FIG. 160.—Pasteurizing apparatus, an arrangement by which milk is conveniently heated to destroy disease germs.FIG. 160.—Pasteurizing apparatus, an arrangement by which milk is conveniently heated to destroy disease germs.
When disease germs are within the body, the problem is far from simple, because chemicals which would effectively destroy the germs would be fatal to life itself. But when germs are outside the body, as in water or milk, or on clothing, dishes, or furniture, they can be easily killed. One of the best methods of destroying germs is to subject them to intense heat. Contaminated water is made safe by boiling for a few minutes, because the strong heat destroys the disease-producing germs. Scalded or Pasteurized milk saves the lives of scores of babies, because the germs of summer complaint which lurkin poor milk are killed and rendered harmless in the process of scalding. Dishes used by consumptives, and persons suffering from contagious diseases, can be made harmless by thorough washing in thick suds of almost boiling water.
The bedding and clothing of persons suffering with diphtheria, tuberculosis, and other germ diseases should always be boiled and hung to dry in the bright sunlight. Heat and sunshine are two of the best disinfectants.
232. Chemicals.Objects, such as furniture, which cannot be boiled, are disinfected by the use of any one of several chemicals, such as sulphur, carbolic acid, chloride of lime, corrosive sublimate, etc.
One of the simplest methods of disinfecting consists in burning sulphur in a room whose doors, windows, and keyholes have been closed, so that the burning fumes cannot escape, but remain in the room long enough to destroy disease germs. This is probably the most common means of fumigation.
For general purposes, carbolic acid is one of the very best disinfectants, but must be used with caution, as it is a deadly poison except when very dilute.
Chloride of lime when exposed to the air and moisture slowly gives off chlorine, and can be used as a disinfectant because the gas thus set free attacks germs and destroys them. For this reason chloride of lime is an excellent disinfectant of drainpipes. Certain bowel troubles, such as diarrhœa, are due to microbes, and if the waste matter of a person suffering from this or similar diseases is allowed passage through the drainage system, much damage may be done. But a small amount of chloride of lime in the closet bowl will insure disinfection.
233. Personal Disinfection.The hands may gather germs from any substances or objects with which they come in contact; hence the hands should be washed with soap and water,and especially before eating. Physicians who perform operations wash not only their hands, but their instruments, sterilizing the latter by placing them in boiling water for several minutes.
Cuts and wounds allow easy access to the body; a small cut has been known to cause death because of the bacteria which found their way into the open wound and produced disease. In order to destroy any germs which may have entered into the cut from the instrument, it is well to wash out the wound with some mild disinfectant, such as very dilute carbolic acid or hydrogen peroxide, and then to bind the wound with a clean cloth, to prevent later entrance of germs.
234. Chemicals as Food Preservatives.The spoiling of meats and soups, and the souring of milk and preserves, are due to germs which, like those producing disease, can be destroyed by heat and by chemicals.
Milk heated to the boiling point does not sour readily, and successful canning consists in cooking fruits and vegetables until all the germs are killed, and then sealing the cans so that germs from outside cannot find entrance and undo the work of the canner.
Some dealers and manufacturers have learned that certain chemicals will act as food preservatives, and hence they have replaced the safe method of careful canning by the quicker and simpler plan of adding chemicals to food. Catchup, sauces, and jellies are now frequently preserved in this way. But the chemicals which destroy bacteria frequently injure the consumer as well. And so much harm has been done by food preservatives that the pure food laws require that cans and bottles contain a labeled statement of the kind and quantity of chemicals used.
Even milk is not exempt, but is doctored to prevent souring, the preservative most generally used by milk dealersbeing formaldehyde. The vast quantity of milk consumed by young and old, sick and well, makes the use of formaldehyde a serious menace to health, because no constitution can endure the injury done by the constant use of preservatives.
The most popular and widely used preservatives of meats are borax and boric acid. These chemicals not only arrest decay, but partially restore to old and bad meat the appearance of freshness; in this way unscrupulous dealers are able to sell to the public in one form or other meats which may have undergone partial decomposition; sausage frequently contains partially decomposed meat, restored as it were by chemicals.
In jams and catchups there is abundant opportunity for preservatives; badly or partially decayed fruits are sometimes disinfected and used as the basis of foods sold by so-called good dealers. Benzoate of soda, and salicylic acid are the chemicals most widely employed for this purpose, with coal-tar dyes to simulate the natural color of the fruit.
