simple science

General Properties of Gases:
If a gas expands suddenly, its temperature falls; for example, if a mass of gas at 87° C. is allowed to expand rapidly to twice its original volume, its temperature falls to 0° C. If the compressed air of a bicycle tire is allowed to expand and a sensitive thermometer is held in the path of the escaping air, the thermometer will show a decided drop in temperature.

The low temperature obtained by the expansion of air or other gases is utilized commercially on a large scale. By means of powerful pistons air is compressed to one third or one fourth its original volume, is passed through a coil of pipe surrounded with cold water, and is then allowed to escape into large refrigerating vaults, which thereby have their temperatures noticeably lowered, and can be used for the permanent storage of meats, fruits, and other perishable material. In summer, when the atmospheric temperature is high, the storage and preservation of foods is of vital importance to factories and cold storage houses, and but for the low temperature obtainable by the expansion of compressed gases, much of our food supply would be lost to use.

General Properties of Gases:
If the pressure on a gas is greatly increased, a sudden transformation sometimes occurs and the gas becomes a liquid. Then, if the pressure is reduced, a second transformation occurs, and the liquid evaporates or returns to its original form as a gas.

We know that a fall of temperature caused water vapor to condense or liquefy. If temperature alone were considered, most gases could not be liquefied, because the temperature at which the average gas liquefies is so low as to be out of the range of possibility; it has been calculated, for example, that a temperature of 252° C. below zero would have to be obtained in order to liquefy hydrogen.

Some gases can be easily transformed into liquids by pressure alone, some gases can be easily transformed into liquids by cooling alone; on the other hand, many gases are so difficult to liquefy that both pressure and low temperature are needed to produce the desired result. If a gas is cooled and compressed at the same time, liquefaction occurs much more surely and easily than though either factor alone were depended upon. The air which surrounds us, and of whose existence we are scarcely aware, can be reduced to the form of a liquid, but the pressure exerted upon the portion to be liquefied must be thirty-nine times as great as the atmospheric pressure, and the temperature must have been reduced to a very low point.

General Properties of Gases:
Ammonia gas is liquefied by strong pressure and low temperature and is then allowed to flow into pipes which run through tanks containing salt water. The reduction of pressure causes the liquid to evaporate or turn to a gas, and the fall of temperature which always accompanies evaporation means a lowering of the temperature of the salt water to 16° or 18° below zero. But immersed in the salt water are molds containing pure water, and since the freezing point of water is 0° C, the water in the molds freezes and can be drawn from the mold as solid cakes of ice.

Ammonia gas is driven by the pump C into the coil D under a pressure strong enough to liquefy it, the heat generated by this compression being carried off by cold water which constantly circulates through B. The liquid ammonia flows through the regulating valve V into the coil E, in which the pressure is kept low by the pump C. The accompanying expansion reduces the temperature to a very low degree, and the brine which circulates around the coil E acquires a temperature below the freezing point of pure water. The cold brine passes from A to a tank in which are immersed cans filled with water, and within a short time the water in the cans is frozen into solid cakes of ice.

Invisible Objects:
We saw in Section 84 that gases have a tendency to expand, but that they can be compressed by the application of force. This observation has led scientists to suppose that substances are composed of very minute particles called molecules, separated by small spaces called pores; and that when a gas is condensed, the pores become smaller, and that when a gas expands, the pores become larger.

The fact that certain substances are soluble, like sugar in water, shows that the molecules of sugar find a lodging place in the spaces or pores between the molecules of water, in much the same way that pebbles find lodgment in the chinks of the coal in a coal scuttle. An indefinite quantity of sugar cannot be dissolved in a given quantity of liquid, because after a certain amount of sugar has been dissolved all the pores become filled, and there is no available molecular space. The remainder of the sugar settles at the bottom of the vessel, and cannot be dissolved by any amount of stirring.

If a piece of potassium permanganate about the size of a grain of sand is put into a quart of water, the solid disappears and the water becomes a deep rich red. The solid evidently has dissolved and has broken up into minute particles which are too small to be seen, but which have scattered themselves and lodged in the pores of the water, thus giving the water its rich color.

There is no visible proof of the existence of molecules and molecular spaces, because not only are our eyes unable to see them directly, but even the most powerful microscope cannot make them visible to us. They are so small that if one thousand of them were laid side by side, they would make a speck too small to be seen by the eye and too small to be visible under the most powerful microscope.

We cannot see molecules or molecular pores, but the phenomena of compression and expansion, solubility and other equally convincing facts, have led us to conclude that all substances are composed of very minute particles or molecules separated by spaces called pores.

Invisible Objects:
If a gas jet is turned on and not lighted, an odor of gas soon becomes perceptible, not only throughout the room, but in adjacent halls and even in distant rooms. An uncorked bottle of cologne scents an entire room, the odor of a rose or violet permeates the atmosphere near and far. These simple everyday occurrences seem to show that the molecules of a gas must be in a state of continual and rapid motion. In the case of the cologne, some molecules must have escaped from the liquid by the process of evaporation and traveled through the air to the nose. We know that the molecules of a liquid are in motion and are continually passing into the air because in time the vessel becomes empty. The only way in which this could happen would be for the molecules of the liquid to pass from the liquid into the surrounding medium; but this is really saying that the molecules are in motion.

From these phenomena and others it is reasonably clear that substances are composed of molecules, and that molecules are not inert, quiet particles, but that they are in incessant motion, moving rapidly hither and thither, sometimes traveling far, sometimes near. Even the log of wood which lies heavy and motionless on our woodpile is made up of countless billions of molecules each in rapid incessant motion. The molecules of solid bodies cannot escape so readily as those of liquids and gases, and do not travel far. The log lies year after year in an apparently motionless condition, but if one's eyes were keen enough, the molecules would be seen moving among themselves, even though they cannot escape into the surrounding medium and make long journeys as do the molecules of liquids and gases.

