neral, very abundant in Derbyshire, in a substance called black wad: it is also found in the greatest purity in the neighbourhood of Exeter. It gives a fine purple tinge to glass, enamels, and earthen-ware; and possessing the seemingly contrary property of purifying glass from any green or yellow tint, it has obtained the name of glass-maker's soap. When manganese, common salt, sulphuric acid (oil of vitriol) and wàter, are combined in certain proportions, the substance now called oxymuriatic acid is produced, which speedily destroys the colouring matter in linen and cotton-cloth, and is now therefore universally employed in manufactories and bleaching. It was first employed in this way at Glasgow by Mr. Watt, afterwards of Birmingham.

It was before noticed that the magnetic meridian varies in many places from the true meridian of the earth; the magnetic north pole, for instance, pointing to the west or to the east of the north pole of the earth; and to account for this fact has hitherto baffled the ingenuity of all enquirers. But this is not all; this variation of the magnetic meridian is itself continually varying at the same place; nor have all the observations hitherto made been sufficient to determine either its rate or its limits. The following table contains a statement of the variation, or declination from the true meridian, of the magnetic needle of the mariner's compass, as observed at London in different years. This table will show that 240 years ago the north pole of the magnet pointed to north by east, or eleven degrees and one-fourth to the east of north: that 160 years ago, it pointed due north; and that moving gradually westward it now points about twenty-four degrees and a half to the west of north, or two degrees to the west of NNW.

From this statement it is impossible to form any estimate of the rate of progression in the variation; partly, it is probable, owing to the imperfection of the instruments by which the variation was observed. For it is only of late years, especially since the introduction of Macculoch's improved sea-compass, that the observations requisite for determining the variation of the magnetic needle could be duly performed. The dip or depression of the magnetic needle increasing as it is

carried nearer to the pole of the earth, could it be placed exactly over one of them, it would there stand perpendicular to the horizon: the instrument for showing this depression is the dipping needle.

In giving to iron or steel the magnetic property, various methods are employed, in all of which a magnet is necessary, although in some cases it may seem not to be requisite. Take a bar of iron three or four feet long, and hold it in a perpendicular position; in a little time the bar becomes magnetic, and acts upon another magnet; the lower end attracting the south pole, and the upper the north pole. If the bar be inverted the polarity will be constantly reversed; the end now lowest becoming a north pole, and the upper a south pole. Bars, or any long pieces of iron not very hard, that have stood for some time in a vertical position, generally become magnetical; such as fire irons, window or rail bars. A long piece of hard iron made red hot, and left to cool in the direction of the magnetic line, becomes magnetic: the same effect is also sometimes produced on an iron bar by a stroke of lightning or of electricity. If a slender steel bar or a large needle be fastened on a poker that has stood a considerable time upright, and stroked upwards by the lower end of a pair of tongs, the whole standing vertically, the needle will, after a dozen strokes all upward, become magnetic. The proper methods of magnetising steel bars or needles for the mariner's compass, are nevertheless of such delicacy as to require the greatest attention, in even those who are professionally engaged in the construction of the magnetic needle, and its application in the mariner's compass, of which this is the construction. The compass for ships consists of the box, the card or fly, and the needle. The circumference of the card is divided into 32 equal parts called points, each containg 11 degrees 15 minutes, and subdivided into quarters. The N. E. S. and W. points are the principal or cardinal points of the compass. Thus a point between and equally distant from S. and W. is called SW.; but another in the middle between W. and SW. becomes WSW. while hat in the middle between S. and SW. is called SSW. To the under side of the card, and in the direction of the line joining the N. and S. points is attached a magnetic bar of hardened steel, and of a rectangular form, called the needle, by which the N. point of the card is directed towards the N. part of the horizon, and of course all the other points to their corresponding parts of the horizon. The card and needle are placed on an upright pin or supporter fixed in the bottom of a brass or wooden circular box; the whole covered with a plate of brass. The box has two pins diametrically opposite, let into a brass ring, moveable in a square wooden box, on two points at right angles to the former. By this contrivance called jimbals, the card preserves in general a horizontal position, even when the ship is considerably agitated. When to this compass are added two perpendicular sights, fixed on a brass bar stretched over the box, by means of which the true bearing of the sun or other celestial object may be observed, for the purpose of determining the variation of the magnetic needle, or how much, and in what direction the N. and S. line pointed out by the needle vary from the true meridian of the ship or place of observation. This is called the azimuth compass, from an Arabic term expressing the angle formed at the observer's station by the true N. and S. line, and a line drawn to the object observed.