Many of the cheap candies sold by street venders are not fit for consumption, since they are not only made of bad material, but are frequently in addition given a light dipping in varnish as a protection against the decaying influences of the atmosphere.
The only wise preservatives are those long known and employed by our ancestors; salt, vinegar, and spices are all food preservatives, but they are at the same time substances which in small amounts are not injurious to the body. Smoked herring and salted mackerel are chemically preserved foods, but they are none the less safe and digestible.
235. The Preservation of Wood and Metal.The decaying of wood and the rusting of metal are due to the action of air and moisture. When wood and metal are surrounded with a covering which neither air nor moisture can penetrate, decayand rust are prevented. Paint affords such a protective covering. The main constituent of paint is a compound of white lead or other metallic substance; this is mixed with linseed oil or its equivalent in order that it may be spread over wood and metal in a thin, even coating. After the mixture has been applied, it hardens and forms a tough skin fairly impervious to weathering. For the sake of ornamentation, various colored pigments are added to the paint and give variety of effect.
Railroad ties and street paving blocks are ordinarily protected by oil rather than paint. Wood is soaked in creosote oil until it becomes thoroughly saturated with the oily substance. The pores of the wood are thus closed to the entrance of air and moisture, and decay is avoided. Wood treated in this way is very durable. Creosote is poisonous to insects and many small animals, and thus acts as a preservation not only against the elements but against animal life as well.
236. Stimulants and Narcotics.Man has learned not only the action of substances upon each other, such as bleaching solution upon coloring matter, washing soda upon grease, acids upon bases, but also the effect which certain chemicals have upon the human body.
Drugs and their varying effects upon the human system have been known to mankind from remote ages; in the early days, familiar leaves, roots, and twigs were steeped in water to form medicines which served for the treatment of all ailments. In more recent times, however, these simple herb teas have been supplanted by complex drugs, and now medicines are compounded not only from innumerable plant products, but from animal and mineral matter as well. Quinine, rhubarb, and arnica are examples of purely vegetable products; iron, mercury, and arsenic are equally well known as distinctly mineral products, while cod-liver oil is the most familiar illustration of an animal remedy. Ordinarily a combination of products best serves the ends of the physician.
Substances which, like cod-liver oil, serve as food to a worn-out body, or, like iron, tend to enrich the blood, or, like quinine, aid in bringing an abnormal system to a healthy condition, are valuable servants and cannot be entirely dispensed with so long as man is subject to disease.
But substances which, like opium, laudanum, and alcohol, are not required by the body as food, or as a systematic, intelligent aid to recovery, but are taken solely for the stimulus aroused or for the insensibility induced, are harmful toman, and cannot be indulged in by him without ultimate mental, moral, and physical loss. Substances of the latter class are known as narcotics and stimulants.
237. The Cost of Health.In the physical as in the financial world, nothing is to be had without a price. Vigor, endurance, and mental alertness are bought by hygienic living; that is, by proper food, fresh air, exercise, cleanliness, and reasonable hours. Some people wish vigor, endurance, etc., but are unwilling to live the life which will develop these qualities. Plenty of sleep, exercise, and simple food all tend to lay the foundations of health. Many, however, are not willing to take the care necessary for healthful living, because it would force them to sacrifice some of the hours of pleasure. Sooner or later, these pleasure-seekers begin to feel tired and worn, and some of them turn to drugs and narcotics for artificial strength. At first the drugs seem to restore the lost energy, and without harm; however, the cost soon proves to be one of the highest Nature ever demands.
238. The Uncounted Cost.The first and most obvious effect of opium, for example, is to deaden pain and to arouse pleasure; but while the drug is producing these soothing sensations, it interferes with bodily functions. Secretion, digestion, absorption of food, and the removal of waste matters are hindered. Continued use of the drug leads to headache, exhaustion, nervous depression, and heart weakness. There is thus a heavy toll reckoned against the user, and the creditor is relentless in demanding payment.