Invisible Objects:
Common sense tells us that a molecule of water is not the same as a molecule of vinegar; the molecules of each are extremely small and in rapid motion, but they differ essentially, otherwise one substance would be like every other substance. What is it that makes a molecule of water differ from a molecule of vinegar, and each differ from all other molecules? Strange to say, a molecule is not a simple object, but is quite complex, being composed of one or more smaller particles, called atoms, and the number and kind of atoms in a molecule determine the type of the molecule, and the type of the molecule determines the substance. For example, a glass of water is composed of untold millions of molecules, and each molecule is a company of three still smaller particles, one of which is called the oxygen atom and two of which are alike in every particular and are called hydrogen atoms.

Invisible Objects:
Generally molecules are composed of atoms which are different in kind. For example, the molecule of water has two different atoms, the oxygen atom and the hydrogen atoms; alcohol has three different kinds of atoms, oxygen, hydrogen, and carbon. Sometimes, however, molecules are composed of a group of atoms all of which are alike. Now there are but seventy or eighty different kinds of atoms, and hence there can be but seventy or eighty different substances whose molecules are composed of atoms which are alike. When the atoms comprising a molecule are all alike, the substance is called an element, and is said to be a simple substance. Throughout the length and breadth of this vast world of ours there are only about eighty known elements. An element is the simplest substance conceivable, because it has not been separated into anything simpler. Water is a compound substance. It can be separated into oxygen and hydrogen.

Gold, silver, and lead are examples of elements, and water, alcohol, cider, sand, and marble are complex substances, or compounds, as we are apt to call them. Everything, no matter what its size or shape or character, is formed from the various combinations into molecules of a few simple atoms, of which there exist about eighty known different kinds. But few of the eighty known elements play an important part in our everyday life. We have seen in an earlier experiment that twice as much hydrogen as oxygen can be obtained from water. Two atoms of the element hydrogen unite with one atom of the element oxygen to make one molecule of water. In symbols we express this H₂O. A group of symbols, such as this, expressing a molecule of a compound is called a formula. NaCl is the formula for sodium chloride, which is the chemical name of common salt.

Light:
Heat keeps us warm, cooks our food, drives our engines, and in a thousand ways makes life comfortable and pleasant, but what should we do without light? How many of us could be happy even though warm and well fed if we were forced to live in the dark where the sunbeams never flickered, where the shadows never stole across the floor, and where the soft twilight could not tell us that the day was done? Heat and light are the two most important physical factors in life; we cannot say which is the more necessary, because in the extreme cold or arctic regions man cannot live, and in the dark places where the light never penetrates man sickens and dies. Both heat and light are essential to life, and each has its own part to play in the varied existence of man and plant and animal.

Light enables us to see the world around us, makes the beautiful colors of the trees and flowers, enables us to read, is essential to the taking of photographs, gives us our moving pictures and our magic lanterns, produces the exquisite tints of stained-glass windows, and brings us the joy of the rainbow. We do not always realize that light is beneficial, because sometimes it fades our clothing and our carpets, and burns our skin and makes it sore. But we shall see that even these apparently harmful effects of light are in reality of great value in man's constant battle against disease.

Light:
Natural heat and light are furnished by the sun, but the absence of the sun during the evening makes artificial light necessary, and even during the day artificial light is needed in buildings whose structure excludes the natural light of the sun. Artificial light is furnished by electricity, by gas, by oil in lamps, and in numerous other ways. Until modern times candles were the main source of light, and indeed to-day the intensity, or power, of any light is measured in candle power units, just as length is measured in yards; for example, an average gas jet gives a 10 candle power light, or is ten times as bright as a candle; an ordinary incandescent electric light gives a 16 candle power light, or furnishes sixteen times as much light as a candle. Very strong large oil lamps can at times yield a light of 60 candle power, while the large arc lamps which flash out on the street corners are said to furnish 1200 times as much light as a single candle. Naturally all candles do not give the same amount of light, nor are all candles alike in size. The candles which decorate our tea tables are of wax, while those which serve for general use are of paraffin and tallow.

Light:
The farther we move from a light, the less strong, or intense, is the illumination which reaches us; the light of the street lamp on the corner fades and becomes dim before the middle of the block is reached, so that we look eagerly for the next lamp. The light diminishes in brightness much more rapidly than we realize, as the following simple experiment will show. Let a single candle serve as our light, and at a distance of one foot from the candle place a photograph. In this position the photograph receives a definite amount of light from the candle and has a certain brightness.

If now we place a similar photograph directly behind the first photograph and at a distance of two feet from the candle, the second photograph receives no light because the first one cuts off all the light. If, however, the first photograph is removed, the light which fell on it passes outward and spreads itself over a larger area, until at the distance of the second photograph the light spreads itself over four times as large an area as formerly. At this distance, then, the illumination on the second photograph is only one fourth as strong as it was on a similar photograph held at a distance of one foot from the candle.

The photograph or object placed at a distance of one foot from a light is well illuminated; if it is placed at a distance of two feet, the illumination is only one fourth as strong, and if the object is placed three feet away, the illumination is only one ninth as strong. This fact should make us have thought and care in the use of our eyes. We think we are sixteen times as well off with our incandescent lights as our ancestors were with simple candles, but we must reflect that our ancestors kept the candle near them, "at their elbow," so to speak, while we sit at some distance from the light and unconcernedly read and sew.

As an object recedes from a light the illumination which it receives diminishes rapidly, for the strength of the illumination is inversely proportional to the square of distance of the object from the light. Our ancestors with a candle at a distance of one foot from a book were as well off as we are with an incandescent light four feet away.