the sensation of red are
invisible, as are those that move faster than the violet rays, and
it is certain that neither the eye of man nor of animal ever will
see but a small part of the beauty of a landscape or the delicate
coloring of a flower. The eye only takes in and renders sensible
to the brain the red, orange, yellow, green, blue, indigo, violet,
and their various tints, but the delicate instruments of science
reveal many other colors. One sees as through a glass darkly, for the
gentle signals that might reveal the beauties of Paradise fall upon
the eye unheeded. A keener vision and a more complete appreciation
of the beauties and the wonders of the universe await one on the
other side of the gauzy veil of immortality. The finger tips of the
outstretched arms may span the river of life and the ethereal breath
of loved ones may be caressing one’s cheek. The music of the spheres
is not a myth; the lily or the rose as it opens its petals to receive
the benediction of the morning sun may give forth a veritable pæan
of joy. A rose bush may be a grander symphony than anything that
Beethoven ever wrote. What to us is the invisible light may be the
illumination that guides the sweep of the angels’ wings.
=How Heat Moves through or Is Transmitted by Matter.= Heat passes
by contact from the warmer to the colder molecules of a body. This
action is called conduction. When one end of a bar of iron is held in
a fire, the end away from the fire soon becomes too hot to hold in
the hand, because heat is rapidly transferred from the hot portion
of the bar to the cooler portion by conduction, showing that iron
is a good conductor. On the other hand, the end of a stick of wood
can be held in the fire until it is completely consumed without
the other end becoming too warm to hold, indicating that wood is
a poor conductor. Metals are the best conductors, silver leading
the list, with copper second. Snow and ice and fibrous and porous
substances are poor conductors, and are called insulators. Air and
water are also poor conductors. The fur of animals and the feathers
of birds protect against the rapid loss of heat because they contain
numerous interstices filled with air, a poor conductor. Heat is lost
by radiation when the molecules of matter set up vibration in the
ether. The atmosphere itself performs this function on a large scale
when the sky is cloudless, so that radiated heat is not absorbed by
the cloud covering and its loss into space restricted. When air or
water is not evenly or homogeneously heated a circulation is set up
in which the colder part settles down and the warmer rises. This is
called convection. The air that is heated by contact with a stove
rises and passes along the ceiling to the colder parts of the room,
gradually parting with its heat until it is no warmer than the air
next adjacent to it, and slowly settling to the floor as the cold
air beneath it moves toward the stove, is warmed and sent aloft,
the first air finally making a complete circuit and returning to
the stove again. In this way the heat is distributed by convection
throughout the whole room. When one part of the earth’s surface
becomes hotter than another a similar action takes place on a large
scale. The region of greater temperature warms the air above it, and
the surrounding denser air flows in along the surface, forcing the
lighter air to rise, when it in turn is similarly warmed and driven
up.
The clear waters of lakes and rivers and of the ocean permit the
passage of heat waves to a considerable depth before they are
completely absorbed. On a cold day in winter, when the sun is shining
brightly, a room with spacious windows may become as warm as though
heated by a furnace, simply by the capacity of the glass in the
windows to transmit the heat waves of the sun without considerable
absorption, and at the same time prevent the escape of the longer
heat waves that are radiated from the interior walls of the room.
This capacity of matter to transmit heat waves without absorption
is called diathermancy. The clear atmosphere is an exceedingly good
transmitter, and rock salt is one of the best of all solids.
The capacity of a body to transmit light without absorbing it and
becoming luminous is called transparency. Air freed of dust motes
is almost perfectly transparent. In this state it is said to be
optically pure. But the ordinary air of nature, with its moisture
and dust, absorbs most of the blue wave lengths and also many of the
longer ones of the other colors of the spectrum.
The capacity of a body for heat is called its specific heat. With
but few exceptions the specific heats of liquids are much greater
than those of solids or gases. It requires ten times the quantity of
heat to raise a pound of water one degree that it does a pound of
iron. Ice has the greatest specific heat of any of the solids, except
paraffin and wood.
When a solid is melted or a liquid vaporized a large amount of heat
becomes latent, insensible to the touch; it disappears as heat. This
is one of the most wonderful of the phenomena of nature. It matters
not how long the time may be, an hour, a day, a year, or a thousand
years after the solid is melted or the liquid turned to vapor, so
soon as the vapor returns to the liquid state or the liquid to a
solid condition, the latent heat becomes sensible in exactly the
same degree in which it previously existed. Let us illustrate with a
pound of ice at zero F. Sixteen heat units, or sixteen times as much
heat as is required to raise one pound of water one degree, must be
absorbed by this pound of ice to raise its temperature to the melting
point (32°); and then one hundred forty-four more heat units must be
absorbed to so far overcome the tendency of the molecules to adhere,
or remain together, that the molecules may roll the one about the
other in the liquid form, but with this important difference: the
one hundred forty-four units become latent and do not, therefore,
cause any increase in temperature, as the sixteen heat units did
in raising the temperature of the ice. The large quantity of heat
required to change the ice to a liquid is called the latent heat of
melting. Any further addition of heat after the melting is complete
causes an increase in temperature, and one hundred eighty heat units
will raise it to the boiling point. Water boils at 212° at sea
level and normal pressure; that is to say, at that temperature the
agitation of the molecules of water is so great as to overcome both
cohesion and the weight with which the air presses down upon them,
and cause them to fly away in the form of steam, which is invisible
when confined inside a boiler. But the entire pound of water is not
instantly changed to the gaseous condition, for with the sending off
of the first few molecules some heat is rendered latent, and more
must be supplied or the boiling ceases; in fact the enormous quantity
of 964.62 heat units must be supplied to entirely change the pound of
water to steam, but at no time does the temperature rise above 212°.