Of the cause of magnetic attraction and repulsion, and of the fluctua

tion of the direction of a magnetic needle, we are still entirely ignorant. Its variation, however, from the true meridian, the continual change in that variation, and the inclination or dipping of the needle, these and other circumstances seem to indicate the cause to exist in the body of the earth. The celebrated Dr. Halley who, above a century ago, in the course of his scientific voyage to St. Helena in the southern ocean, took particular pains to observe the effects of magnetism on the compass, adopted the following ingenious hypothesis. The globe of the earth he supposed to be one great magnet, having four magnetical poles or points of attraction, two near each pole of the earth; and that in the parts of the world adjacent to any one of the magnetic poles, the needle is chiefly governed thereby, the nearest pole being always predominant over the more remote. Of the N. poles, that which is nearest to us the doctor supposed to be situated about seven degrees from the true N. pole, in the meridian of the Land's-end in Cornwall, in W. longitude from Greenwich observatory five degrees forty-two minutes. The other about fifteen degrees from the true N. pole, in a meridian passing over the W. parts of North America, in 120 degrees of W. longitude. The variation of the needle from the true N. and S. points of the world not being uniform, but variable in different years, and in a different manner in various parts of the world, Dr. Halley imagined two of the magnetic poles to be fixed, and the other two to be moveable. To account for this, he conceived the external part of the earth to be a hollow spherical shell, containing within it a magnetic globe, detached from it, but having the same centre of gravity, and revolving round like the earth, but with a motion so small a matter slower as scarcely to become sensible even in a year. Hence would be produced a variation between the true and the magnetic poles, and consequently in the direction of the needle. Different from this fanciful scheme of a most able natural philosopher is that of Epinus, who in 1759 gave it as his opinion, that magnetism was produced by a peculiar fluid, so subtile as to penetrate all substances, and of an elastic nature, its particles mutually repelling one another. Between this fluid and iron, a mutual attraction subsists; but on it no other substance has any action. According to this scheme, a body containing or consisting of iron in its metallic state (not in that of an oxyde or rust) is rendered magnetic by having the equal quantity of fluid diffused through it disturbed, so that it exceeds in one part, and is deficient in another; and its magne tism continues so long as this unequal diffusion lasts, and until the balance between the overcharged and the undercharged parts be restored.

By the following simple experiment the effects of the magnet upon iron may be very perceptible. Strew some iron or steel filings lightly over a sheet of paper on a table, and among them place a small magnet. If by knocking on the table the filings are made to move, they will cling to one another forming lines of different sorts. Those at the two poles arrange themselves in straight lines in the direction of the axis of the magnet: but those on each side begin to bend outward more and more, as they recede from the poles, until at last they form complete arches, mecting together as they bend round from the opposite poles of the magnet. Again, find out by trial a piece of iron a little heavier than can be supported by

P

a magnet, and apply it to one of the poles. When the hand is removed, the iron must of course drop off; but if before the hand be removed another larger piece of iron be brought within half an inch of the under side of the first piece, the magnet will then be found to support the first piece. Hence it appears that a magnet will support a greater weight of iron over an anvil or other piece of iron, than over a table; for this reason, that the lower iron becoming magnetic in this situation, increases the magnetism of the upper iron, and thereby increases the attraction for, and adherence to the original magnet.

