Thou knowest not! 'tis doubt, 'tis darkness all! E'en here on earth our thoughts benighted stray, And all is mystery through this worldly ballWho then can reach or read yon milky way ? Creation's heights and depths are all unknown, untrod; Who then shall say how vast, how great, creation's God? QUESTIONS.-1. Why is it supposed that those meteoric appearances called falling stars owe their origin to electricity? 2. How may the analogy between electricity and the water-spout be made visible? 3. Describe the northern light. 4. What is Dr. Franklin's idea of it? [NOTE. A similar light called aurora australis has been long since observed towards the south pole.]

LESSON 70.
Galvanism.

Mus'cle, the fleshy fibrous part of an animal body. GALVANISM is another mode of exciting electricity. In electricity the effects are chiefly produced by mechanical action; but the effects of galvanism are produced by the chemical action of bodies upon each other. This branch of philosophy has been denominated galvanism, from Galvani, an Italian professor, whose experiments led to its discovery. In 1789, he was by accident led to the fact of electricity having the property of exciting contractions in the muscles of animals. After having observed that common electricity, even that of lightning, produced vivid convulsions in the limbs of recently killed animals, he ascertained that metallic substances, by mere contact, under particular circumstances, excited similar commotions. He found it to be essential that the forces of metals employed should be of different kinds. He applied one piece of metal to the nerve of the part, and the other to the muscle, and afterwards connected the metals, either by bringing them together, or by connecting them by an arch of a metallic substance; every time this connexion was formed, the convulsions took place. The greatest muscular contractions were found to be produced by zinc, silver, and gold. A person may be made sensible of this kind of electric action by the following experiments. If he place a piece of one metal, as a half crown above, and a piece of some other metal, as

zinc, below his tongue, by bringing the outer edge of these pieces in contact, he will perceive a peculiar taste, and in the dark will see a flash of light. If he put a slip of tin-foil upon the bulb of one of his eyes, and a piece of silver in his mouth, by causing these pieces to communicate, in a dark place, a faint flash will appear before his eyes. Galvani sup

posed that the virtues of this new agent resided in the nerves of the animal, but Volta, who prosecuted this subject with much greater success, showed that the phenomena did not depend on the organs of the animal, but upon the electrical agency of the metals, which is excited by the moisture of the animal, whose organs were only a delicate test of the presence of electric influence. In exciting the electricity of the pieces of silver and zinc, the saliva of the mouth answers the same purpose as the moisture of the animal.

The conductors of the galvanic fluid are divided into the perfect and imperfect. The perfect conductors consist of metallic substances and charcoal: the imperfect are water and oxydated fluids, as the acids and all the substances that contain these fluids. To render the Galvanic, or more properly the Voltaic power sensible, the combination must consist of three conductors of the different classes. When two of the three conductors are of the first class, the combination is said to be of the first order; when otherwise, it is said to be of the second order. If a piece of zinc be laid upon a piece of copper, and upon the copper a piece of flannel, moistened with a solution of salt in water a circle of the first class is formed; and then if three other pieces be laid on these in the same order, and repeated several times, the whole will form a pile or battery of the first order. The effects may be increased to any degree, by a repetition of the same simple combination. The following is a cheap and easy method of constructing a Voltaic pile, for zinc is one of the cheapest of metals, and may be easily melted, like lead. Let a person cast twenty or thirty pieces of zinc, of the size of a cent, which may easily be done in moulds made of clay. Let him then take as many cents, and as many pieces of paper or woollen cloth cut in the same shape, and which he is to dip in a solution of salt and water. In building the pile, let him place a piece of zinc, wet paper, the superabundant water being pressed out, after which the copper; then zinc, paper, copper, and so on, until the whole be

finished. The sides of the pile may be supported with rods of glass, or varnished wood, fixed in the board on which it is built. Having wet both hands, touch the lower part of the pile with one hand, and the upper part with the other, constant little shocks of electricity will be felt until one hand be removed. If the hand be brought back a similar repetition of shocks will be experienced. Hold a silver spoon in one hand, and touch with it the battery in the lower part, then touch the upper part with the tongue; the bitter taste is extreme. If the end of the spoon be put under the eyebrow, close to the ball of the eye, a sensation will be felt like the burning of red-hot iron, but which ceases the instant the spoon is removed. The plates will soon become oxydated, and require cleaning in order to make them act.

QUESTIONS-1. What is galvanism? 2. Give an account of the origin of this branch of philosophy. 3. How may a person be made sensible of this kind of electric action? 4. What was the discovery of Volta? 5. What are perfect conductors of galvanic fluid ?-imperfect? 6. What is necessary in order to render the galvanic or voltaic power sensible? 7. When is the combination said to be of the first order?-second order? 8. How may a pile or battery of the first order be formed? 9. What is a cheap and easy method of forming a voltaic pile? 10. What experiments may be formed with such a pile? 11. Why do the plates require cleaning? (See Voltaic pile, fig. 47.)

LESSON 71.

Galvanism (continued.)

Lab'oratory, a room fitted up with apparatus for the performance of chemical operations.

Deflagrate, to burn rapidly: nitre thrown on hot coals deflagrates. When accompanied with a loud noise it is termed děto-na'tion.

THE most convenient kind of galvanic battery consists of a trough made of baked wood, three inches broad, and about as deep; in the sides of the trough are grooves opposite to each other; into each pair of grooves is fixed, by cement, a plate of zinc and silver soldered together, and in the order of silver and zinc; the cement must be filled in so as to prevent any communication between the different cells. The cells are to be filled with water and nitrous acid, and then

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