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if a communication be made between the first and last cell, by means of the hands, a strong shock will be felt, which will be repeated as often as the contact is renewed. Several persons, by joining hands, having first wetted them with water, may receive the shock.

The spark from a powerful galvanic battery acts upon and inflames gun-powder, charcoal, cotton, and other inflammable substances, melts all metals and disperses diamonds. Fill the battery, described above, with water and nitrous acid in the proportion of nine parts of water and one of acid, and wipe the edges of the plates very dry; then fasten two wires to pieces of copper, which are to be put into the outer cells, and in order to hold the wires they must be surrounded to a sufficient extent with little glass tubes. If the ends of the wires be brought together on a plate of glass, a spark will be perceived; and if gun-powder be laid on the glass between the points of the wires, it will be exploded.

The galvanic battery in the laboratory of the Royal Institution at London consists of two hundred instruments, connected together in regular order, each composed of ten double plates arranged in cells of porcelain, and containing in each plate thirty-two square inches; so that the whole number of double plates is two thousand, and the whole surface one hundred and twenty-eight thousand square inches. This battery, when the cells are filled with sixty parts of water, mixed with one part of nitric acid, and one part of sulphuric acid, affords a series of impressive and brilliant effects. When pieces of charcoal, about one inch long and one-sixth of an inch in diameter, are brought near each other, a bright spark is produced, and more than half the volume of charcoal becomes ignited to whiteness, and by withdrawing the points from each other, a constant discharge takes place through the heated air, in a space equal at least to four inches, producing a most brilliant ascending arch of light, broad and conical in form in the middle. When any substance is introduced into this arch, it instantly becomes ignited; platina melts as readily in it as wax in the flame of a common candle; fragments of diamond, and points of charcoal, and plumbago, rapidly disappear, and seem to evaporate in it. Such are the decomposing powers of electricity, that not even insoluble compounds are capable of resisting their energy; for glass, when moistened and placed in contact

NEW DEFLAGRATOR.

161 with electrified surfaces from the voltaic apparatus is slowly acted upon, and the alkaline, earthy, or acid matter carried to the poles in the common order. Not even the most solid aggregates, nor the firmest compounds, are capable of resisting this mode of attack; its operation is slow, but the results are certain; and sooner or later, by means of it, bodies are resolved into simpler forms of matter.

The effects of galvanism on metallic bodies are greatly increased by using plates of a large size; and on the contrary, the shock is increased by multiplying the pairs of plates. The shock of a battery containing eighty or a hundred pairs of plates, of three or four inches in diameter, is such as few persons would be willing to bear more than once. At the same time such a battery produces but feeble effects when passed through a metallic wire. On the contrary, if one or two pairs of plates containing the same extent of surface be used, the sensation it gives is hardly to be felt, while it will deflagrate a metallic wire of considerable size.

Professor Hare, of Philadelphia, has invented a new method of extricating the Voltaic influence, by so connecting the plates, that, in effect, only two great surfaces of the metals are presented to each other. By this arrangement, the galvanic action on different substances has presented some new phenomena, and the common theory of galvanism must undergo, it is thought, a radical change. The calorific principle is immensely increased, while the electric shock is hardly to be perceived. Charcoal exposed to the effects of this new deflagrator melts into globules resembling diamond, and the process is attended with a most intense light. If mercury be placed in the hand, and the back side of the hand be applied to the negative pole, and the positive pole be brought、 to the surface of the mercury, it will be inflamed, and the hand will be affected with no disagreeable sensation, till the mass of mercury becomes heated. The new view, which Professor Hare has been induced to offer, is, that galvanism is a compound of electricity and caloric, and this is thought to be confirmed by the action of his machine."

QUESTIONS.-1. What is the most convenient kind of galvanic battery? 2. What is the effect of a powerful galvanic battery upon inflammable substances? 3. Describe the battery at the Royal Institution in London. 4. What effect does it produce upon charcoal?other substances? 5. What is said of the effects of galvanism by

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using plates of a large size?-by multiplying the pairs of plates? 6 What is the invention of Prof. Hare? 7. What experiments may be performed with it? 8. What new view of the subject has Prof. Hare offered? (See the galvanic or voltaic battery described at the beginning of this lesson, fig. 46.) [NOTE. Prof. Hare has named his new apparatus Calorimotor, or heat mover.]

LESSON 72.

Magnetism.

Polarity, that property of a magnet, by which, if left at liberty, it will point towards the poles of the earth, or nearly so the same end alway's points to the same pole.