CHAPTER II.

SECT. I.

CHEMISTRY.

THE name of this branch of natural knowledge has been differently written at different epochs. Until within our own days the usual orthography was in English chymistry, in French chymie, in Latin, Italian, and Spanish, chymia, all words derived from chymos, a Greek term, signifying juice, liquor, a fluid. The present term chemistry, may also be formed from the Greek verb cheo, or more properly heo, to pour out; all significations very applicable to a science which treats of the fusion and liquifaction of solid substances, and of the intermixture of fluid substances.

All natural events consist of one of two things, either of those produced or accompanied by motions perceptible in their progress by our senses, or of those in the progress of which no motions or changes are perceptible by our senses. The first class of objects form natural, or more properly mechanical philosophy; the second class belong to chemistry. Of the several branches of mechanical philosophy, a general notion has been given in the preceding chapter of this work: the present chapter will contain a summary statement of the principles and phenomena of chemistry.

Rise and progress of chemistry. The beginnings of every art, tending either to supply the necessities or to alleviate the inconveniences of human life, were probably coeval with the first establishment of civil society, and preceded by many ages the invention of letters, of hieroglyphics, and of every other mode of transmitting to pos

terity the memory of past transactions. In vain should we inquire who made the first plough, baked the first bread, formed the first pot, wove the first garment, or hollowed out the first canoe from the trunk of a tree. Whether men were originally left to pick up casual information concerning objects around them, or were supernaturally assisted in the discovery of matters necessary for their well-being, and even for their existence: these are questions which we shall never be able to solve.

It is not to be doubted that, in the period which intervened between the formation of man and the deluge, a great variety of economical arts were carried to a considerable degree of perfection. Of these, however, many must have been lost with the great body of the inhabitants of the earth: for it is scarcely possible that the single and not numerous family which survived the general ruin, had either practised, or even been slightly acquainted with every art in use among the great body of their fellow creatures. Were the inhabitants of the earth to be now swept away by some overpowering calamity, a few persons only being preserved, it will not be difficult to imagine what would be the fate of the various arts by which the elegancies, the comforts, and even the necessities of life are supplied. Centuries might again pass away before the new inhabitants of our globe could become acquainted with the nature and uses of the mariner's compass, with the arts of painting, dyeing, printing, of making porcelain, gun-powder, steel, brass, &c.

Of the events which took place between the creation of man and his destruction by the deluge, the only account on which we can rely is contained in the first six chapters of the book of Genesis. In that brief summary of the history of the infancy of society, we cannot reasonably expect to find details, or even the dates of the various useful discoveries in art and science, which were made by men in that period. Short however as is that account, chemists may, not improperly carry, back to the very earliest epochs the antiquity of their art. Tubal Cain is there mentioned as an instructor of every artificer in copper and iron: a proof beyond controversy, that one part of chemistry, that concerned in the management of metals, was then well understood. Copper and iron, it is well known, are with great difficulty extracted from their ores, and cannot, without much skill and trouble, be rendered malleable. Cain, we are informed, built a city; and thence some would infer, that the use of metals was known and familiar, even prior to Tubal.

For many ages after the flood, we have no certain accounts of the state of chemistry. The art of making wine, and the inebriating quality of the juice of the grape, when suffered to ferment, seem to have been unknown to the family preserved in the ark. The Egyptians must have been well skilled in the management of metals, in medical chemistry, and the art of preserving dead bodies, long before the time of Moses, the earliest historian of that singular country, who was born about 1635 years before Christ. This is evident from the account of Joseph's cup, of the embalming of Jacob, &c. Moses also mentions furnaces for working iron, ores from which iron was extracted, and swords, knives, axles, tools for cutting stones, all made of that metal. That the Egyptians at a very early period, understood the art of dyeing and of making coloured glass, appears from history, and from the