CHAPTER V
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.