heat waves, can be reflected or thrown back by objects upon which they strike. Thus, the presence of a sounding board behind a speaker makes it easier to hear him, since the reflected waves serve to reinforce or strengthen the original waves. The walls of a room, if they are not too far from the speaker, act in the same way, and thus it is usually easier to hear in a room than in the open air. If, however, the walls of a room or hall are more than fifty-six feet from the speaker there may be an echo. The reason for this is as follows: The sensation of sound persists for about 1 of a second, and as sound waves, under ordinary atmospheric conditions of temperature and pressure, travel at the rate of about 1100 feet per second, or 100 feet in the tenth of a second, if the walls are not more than fifty-six feet from the speaker, the waves can reach the walls and be thrown back to the audience before the sensation caused by the original waves has ceased. If, however, the hall is so large that the reflected waves do not reach the audience before the original sensation has ceased and yet is not large enough to prevent reflection, the reflected waves will produce a secondary sensation like unto the first and this is what constitutes an echo. The nature of the reflecting surfaces greatly influences the reflection of sound. Sound waves are reflected much more strongly from smooth, hard walls than from those which are carved or hung with draperies. For this reason, draperies are sometimes used to prevent echoes.

Megaphone. Stethoscope. Ear-Trumpet.-These are three examples of appliances used to intensify sounds by concentration of the sound waves and, to some extent, by reflection.

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telephone, omitting details, may be said to consist of: (1) a box-like portion known as the transmitter—this has a mouthpiece in front and a diaphragm at the back and a portion of it is filled with granular pieces of carbon into which the electric circuit wires enter; (2) wires, which are in connection with others from the electric current generator and from the receiver; (3) the receiver, i. e., the part from which the sound is heard. The structure of the receiver is similar to that of the transmitter. When a person speaks into the mouthpiece of the transmitter, the sound waves produced start the diaphragm at the back of the transmitter vibrating; this causes fluctuations in the electric current that correspond with the sound waves produced by the voice, and, though transmitted from one wire to another at the central station, these cause the diaphragm in the receiver to vibrate and give rise to sound waves in the air in the receiver similar to those started in the transmitter by the voice of the speaker.

The phonograph.-The phonograph is still another example of the manner in which sound waves can act upon and be reproduced by different media. Records are made by talking, singing, etc., in front of a cone at the back of which is a membranous diaphragm. The latter is set in motion by the sound waves produced by talking, etc., and transmits its motion to a sharppointed instrument, known as the stylus, which is attached to its under surface. While the record is being made a wax-coated disc or cylinder is kept rotating under the stylus and the latter cuts into the wax the impressions of the wave forms received from the diaphragm.

The essential parts of phonographs used to repro

duce sounds are the same as those on which the records are made, and when a record is placed and made to revolve under the stylus, the latter passes in the traces made by the other stylus and, by so doing, moves the diaphragm, causing it to produce the same form of waves as those which made the tracings on the record and it thus gives rise to similar sounds.

The cones of the phonograph being different in every respect from the human pharynx, there is, however, a varying amount of difference in the tone of sounds reproduced by phonographs.

Difference in sounds.-The differences in sound are classed under three headings-viz., pitch, quality, and loudness.

Pitch. By pitch is meant the position of a sound in the musical scale. The pitch is said to be high when the note is up toward the treble and low when it is down toward the base. The greater the number of vibrations per second, the higher will be the pitch of a sound. The difference between the pitch of male and female voices is due primarily to the different size of the larynx and vocal cords in the two sexes, the larynx being larger and the cords longer in men than in women. A boy's larynx resembles the female larynx and therefore his voice is similar. The socalled breaking of the boy's voice, which occurs at puberty, is due to faulty neuro-muscular control brought about in part by the very rapid growth of the larynx and vocal cords which takes place at that time.

Quality, tone or timbre.-The quality, known also as the tone or timbre, of sound constitutes the various characteristics other than pitch by which one person's voice can be distinguished from another's and one type of musical instrument from another. Na

The megaphone, a hollow, cone-shaped appliance, starts the waves produced by anyone speaking into it in one direction and thus keeps them from spreading as widely as usual; this greatly intensifies the sound and increases the distance that it travels in the direction in which it was started.

The stethoscope is an instrument used for distinguishing sounds produced within the body. Stethoscopes vary in their structure, but usually consist of a tube on one end of which is an enlarged portion containing a very delicate diaphragm and on the other end one or two projections that fit into the ears of the listener. The portion containing the diaphragm is placed over the part of the patient's body that is being examined, and the impulses of the sounds produced by the heart, lungs, air in the cavities, or whatever is causing sounds in the part of the body being examined, start similar vibrations in the diaphragm of the stethoscope, and the waves produced in the air in the tube by these vibrations, being confined within a limited area, are so intensified that even slight abnormal differences of sound can be distinguished.

The concentration of sound waves by the eartrumpet so increases their force that they are able to produce vibration in the membranes and small bones of the ear when, as the result of disease or injury, these parts have become so stiffened that they are not acted upon by less forcible waves.

CHAPTER X

CHEMICAL REACTIONS. VALENCE. RADICALS.
CHEMICAL FORMULAS AND EQUATIONS

Nature and Causes of Chemical Reactions-Nature of ValenceRadicals-Chemical Formulæ and Equations.

Chemical reactions. By a chemical reaction is meant the chemical changes that take place between certain substances when they are brought together.

Agents which promote chemical reactions. Catalyzers. Certain elements have such a strong affinity for each other that, if brought into contact, they will unite, sometimes even leaving other elements with which they are already in combination to do so. Many other elements, however, especially when already combined, do not unite thus readily, and the help of agents, such as light, heat, electricity, is necessary to promote chemical action. As a rule, solution facilitates-in fact, is often necessary forchemical reactions, and there are many chemical substances and ferments that, in some unknown way, hasten them, but do not themselves enter into the reaction; such substances are called catalyzers, and the action is termed catalysis. The action of catalyzers will be demonstrated in Experiment 13.