ALTHOUGH the phenomena of the magnet have, for many ages, engaged the attention of natural philosophers, not only by their singularity and importance, but also by the obscu rity in which they are involved; yet very few additions have been made to the discoveries of the first inquiries into the subject. No hypothesis has hitherto been framed, that will account in an easy and satisfactory manner, for all the various properties of the magnet, nor have the links of the chain, which connect it with the other phenomena of the universe, ever been pointed out. It is certain, indeed, that both natural and artificial electricity will give polarity to needles, and even reverse a given polarity; and hence it may be inferred that there is a considerable affinity between the electric and magnetic powers, but in what manner electricity acts in producing magnetism, is still utterly unknown..

The ancients were acquainted with the attractive and repulsive powers of the magnet; but it does not appear that they knew of its tendency to the pole: this very fortunate discovery was made about the beginning of the fourteenth century, when the spirit of exploring distant regions was gradually forming in Europe. The use which might be made of it in directing navigation was immediately perceived, and that most valuable, but now familiar instrument, the mariner's compass, invented. When navigators found that they could, at all seasons, and in every place, discover the north and south with the greatest ease and accuracy, it be came no longer necessary to depend, like the voyagers of former ages, merely on the light of the stars, and the obser

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vation of the sea-coast. They gradually abandoned their ancient timid and lingering course along the shore and ventured boldly into the ocean. Relying on this new guide, they could steer in the darkest night, and under the most cloudy sky, with a security and precision till then unknown. The compass may be said to have opened to man the dominion of the sea, and to have put him in full possession of the earth, by enabling him to visit every part of it.

Nearly half a century elapsed, from the time of this discovery, before navigators ventured into any seas which they had not been accustomed to frequent. But in the course of the fifteenth century, discoveries were made far beyond the conception of all former ages. In the first voyage of Columbus, the Spaniards were struck with an appearance, not less astonishing than new. They observed that the magnetic needle, in their compasses, did not point exactly to the polar star, but varied toward the west; and, as they proceeded, this variation increased. This appearance, which filled the companions of Columbus with terror, and which still remains one of the mysteries of nature, is that deviation from the meridian which is called the variation of the needle. It is different in different parts of the world; being west at some places, east at others, and in parts where the variation is of the same name, its quantity is very dif ferent. It is the same to all needles in the same place; and for a long time, it was thought to be invariably the same, at the same place, in all ages; but it was discovered, about the year 1625, that it was different at different times, in the same place. From subsequent observations, it appears, that this deviation was not a constant quantity, but that it gradually diminished; and at last, about the year 1660, it was found that the needle at London pointed exactly north. At present the declination at London is about twenty-four degrees west. For some years, it has been nearly stationary; but it is understood now to be returning in an easterly direction. Knowing the variation, or declination of the magnetic needle, that is, the angle which the magnetic meridian makes with the meridian of the place, mariners are able to sail by the compass with as much accuracy as if it pointed exactly north.

The inclination, or dipping of the magnetic needle, expresses the property which the magnet possesses of inclining

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MAGNETICAL EXPERIMENTS.

one of its poles towards the horizon, and of elevating the other pole above it. This property was discovered in the year 1576. It is found to be always the same at the same place, but different in different places.

QUESTIONS.-1. When was the polarity of the magnet discovered? 2. What use was made of this property of the magnet? 3. When and by whom was the deviation of the needle from the meridian discovered? 4. What is said of this variation with respect to the same place ?--to different places? 5. What is said of the declination of the needle at London? 6. What is the inclination or dipping of the magnetic needle ?

LESSON 73.

Magnetical Experiments.

THE natural magnet, or loadstone, is found in the earth, generally in iron mines, in a hard and brittle state, and for the most part, more vigorous in proportion to the degree of hardness. Artificial magnets, which must be made of hard or highly tempered steel, are now generally used in preference to the natural magnet; not only, as they may be procured with greater ease, but because they are far superior to the natural magnet in strength, communicate the magnetic virtue more powerfully, and may be varied in their form more easily. In making artificial magnets, care should be taken to apply the north pole of the natural magnet or magnets to that extremity of the steel which is required to be made the south pole, and to apply the south pole of the magnet to the opposite extremity of the piece of steel. Very powerful magnets may be formed by first constructing several weak magnets, and then joining them together to form a compound one.

The north or south poles of two magnets repel each other; but the north pole of one attracts the south pole of another. The attraction between the magnet and iron is mutual, for the iron attracts the magnet as much as the magnet attracts the iron; since if they be placed on pieces of wood, so as to float upon the surface of the water, it will be found that the iron advances towards the magnet as well as the magnet towards the iron, or, if the iron be kept steady, then the mag

net will move towards it.

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