QUESTIONS.-1. What is an oxyd? 2. What are the principal ways by which metallic oxyds are formed? 3. What is said of iron as an example? 4. What is red lead and how is it made? 5. What is said of the different capacity and attraction of metals for oxygen? 6. What experiment is given for illustration? 7. What is said of the properties of nitrous oxyd gas? 8. What effects does it produce on being inhaled? 9. How may it be procured? 10. How may combustion be defined? 11. How is the process of combustion explained? 12. What remains when the combustion is over? 13. What is smoke? 14. How may the agency of oxygen in combustion be demonstrated? 15. What becomes of the nitrogen gas? 16. What is said of the indestructibility of matter? 17. What is a retort? (see Appendix.) 18. How may chlorine be procured? 19. What is said of the attraction of chlorine for the metals? 20. How is combustion defined in the Appendix, and on what grounds is it so defined?

LESSON 67.

Electricity.

Electric. The first electrical phenomena are supposed to have been observed in a mineral substance called arber, in Greek elektron, and hence the fluid or power has been denominated electric.

THE surface of the earth, and of all the bodies with which we are acquainted, is supposed to contain or possess a power of exciting or exhibiting a certain quantity of an exceedingly subtile agent, called the electric fluid or power. The quantity usually belonging to any surface, is called its natural share, and then it produces no sensible effects; but when any surface becomes possessed of more, or of less, than its natural quantity, it is electrified, and it then exhibits a variety of peculiar and surprising phenomena ascribed to the power called electric. If you take a stick of sealing-wax and rub it on the sleeve of your coat, it will have the power of attracting small pieces of paper, or other light substances, when held near them. If a clean and dry glass tube be briskly rubbed with the hand, or with a piece of flannel, and then presented to any small light substances, it will immediately attract and repel them alternately for a considerable time. The tube is then said to be excited. If an excited glass tube, in a dark room, be brought within about half an inch of the finger, a lucid spark will be seen between the finger and the tube, accompanied with a snapping noise, and a peculiar sensation of the finger. Dry flannel clothes,

when handled in the dark, frequently exhibit a sparkling appearance, attended with the same kind of noise that is heard in the experiment of the glass tube.

All those bodies which transmit or conduct electricity from one surface to another, are called conductors, and those surfaces that will not transmit the electric power, are called electrics or non-conductors. The general class of conductors comprehends metals, ores, and fluids in their natural state, except air and oils. Vitrified and resinous substances, amber, sulphur, wax, silk, cotton, and feathers, are electrics or non-conductors. Many of these, such as glass, resin, and air, become conductors by being heated. When a surface is supposed to have more than its natural quantity of this fluid, it is said to be positively electrified; and when less than its natural share, to be negatively electrified. When any electrified conductor is wholly surrounded by non-conductors, so that the electric fluid cannot pass from it along conductors to the earth, it is said to be insulated. The human body is a good conductor of electricity; but if a person stand on a cake of resin, or on a stool supported by glass legs, the electric fluid cannot pass from him to the earth, and if he is touched by another person standing on the ground, the same sparkling appearance and noise, as mentioned above, will be exhibited. Two surfaces, both positively, or both negatively electrified, repel each other; and two substances, of which one is positively, and the other negatively electrified, attract each other. Opposite electricities always accompany each other, for if any surface become positive, the surface with which it is rubbed becomes negative; and if any surface be rendered positive, the nearest conducting surface will become negative. When one side of a conductor receives the electric fluid, its whole surface is instantly pervaded; but when an electric or nonconductor is presented to an electrified body, it becomes electrified on a small spot only. If to one side of a pane of glass, you communicate positive electricity, the opposite side will become negatively electrified, and the plate is then said to be charged. These electricities cannot come together, unless a communication, by means of conductors, is made between the sides of the glass; and if their union be made through the human body, it produces an affection of the nerves called an electric shock.

As the excitation which is produced by rubbing with the hand on a tube or plate of glass, is not only very laborious, but inadequate to the production of any material quantity of electric fluid, machines have been constructed of various forms for this purpose. The most common machine consists of a glass cylinder, supported by two glass pillars, and made to turn by a crank or handle. A rubber, or cushion, of leather, spread with an amalgam of mercury and zinc or tin is fastened to a spring, which proceeds from a socket cemented on the top of another glass pillar. A piece of black silk is fastened to the cushion and extended over the cylinder, nearly to the receiving points, to prevent the fluid from flying off. A fourth glass pillar supports what is called the prime conductor, which is made of hollow brass or tin plate, and, at the end towards the cylinder, has a collection of pointed wires, and at the other end, a single wire terminated by a brass ball. A small chain is fastened to the cushion, one end of which extends to the floor or table. It serves to conduct the fluid in passing from the earth to supply the machine. When the cylinder is turned swiftly, the electric fluid passes from the rubber to the glass, and is thence conveyed to the points of the prime conductor, which is thus positively electrified. While the electric fluid is collecting, it produces a crackling noise, and in a darkened room the flame will be seen spread on the surface of the cylinder. If a cylinder be made of resin, the electricity is the reverse of that which is produced by the smooth glass cylinder and rubber of the usual machines; for in this case the rubber partakes of the positive, and the cylinder, and prime conductor, is electrified with the negative. This difference between the resin and glass has given rise to what is called the double current, or vitreous and resinous electricity; but it is generally supposed that the difference arises more from the effect of the surfaces that act on each other, than from any peculiar qualities in the different bodies.