Moreover, the respite allowed by a narcotic is exceedingly brief, and a depression which is long and deep inevitably follows. In order to overcome this depression, recourse is usually had to a further dose, and as time goes on, the intervals of depression become more frequent and lasting, and the necessity to overcome them increases. Thus withoutintention one finds one's self bound to the drug, its fast victim. The sanatoria of our country are crowded with people who are trying to free themselves of a drug habit into which they have drifted unintentionally if not altogether unknowingly. What is true of opium is equally applicable to other narcotics.
239. The Right Use of Narcotics.In the hands of the physician, narcotics are a great blessing. In some cases, by relieving pain, they give the system the rest necessary for overcoming the cause of the pain. Only those who know of the suffering endured in former times can fully appreciate the decrease in pain brought about by the proper use of narcotics.
240. Patent Medicines, Cough Sirups.A reputable physician is solicitous regarding the permanent welfare of his patient and administers carefully chosen and harmless drugs. Mere medicine venders, however, ignore the good of mankind, and flood the market with cheap patent preparations which delude and injure those who purchase, but bring millions of dollars to those who manufacture.
Practically all of these patent, or proprietary, preparations contain a large proportion of narcotics or stimulants, and hence the benefit which they seem to afford the user is by no means genuine; examination shows that the relief brought by them is due either to a temporary deadening of sensibilities by narcotics or to a fleeting stimulation by alcohol and kindred substances.
Among the most common ailments of both young and old are coughs and colds; hence many patent cough mixtures have been manufactured and placed on the market for the consumption of a credulous public. Such "quick cures" almost invariably contain one or more narcotic drugs, and not only do not relieve the cold permanently, but occasion subsequent disorders. Even lozenges and pastilles are not free from fraud, but have a goodly proportion of narcotics, containing in somecases chloroform, morphine, and ether.
The widespread use of patent cough medicines is due largely to the fact that many persons avoid consulting a physician about so trivial an ailment as an ordinary cold, or are reluctant to pay a medical fee for what seems a slight indisposition and hence attempt to doctor themselves.
Catarrh is a very prevalent disease in America, and consequently numerous catarrh remedies have been devised, most of which contain in a disguised form the pernicious drug, cocaine. Laws have been enacted which require on the labels a declaration of the contents of the preparation, both as to the kind of drug used and the amount, and the choice of accepting or refusing such mixtures is left to the individual. But the great mass of people are ignorant of the harmful nature of drugs in general, and hence do not even read the self-accusing label, or if they do glance at it, fail to comprehend the dangerous nature of the drugs specified there. In order to safeguard the uninformed purchaser and to restrict the manufacture of harmful patent remedies, some states limit the sale of all preparations containing narcotics and thus give free rein to neither consumer nor producer.
241. Soothing Sirups; Soft Drinks.The development of a race is limited by the mental and physical growth of its children, and yet thousands of its children are annually stunted and weakened by drugs, because most colic cures, teething concoctions, and soothing syrups are merely agreeably flavored drug mixtures. Those who have used such preparations freely, know that a child usually becomes fretful and irritable between doses, and can be quieted only by larger and more frequent supplies. A habit formed in this way is difficult to overcome, and many a child when scarcely over its babyhood had a craving which in later years may lead to systematic drug taking. And even though the pernicious drug craving is not created, considerable harm is done to the child, becauseits body is left weak and non-resistant to diseases of infancy and childhood.
Many of our soft drinks contain narcotics. The use of the coca leaf and the kola nut for such preparations has increased very greatly within the last few years, and doubtless legislation will soon be instituted against the indiscriminate sale of soft drinks.
242. Headache Powders.The stress and strain of modern life has opened wide the door to a multitude of bodily ills, among which may be mentioned headache. Work must be done and business attended to, and the average sufferer does not take time from his vocation to investigate the cause of the headache, but unthinkingly grasps at any remedy which will remove the immediate pain, and utterly disregards later injury. The relief afforded by most headache mixtures is due to the presence of antipyrin or acetanilid, and it has been shown conclusively that these drugs weaken heart action, diminish circulation, reduce the number of red corpuscles in the blood, and bring on a condition of chronic anemia. Pallid cheeks and blue lips are visible evidence of the too frequent use of headache powders.
The labels required by law are often deceptive and convey no adequate idea of the amount of drug consumed; for example, 240 grains of acetanilid to an ounce seems a small quantity of drug for a powder, but when one considers that there are only 480 grains in an ounce, it will be seen that each powder is one half acetanilid.
