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ergy: for even glass, sulphate of barytes, fluor spar, &c. when moistened and placed in contact with electrified surfaces from the Galvanic apparatus, are 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.
550. It is ascertained, that during these shocks, an oxidation of the metallic plates takes place; and, after their surfaces become tarnished, the shock diminishes; but, on being wiped, its force is renewed.
A combination of these troughs forms a galvanic battery, the force of which has produced the most surprising effects; and, by disturbing the close affinity of the constituent parts of many bodies, has led to the analysis of substances, hitherto deemed simple and elementary
Obs.--Some fishes, as the torpedo, the gymnotus electricus, and the silarus electricus, when touched, communicate shocks to the human body like those of electricity : but as there is no circuit for the fluid, in these cases, no adequate solution has yet been found of this strange phenomenon.
551. Since the identity of lightning and electric matter has been ascertained, philosophers have been led to seek the explication of aerial meteors in principles of electricity : and there is no doubt, that the greater part of them, and especially the aurora borealis, are electrical, or gaseous phenomena.
It has been observed, that the aurora borealis produces a very sensible fluctuation in the magnetic needle; and that the flashes have been attended with various rumbliny and hissing sounds.
Now black, and deep, the night begins to fall,
Now lost, and now renew'd, he sinks absorpt,
552. Earthquakes, the most dreadful phenomena of nature, have been ascribed, by some naturalists, to water, fire, steam, and electricity ; each of these powerful agents being supposed to operate in the bowels of the earth.
Subterraneous fire, and steam generated from it, are supposed to be the true causes of earthquakes. The elasticity of steam, and its expansive force, are every way capable of producing the stupendous effects attributed to earthquakes ; the force of steam being 28 times greater than that of gunpowder.
Can the poor brittle tenements of man
553. The most remarkable changes in the form and constitution of the earth, since the deluge, have probably been produced by subterraneous fires in volcanoes, and by earthquakes ; by which plains are converted into mountains, the ocean into islands, and dry land into pools.
Obs.--Half a pound of steel-filings, half a pound of brimstone, and a pint of water, will, when well mixed, acquire heat enough to make the mass take fire: and it appears, that volcanic mountains abound in snch mixtures: which are is.
nited, no doubt, by rain falling into their craters, or by tbe sea communicating with their bases and lower cavities.
The fluid lake that works below,
554. The eruptions of volcanoes exhibit dreadful phenomena, in prodigious inundations of liquid fire, which bear inevitable destruction with them.
The name of lava is given to these fiery streams, consisting of a mixture of stones, sand, earth, metallic substances, salt, &c. calcined and vitrefied.
The last great eruption of Etna, was in 1669; and the progress of the lava was at the rate of a furlong a day it destroyed, in forty days, the habitations of 27,000 persons ; and of 20,000 inhabitants of the city of Catanea, only 3,000 escaped.
The other great volcanoes in Europe are, Vesuvius, and Hecla; but there are two or three hundred in diferent parts of the world.
XXIII. Magnetism. 555. The power of certain ores of iron to attract pieces of iron, was known by the Greeks ; but accident led an Italian to discover, that if suspended on a centre, and allowed to turn in any direction, one end would constantly point northward, and, of course, the other southward.
This ore of iron was called loadstone; and it soon became evident, that the property of distinguishing the north and south, was of the highest utility to ships 556. In the course of time, it was discovered, that loadstone communicated its property of turning northward to all pieces of iron and steel; and that these could communicate it to others, in a degree of strength proportioned to the number of magnets employed in transferring the property.
Natural magnets are not therefore now used; but needles are first made of the most convenient shape, and the magnetic property communicated to them, by contact with other natural, or artificial magnets.
557. The two poles or points of the magnetic needle have different properties, the south poles of magnets attracting the north poles of others ; and the same poles acting repulsively towards each other; i. e. a north pole repels a north pole, and a south pole, a south pole.
A white heat destroys the polarity; and so will strokes of the hammer. But it will be acquired, by iron bars that long stand upright, or that are heated redhot, and left to cool in a polar direction. 558. If a magnet be laid
and some steel-dust be suffered to fall gently upon it, the dust will
arrange itself on the paper in regular curves, under the influence of the magnetic attraction.
If the magnetic bar be bent round into the form of a horse-shoe, its poles in that position will attract and operate in a higher degree than when used sepa. rately; and the strength of all magnets is increased by thus keeping their powers in action.
559. Magnets do not point exactly north and south; but in different parts of the world, with a different inclination eastward or westward of the north; and different in each place, at different times.
In London at this time, the needle points 26 degrees to the west of the north; or rather more westerly than north north-west; and the inclination has increased to the west from due north, for 160 years past.
Nor does the needle lie parallel, or flat, in any place; but the north point is turned upward or downward, and in London at this time it dips 72 degrees.
Obs.--The cause of the phenomenon of magnetism has, hitherto, baffled the investigation of philosophers; and the more we think of them, the greater becomes our embarrassment: but some accidental discovery will effect more than all reasoning; and till that occur, we must be content to class this, among the many unaccountable wonders of nature,
XXIV. Mathematics. 560. Mathematics, or the mathematical sciences, are divided into
1. Pure mathematics ; containing arithmetic and geometry, which treat only of number and magnitude.
2. Mixed mathematics; which treat of the properties of quantity applied to matter, as astronomy, geography, &c.
3. Speculative mathema s; which contemplate the proportions, relations, &c. of bodies.
4. Practical mathematics; or their application to the practical uses of life.
561. Geometry is an ancient, perfect, and beautiful science; which enables us to determine the relations and proportions of superficies and solids.
Superficies consist of figures of three sides, called triangles; of four sides, called quadrangles, squares, parallelograms, and trapeziums : of five sides, called pentagons ; of six sides, called hexagons; and of many sides, called polygons.
Superficies also are circles, ovals, or ellipses ; sectors of circles, or parts cut out from the centre ; and segments of circles, cut off by a strait line, called a chord.