As in the former case of changing the solid to a liquid, a large
amount of heat becomes latent; in this case it is called the latent
heat of vaporization.
Now carefully fix in the mind that a liquid does not need to be
raised to its boiling point before vaporization begins, for it
operates at all temperatures, even after the liquid is frozen, but
much more rapidly from the liquid. If one wishes to test this: weigh
a piece of ice during very cold weather. Then leave it out in a
temperature that is below freezing for several days, and on weighing
again it will be found that the ice has lost weight. All evaporation
produces a cooling effect because of the heat that is rendered latent
in the process of changing the liquid or the solid to a gaseous form.
The drier the air the greater is the cooling effected by keeping the
surface wetted, and the cooling is accelerated by placing the wet
object where there is a free circulation of air.
A wooden water bucket that has been soaked for a day or two so
that every part of the wood is saturated with water, will, if kept
closed, keep water all day in the open field practically as cool as
when it left the deep well, and often cooler. Not enough use is made
of cooling by evaporation by those who have not ice in the summer.
Inexpensive and fairly effective refrigerators may be made, by any
mechanic, of lattice-work sides covered with any thick fabric and
kept moist, which would keep milk, butter, fruit, vegetables, and
cooked meats in good condition if placed in a hallway with a good
circulation of air, or in any shady place with good ventilation.
Most solids expand with gain in temperature and therefore possess
greater volume in the liquid form than in the solid, and the
temperature of their melting points rises as they are subjected to
increasing pressure. The law reverses when applied to ice, which
contracts in melting. To few is it known that a skater on ice really
rides upon water molecules, for the sharp edge of the skate, when
applied to the ice under the weight of one’s body, is lubricated by
the slight melting of the ice in immediate contact with the skate,
the molecules of water returning to the form of ice as soon as the
skater passes and the pressure is relieved. The strange phenomenon
may be witnessed by passing a wire through a block of ice without
severing it into two pieces, by attaching heavy weights to the two
ends of the wire and suspending it across the ice, the ice slowly
melting as the result of the pressure applied by the underside of the
wire and freezing molecules closing the space on top of the wire. By
this process do we account for the moving of glaciers down tortuous
valleys as though they were liquids.
=Altitude Measured by Change in Boiling Point of Water.= The boiling
point of water at sea level and ordinary air pressure is 212°. If the
pressure of the atmosphere were increased to about thirty pounds,
instead of about fifteen to the square inch it would be necessary to
raise water to 250° before boiling would begin. The changes of air
pressure due to the passage of the severe storms of winter may cause
the boiling point of water to vary from 207° to 215°. This knowledge
may be useful in measuring the heights of mountains, although the
method does not give close results. The decrease of pressure with
altitude lowers the boiling point, the amount being approximately one
degree for each 555 feet of ascent. The best results may be secured
by having a person at the base of the mountain, where the elevation
above sea level is known, determine the boiling point at the same
time that a person on the mountain top does. The thermometers should
be read closely to the fraction of a degree.
With the barometer at its normal height of thirty inches, air at 60°
will instantly rise to the phenomenal temperature of 175.50 if it be
confined and its pressure doubled, and it will diminish to one half
of its former volume. But if its pressure be diminished one half, its
volume will expand to double its original size and its temperature
will fall from 60° to 2.4°. From these facts the reader would
naturally expect to find low pressure of the atmosphere accompanying
cold waves and high pressure to be coincident with warm conditions,
which is exactly the reverse of what actually occurs in the free air
of nature. This apparent contradiction will be made plain in the
treatment of cold waves, page 124.
A temperature of -459° on the Fahrenheit scale and -273.1° on the
Centigrade represents what is called absolute zero. It is supposed
to be the temperature at which there is no motion of the molecules
of matter. Bodies or planets without atmospheres have temperatures
approaching absolute zero, for there is no protecting envelope to
absorb heat or to prevent the instant radiation into space of that
which impinges upon the body. Our moon is an illustration, and
notwithstanding the fierce beating upon its surface of the solar
energy it remains incased in the intense cold of space.
The thermometer is the instrument that measures temperature. It was
not until eighty-seven years after Columbus discovered America that
Galileo discovered the principle of the thermometer. This first
instrument was crude. It consisted of a glass bulb, containing air,
terminating below in a long glass tube, which dipped into a vessel
containing colored water. When the temperature fell the contraction
of the air in the bulb caused the water to rise in the tube, and
when the temperature rose the expansion of the air forced the water
to a lower level. Galileo also invented the alcohol thermometer in
1611, but the determination of the zero and some fixed point above
it, by which to graduate the scale, took years to evolve. The modern
alcohol and mercury thermometers consist of a bulb filled with the
liquid, and a tube partly filled, the upper part being a tolerably
complete vacuum, allowing the liquid freedom of movement up and down
the tube. When a tube is broken one is surprised to see that the
diameter of the bore is less than that of the smallest fuzzy hair
from the back of the hand. The size of the column of mercury is
magnified by the action of light passing through the glass of the
tube.
Temperatures are usually taken in the shade. The instrument should
be free from all bodies that could conduct heat to it, and it should
have free circulation of air about it.
In a complete meteorological station automatically recording
instruments, too complicated for the use of the layman, record for
each moment of time the temperature of the air and its pressure, the
periods of sunshine, the duration and the amount of rainfall, and the
direction and velocity of the wind.