Powders taken in small quantities and at rare intervals are apparently harmless; but they never remove the cause of the trouble, and hence the discomfort soon returns with renewed force. Ordinarily, hygienic living will eliminate the source of the trouble, but if it does not, a physician should be consulted and medicine should be procured from himwhich will restore the deranged system to its normal healthy condition.
243. Other Deceptions.Nearly all patent medicines contain some alcohol, and in many, the quantity of alcohol is far in excess of that found in the strongest wines. Tonics and bitters advertised as a cure for spring fever and a worn-out system are scarcely more than cheap cocktails, as one writer has derisively called them, and the amount of alcohol in some widely advertised patent remedies is alarmingly large and almost equal to that of strong whisky.
FIG. 161.—Diagram showing the amount of alcohol in some alcoholic drinks and in one much used patent medicine.FIG. 161.—Diagram showing the amount of alcohol in some alcoholic drinks and in one much used patent medicine.
Some conscientious persons who would not touch beer, wine, whisky, or any other intoxicating drink consume patent remedies containing large quantities of alcohol and thus unintentionally expose themselves to mental and physical danger. In all cases of bodily disorder, the only safe course is to consult a physician who has devoted himself to the study of the body and the methods by which a disordered system may be restored to health.
244. Nitrogen.A substance which plays an important part in animal and plant life is nitrogen. Soil and the fertilizers which enrich it, the plants which grow on it, and the animals which feed on these, all contain nitrogen or nitrogenous compounds. The atmosphere, which we ordinarily think of as a storehouse of oxygen, contains far more nitrogen than oxygen, since four fifths of its whole weight is made up of this element.
Nitrogen is colorless, odorless, and tasteless. Air is composed chiefly of oxygen and nitrogen; if, therefore, the oxygen in a vessel filled with air can be made to unite with some other substance or can be removed, there will be a residue of nitrogen. This can be done by floating on water a light dish containing phosphorus, then igniting the phosphorus, and placing an inverted jar over the burning substance. The phosphorus in burning unites with the oxygen of the air and hence the gas that remains in the jar is chiefly nitrogen. It has the characteristics mentioned above and, in addition, does not combine readily with other substances.
245. Plant Food.Food is the course of energy in every living thing and is essential to both animal and plant life. Plants get their food from the lifeless matter which exists in the air and in the soil; while animals get their food from plants. It is true that man and many other animals eatfleshy foods and depend upon them for partial sustenance, but the ultimate source of all animal food is plant life, since meat-producing animals live upon plant growth.
Plants get their food from the air, the soil, and moisture. From the air, the leaves take carbon dioxide and water and transform them into starchy food; from the soil, the roots take water rich in mineral matters dissolved from the soil. From the substances thus gathered, the plant lives and builds up its structure.
A food substance necessary to plant life and growth is nitrogen. Since a vast store of nitrogen exists in the air, it would seem that plants should never lack for this food, but most plants are unable to make use of the boundless store of atmospheric nitrogen, because they do not possess the power of abstracting nitrogen from the air. For this reason, they have to depend solely upon nitrogenous compounds which are present in the soil and are soluble in water. The soluble nitrogenous soil compounds are absorbed by roots and are utilized by plants for food.
246. The Poverty of the Soil.Plant roots are constantly taking nitrogen and its compounds from the soil. If crops which grow from the soil are removed year after year, the soil becomes poorer in nitrogen, and finally possesses too little of it to support vigorous and healthy plant life. The nitrogen of the soil can be restored if we add to it a fertilizer containing nitrogen compounds which are soluble in water. Decayed vegetable matter contains large quantities of nitrogen compounds, and hence if decayed vegetation is placed upon soil or is plowed into soil, it acts as a fertilizer, returning to the soil what was taken from it. Since man and all other animals subsist upon plants, their bodies likewise contain nitrogenous substances, and hence manure and waste animal matter is valuable as a fertilizer or soil restorer.
