It

body, while its caloric, becoming free, is diffused among the surrounding bodies. Whenever we burn a combustible body, a continued stream of atmospheric air flows towards the fire place, to occupy the vacancy left by the air that has undergone decomposition, and which, in its turn, becomes decomposed also. Hence a supply of caloric is furnished without intermission, till the whole of the combustible is saturated with oxygen. As the combustible burns, light is disengaged, and the more subtile parts, now converted by caloric into gas, are dissipated in that state. When the combustion is over, nothing remains but the earthy parts of the combustible, and that portion which is converted, by the process, into an oxyd, or an acid. The smoke which arises from a common fire is chiefly water in the state of vapour, with a mixture of carburetted hydrogen and bituminous substances; part of the water comes from the moisture of the fuel; the other part is formed during combustion, by the union of the hydrogen of the combustible with the oxygen of the atmosphere. The agency of oxygen in combustion may be demonstrated by placing a lighted candle under a glass vessel inverted upon a plate of water. will be seen that the candle will go out as soon as it has consumed all the oxygen contained in the included air, and that the water will rise up in the vessel to fill the vacancy. In the decomposition of atmospheric air by combustion, it is natural to ask what becomes of the nitrogen gas? As the oxygen becomes fixed in the combustible body, its caloric is disengaged, a part of which combines with the nitrogen, and carries it off in the form of rarefied nitrogen gas. bodies are burnt, none of their principles are destroyed. We have reason to think that every particle of matter is indestructible, and that the process of combustion merely decomposes the body, and sets its several component parts at liberty, to separate from each other, to form other new and varied combinations. It was said of old, that the Creator weighed the dust, and measured the water, when he made the world. The first quantity is here still; and though man can gather and scatter, move, mix, and unmix, yet he can destroy nothing: the dissolution of one thing is a preparation for the being, and the bloom, and the beauty of another. Something gathers up all the fragments, and nothing is lost.

When

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 attrac tion 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 araber, 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 non-. conductors? 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.]