Some of the experiments which may be made with an electrical machine are necessary for illustrating the laws of electricity, and others are merely entertaining. If the inside of a glass tumbler be electrified by presenting it to a pointed wire extending from the prime conductor, and then placed over a few pith-balls laid upon a table, the balls will immediately begin to leap up along the sides of the glass, and then

back to the table; they are attracted and repelled by the electrified inside surface of the glass, the electricity of which they gradually conduct to the table. If a person having long hair, not tied up, be placed upon an insulated stand, and, by means of a chain be connected with the prime conductor, when the machine is put in motion, the hairs on his head, by repelling each other, will stand out in a most surprising manner. A piece of sponge, filled with water, and hung to a conductor, when electrified in a dark room, exhibits a most beautiful appearance. If a piece of sealing-wax be fastened to a wire, and the wire be fixed into the end of the conductor, and the wax lighted, the moment the machine is worked, the wax will fly off in the finest threads imaginable. Take a two ounce phial, half full of olive-oil, pass a slender wire through the cork, and let the end of it be so bent as to touch the glass just below the surface of the oil; then place your thumb opposite the point of the wire in the phial, and if, in that position, you take a spark from the charged conductor, the spark, in order to reach your thumb, will actually perforate the glass. In this way holes may be made all round the phial.

room?

QUESTIONS.-1. What parts of bodies contain the electric fluid? 2. When is a body said to be electrified? 3. What experiment may be made with sealing-wax? 4. When is a glass tube said to be excited? 5. What is said respecting an excited tube when in a dark 6. What are conductors of electricity? 7. Electrics, or nonconductors? 8. When is a surface positively, and when negatively electrified? 9. When is a conductor said to be insulated? 10. What is said of the human body as a conductor? 11. When do surfaces repel, and when attract each other? 12. What takes place when a conductor receives the electric fluid ?-non-conductor? 13. When is a plate of glass said to be charged? 14. What is an electric shock? 15. Describe the electrical machine. 16. What are some of the experiments that may be made with it? (See Electrical Machine, fig. 49.) [NOTE. The earliest account of any known electrical effect is by the ancient naturalists, Thales and Theophrastus, who flourished, the first 600, and the latter 300 years before the present era.]

152

DR. FRANKLIN'S DISCOVERY.

LESSON 68.

Electricity (continued.)

A queous, watery. Collapse', to fall together.

after

THE Leyden phial is a glass jar coated with tin foil on the inside and outside within about three inches of the top of its cylindrical part, and having a wire with a brass ball at its extremity. This wire passes through a cork or piece of wood, and at its lower extremity is a small chain, or wire, that touches the inside coating in several places, and serves as a conductor to charge the jar with electric fluid. On bringing the ball of the jar near the prime conductor, a few turns of the machine, the jar will be charged. The discharging rod consists of two brass balls attached to the ends of a wire, bent in the form of a semicircle, and fixed to a glass handle. When one of the balls of the discharging rod is applied to the ball of the jar, and the other to the outside coating, a communication is made between the outside and inside of the jar, by which the equilibrium is instantly restored by the superabundant electricity passing from one side to the other, appearing in the form of a vivid flash, and accompanied with a loud report. Any number of persons may receive the shock together by laying hold of each other's hands, the person at one end touching the outside of the jar, and the person at the other end bringing his hand near the ball of the jar. If there were a hundred persons so situated, they would every one feel the shock at the same instant. The electric fluid may be thus conveyed many miles in a moment of time. When great force is required from the electric fluid, a number of jars of the above description are connected together by making a communication between all their outsides, and another between all their insides. In this manner any number of jars may be charged with the same facility as a single one, and from the powerful effect of the electric fluid, when it is thus collected, it is called an electrical battery.

The Leyden phial received its name from the birth-place of the discoverer, who was a native of Leyden in Holland. But the greatest discovery that was ever made in electricity was reserved for Dr. Franklin, in America. It had been