247. Bacteria as Nitrogen Gatherers.Soil from which crops are removed year after year usually becomes less fertile, but the soil from which crops of clover, peas, beans, or alfalfa have been removed is richer in nitrogen rather than poorer. This is because the roots of these plants often have on them tiny swellings, or tubercles, in which millions of certain bacteria live and multiply. These bacteria have the remarkable power of taking free nitrogen from the air in the soil and of combining it with other substances to form compounds which plants can use. The bacteria-made compounds dissolve in the soil water and are absorbed into the plant by the roots. So much nitrogen-containing material is made by the root bacteria of plants of the pea family that the soil in which they grow becomes somewhat richer in nitrogen, and if plants which cannot make nitrogen are subsequently planted in such a soil, they find there a store of nitrogen. A crop of peas, beans, or clover is equivalent to nitrogenous fertilizer and helps to make ready the soil for other crops.
FIG. 162.—Roots of soy bean having tubercle-bearing bacteria.FIG. 162.—Roots of soy bean having tubercle-bearing bacteria.
248. Artificial Fertilizers.Plants need other foods besides nitrogen, and they exhaust the soil not only of nitrogen, but also of phosphorus and potash, since large quantities of these are necessary for plant life. There are many other substances absorbed from the soil by the plant, namely, iron, sodium, calcium, magnesium, but these are used in smaller quantities and the supply in the soil does not readily become exhausted.
Commercial fertilizers generally contain nitrogen, phosphorus, and potash in amounts varying with the requirements of the soil. Wheat requires a large amount of phosphorus and quickly exhausts the ground of that food stuff; a field which has supported a crop of wheat is particularly poor in phosphorus, and a satisfactory fertilizer for that land would necessarily contain a large percentage of phosphorus. The fertilizer to be used in a soil depends upon the character of the soil and upon the crops previously grown on it.
FIG. 163.—Water cultures of buckwheat: 1, with all the food elements; 2, without potash; 3, without nitrates.FIG. 163.—Water cultures of buckwheat: 1, with all the food elements; 2, without potash; 3, without nitrates.
The quantity of fertilizer needed by the farmers of the world is enormous, and the problem of securing the necessary substances in quantities sufficient to satisfy the demand bids fair to be serious. But modern chemistry is at work on the problem, and already it is possible to make some nitrogen compounds on a commercial scale. When nitrogen gas is in contact with heated calcium carbide, a reaction takes place which results in the formation of calcium nitride, a compound suitable for enriching the soil. There are other commercial methods for obtaining nitrogen compounds which are suitable for absorption by plant roots.
Phosphorus is obtained from bone ash and from phosphate rock which is widely distributed over the surface of the earth. Bone ash and thousands of tons of phosphate rock are treated with sulphuric acid to form a phosphorus compound which is soluble in soil water and which, when added to soil, will be usable by the plants growing there.
The other important ingredient of most fertilizers is potash.Wood ashes are rich in potash and are a valuable addition to the soil. But the amount of potash thus obtained is far too limited to supply the needs of agriculture; and to-day the main sources of potash are the vast deposits of potassium salts found in Prussia.
Although Germany now furnishes the American farmer with the bulk of his potash, she may not do so much longer. In 1911 an indirect potash tax was levied by Germany on her best customer, the United States, to whom 15 million dollars' worth of potash had been sold the preceding year. This led Americans to inquire whether potash could not be obtained at home.
Geologists say that long ages ago Germany was submerged, that the waters slowly evaporated and that the various substances in the sea water were deposited in thick layers. The deposits thus left by the evaporation of the sea water gradually became hidden by sediment and soil, and lost to sight. From such deposits, potash is obtained. Geologists tell us that our own Western States were once submerged, and that the waters evaporated and disappeared from our land very much as they did from Germany. The Great Salt Lake of Utah is a relic of a great body of water. If it be true that waters once covered our Western States, there may be buried deposits of potash there, and to-day the search for the hidden treasure is going on with the energy and enthusiasm characteristic of America.
Another probable source of potash is seaweed. The sea is a vast reservoir of potash, and seaweed, especially the giant kelp, absorbs large quantities of this potash. A ton of dried kelp (dried by sun and wind) contains about 500 pounds of pure potash. The kelps are abundant, covering thousands of square miles in the Pacific Ocean, from Mexico to the Arctic Ocean.
249. The Senses.All the information which we possess of the world around us comes to us through the use of the senses of sight, hearing, taste, touch, and smell. Of the five senses, sight and hearing are generally considered the most valuable. In preceding Chapters we studied the important facts relative to light and the power of vision; it remains for us to study Sound as we studied Light, and to learn what we can of sound and the power to hear.
250. How Sound is Produced.If one investigates the source of any sound, he will always find that it is due to motion of some kind. A sudden noise is traced to the fall of an object, or to an explosion, or to a collision; in fact, is due to the motion of matter. A piano gives out sound whenever a player strikes the keys and sets in motion the various wires within the piano; speech and song are caused by the motion of chest, vocal cords, and lips.
FIG. 164.—Sprays of water show that the fork is in motion.FIG. 164.—Sprays of water show that the fork is in motion.
If a large dinner bell is rung, its motion or vibration may be felt on touching it with the finger. If a tuning fork is made to give forth sound by striking it against the knee, or hitting it with a rubber hammer, and is then touched to thesurface of water, small sprays of water will be thrown out, showing that the prongs of the fork are in rapid motion. (A rubber hammer is made by putting a piece of glass tubing through a rubber cork.)
If a light cork ball on the end of a thread is brought in contact with a sounding fork, the ball does not remain at rest, but vibrates back and forth, being driven by the moving prongs.
FIG. 165.—The ball does not remain at restFIG. 165.—The ball does not remain at rest
These simple facts lead us to conclude that all sound is due to the motion of matter, and that a sounding body of any kind is in rapid motion.
251. Sound is carried by Matter.In most cases sound reaches the ear through the air; but air is not the only medium through which sound is carried. A loud noise will startle fish, and cause them to dart away, so we conclude that the sound must have reached them through the water. An Indian puts his ear to the ground in order to detect distant footsteps, because sounds too faint to be heard through the air are comparatively clear when transmitted through the earth. A gentle tapping at one end of a long table can be distinctly heard at the opposite end if the ear is pressed against the table; if the ear is removed from the wood, the sound of tapping is much fainter, showing that wood transmits sound more readily than air. We see therefore that sound can be transmitted to the ear by solids, liquids, or gases.
Matter of any kind can transmit sound to the ear. The following experiments will show that matter is necessary for transmission. Attach a small toy bell to a glass rod (Fig. 166) by means of a rubber tube and pass the rod through one of two openings in a rubber cork. Insert the cork in a strong flask containing a small quantity of water and shake the bell,noting the sound produced. Then heat the flask, allowing the water to boil briskly, and after the boiling has continued for a few minutes remove the flame and instantly close up the second opening by inserting a glass stopper. Now shake the flask and note that the sound is very much fainter than at first. As the flask was warmed, air was rapidly expelled; so that when the flask was shaken the second time, less air was present to transmit the sound. If the glass stopper is removed and the air is allowed to reenter the flask, the loudness of the sound immediately increases.
FIG. 166.—Sound is carried by the air.FIG. 166.—Sound is carried by the air.
Since the sound of the bell grows fainter as air is removed, we infer that there would be no sound if all the air were removed from the flask; that is to say, sound cannot be transmitted through empty space or a vacuum. If sound is to reach our ears, it must be through the agency of matter, such as wood, water, or air, etc.
252. How Sound is transmitted through Air.We saw in Section 250 that sound can always be traced to the motion or vibration of matter. It is impossible to conceive of an object being set into sudden and continued motion without disturbing the air immediately surrounding it. A sounding body always disturbs and throws into vibration the air around it, and the air particles which receive motion from a sounding body transmit their motion to neighboring particles, these in turn to the next adjacent particles, and so on until the motion has traveled to very great distances. The manner in which vibratory motion is transmitted by the atmosphere must be unusual in character, since no motion of the air is apparent, and since in the stillness of night when "not a breath of air" isstirring, the shriek of a railroad whistle miles distant may be heard with perfect clearness. Moreover, the most delicate notes of a violin can be heard in the remotest corners of a concert hall, when not the slightest motion of the air can be seen or felt.
In our study of the atmosphere we saw that air can be compressed and rarefied; in other words, we saw that air is very elastic. It can be shown experimentally that whenever an elastic body in motion comes in contact with a body at rest, the moving body transfers its motion to the second body and then comes to rest itself. Let two billiard balls be suspended in the manner indicated in Figure 167. If one of the balls is drawn aside and is then allowed to fall against the other, the second ball is driven outward to practically the height from which the first ball fell and the first ball comes to rest.
FIG. 167.—Elastic balls.FIG. 167.—Elastic balls.
FIG. 168.—Suspended billiard balls.FIG. 168.—Suspended billiard balls.
If a number of balls are arranged in line as in Figure 168 or Figure 169, and the end ball is raised and then allowed to fall, or ifAis pushed againstC, the last ballBwill move outward alone, with a force nearly equal to that originally possessed byAand to a distance nearly equal to that through whichAmoved. But there will be novisiblemotion of the intervening balls. The force of the moving ballAis given to the second ball, and the second ball in turn gives the motion to the third, and so on throughout the entire number, untilBis reached. ButBhas no ball to give its motion to, henceBitself moves outward, and moves with a force nearly equal to that originally imparted byAand to a distance nearly equal to that through whichAfell. MotionatAis transmitted toBwithout any perceptible motion of the balls lying between these points. Similarly the particles of air set into motion by a sounding body impart their motion to each other, the motion being transmitted onward without any perceptible motion of the air itself. When this motion reaches the ear, it sets the drum of the ear into vibration, and these vibrations are in turn transmitted to the auditory nerves, which interpret the motion as sound.
FIG. 169.—Elastic balls transmit motion.FIG. 169.—Elastic balls transmit motion.
FIG. 170.—When a ball meets more than one ball, it divides its motion.FIG. 170.—When a ball meets more than one ball, it divides its motion.
253. Why Sound dies away with Distance.Since the last ballBis driven outward with a force nearly equal to that possessed byA, it would seem that the effect on the ear drum should be independent of distance and that a sound should be heard as distinctly when remote as when near. But we know from experience that this is not true, because the more distant the source of sound, the fainter the impression; and finally, ifthe distance between the source of sound and the hearer becomes too great, the sound disappears entirely and nothing is heard. The explanation of this well-known fact is found in a further study of the elastic balls (Fig. 170). IfAhits two balls instead of one, the energy possessed byAis given in part to one ball, and in part to the other, so that neither obtains the full amount. These balls, having each received less than the original energy, have less to transmit; each of these balls in turn meets with others, and hence the motion becomes more and more distributed, and distant balls receive less and less impetus. The energy finally given becomes too slight to affect neighboring balls, and the system comes to rest. This is what occurs in the atmosphere; a moving air particle meets not one but many adjacent air particles, and each of these receives a portion of the original energy and transmits a portion. When the original disturbance becomes scattered over a large number of air particles, the energy given to any one air particle becomes correspondingly small, and finally the energy becomes so small that further particles are not affected; beyond this limit the sound cannot be heard.
If an air particle transmitted motion only to those air particles directly in line with it, we should not be able to detect sound unless the ear were in direct line with the source. The fact that an air particle divides its motion among all particles which it touches, that is, among those on the sides as well as those in front, makes it possible to hear sound in all directions. A good speaker is heard not only by those directly in front of him, but by those on the side, and even behind him.
254. Velocity of Sound.The transmission of motion from particle to particle does not occur instantaneously, but requires time. If the distance is short, so that few air particles are involved, the time required for transmission is very brief,and the sound is heard at practically the instant it is made. Ordinarily we are not conscious that it requires time for sound to travel from its source to our ears, because the distance involved is too short. At other times we recognize that there is a delay; for example, thunder reaches our ears after the lightning which caused the thunder has completely disappeared. If the storm is near, the interval of time between the lightning and the thunder is brief, because the sound does not have far to travel; if the storm is distant, the interval is much longer, corresponding to the greater distance through which the sound travels. Sound does not move instantaneously, but requires time for its transmission. The report of a distant cannon is heard after the flash and smoke are seen; the report of a near cannon is heard the instant the flash is seen.
The speed with which sounds travels through the air, or its velocity, was first measured by noting the interval (54.6 seconds) which elapsed between the flash of a cannon and the sound of the report. The distance of the cannon from the observer was measured and found to be 61,045 feet, and by dividing this distance by the number of seconds, we find that the distance traveled by sound in one second is approximately 1118 feet.
High notes and low notes, soft notes and shrill notes, all travel at the same rate. If bass notes traveled faster or slower than soprano notes, or if the delicate tones of the violin traveled faster or slower than the tones of a drum, music would be practically impossible, because at a distance from the source of sound the various tones which should be in unison would be out of time—some arriving late, some early.
255. Sound Waves.Practically everyone knows that a hammock hung with long ropes swings or vibrates more slowlythan one hung with short ropes, and that a stone suspended by a long string swings more slowly than one suspended by a short string. No two rocking chairs vibrate in the same way unless they are exactly alike in shape, size, and material. An object when disturbed vibrates in a manner peculiar to itself, the vibration being slow, as in the case of the long-roped swing, or quick, as in the case of the short-roped swing. The time required for a single swing or vibration is called theperiodof the body, and everything that can vibrate has a characteristic period. Size and shape determine to a large degree the period of a body; for example, a short, thick tuning fork vibrates more rapidly than a tall slender fork.
FIG. 171.—The two hammocks swing differently.FIG. 171.—The two hammocks swing differently.
Some tuning forks when struck vibrate so rapidly that the prongs move back and forth more than 5000 times per second, while other tuning forks vibrate so slowly that the vibrations do not exceed 50 per second. In either case the distance through which the prongs move is very small and the period is very short, so that the eye can seldom detect the movement itself. That the prongs are in motion, however, is seen by the action of a pith ball when brought in contact with the prongs (see Section 250).
FIG. 172.—The pitch given out by a fork depends upon its shape.FIG. 172.—The pitch given out by a fork depends upon its shape.
The disturbance created by a vibrating body is called a wave.
256. Waves.While the disturbance which travels out from a sounding body is commonly called a wave, it is by no means like the type of wave best known to us, namely, the water wave.
If a closely coiled heavy wire is suspended as in Figure 173 and the weight is drawn down and then released, the coil will assume the appearance shown; there is clearly an overcrowding or condensation in some places, and a spreading out or rarefaction in other places. The pulse of condensation and rarefaction which travels the length of the wire is called a wave, although it bears little or no resemblance to the familiar water wave. Sound waves are similar to the waves formed in the stretched coil.
FIG. 173.—Waves in a coiled wire.FIG. 173.—Waves in a coiled wire.
Sound waves may be said to consist of a series of condensations and rarefactions, and the distance between two consecutive condensations and rarefactions may be defined as the wave length.
257. How One Sounding Body produces Sound in Another Body.In Section 255 we saw that any object when disturbed vibrates in a manner peculiar to itself,—its natural period,—a long-roped hammock vibrating slowly and a short-roped hammock vibrating rapidly. From observation we learn that it requires but little force to cause a body to vibrate in its natural period. If a sounding body is near a body which has the same period as itself, the pulses of air produced by the sounding body will, although very small, set the second body into motion and causeit to make a faint sound. When a piano is being played, we are often startled to find that a window pane or an ornament responds to some note of the piano. If two tuning forks of exactly identical periods (that is, of the same frequency) are placed on a table as in Figure 174, and one is struck so as to give forth a clear sound, the second fork will likewise vibrate, even though the two forks may be separated by several feet of air. We can readily see that the second fork is in motion, although it has not been struck, because it will set in motion a pith ball suspended beside it; at first the pith ball does not move, then it moves slightly, and finally bounces rapidly back and forth. If the periods of the two forks are not identical, but differ in the slightest degree, the second fork will not respond to the first fork, no matter how long or how loud the sound of the first fork. If we suppose that the fork vibrates 256 times each second, then 256 gentle pulses of air are produced each second, and these, traveling outward through the air, reach the silent fork and tend to set it in motion. A single pulse of air could not move the solid, heavy prongs, but the accumulated action of 256 vibrations per second soon makes itself felt, and the second fork begins to vibrate, at first gently, then gradually stronger, and finally an audible tone is given forth.
FIG. 174.—When the first fork vibrates, the second responds.FIG. 174.—When the first fork vibrates, the second responds.
The cumulative power of feeble forces acting frequently at definite intervals is seen in many ways in everyday life. Asmall boy can easily swing a much larger boy, provided he gives the swing a gentle push in the right direction every time it passes him. But he must be careful to push at the proper instant, since otherwise his effort does not count for much; if he pushes forward when the swing is moving backward, he really hinders the motion; if he waits until the swing has moved considerably forward, his push counts for little. He must push at the proper instant; that is, the way in which his hand moves in giving the push must correspond exactly with the way in which the swing would naturally vibrate. A very striking experiment can be made by suspending from the ceiling a heavy weight and striking this weight gently at regular, properly timed intervals with a small cork hammer. Soon the pendulum, or weight, will be set swinging.