TH LECTURE L. ON MAGNETISM.* HOUGH the phenomena of the magnet have for many ages engaged the attention of natural philofophers, both from their fingularity and importance; we are not yet in poffeffion of any hypothefis, that will fatisfactorily account for the various properties of the magnet, or point out thofe links of the chain, that connect it with the other phenomena of the universe. It is known by the works of Plato and Arif-, totle, that the ancients were acquainted with the attractive and repulfive powers of the magnet; but it does not appear, that they knew of it's pointing to the pole, or the ufe of the compafs. That property of the magnet, whereby, when properly fufpended, it turns towards the north pole, renders it of the utmost service to mankind in general, but more particularly to an Englishman; the riches and power of whose country depend on navigation. The powers of the magnet excited the wonder of the ancients; they were to them inexplicable, and remain fo. Pofterity, instead of being able to remove the difficulties, have only by their researches, found out new wonders equally inexplicable. All, therefore, that I fhall be able to do, will be to relate to you the principal qualities of this curious phenomenon. "The magnet is a proof, that nature has many fecrets, and that philofophy, if contented with prefent knowledge, forgoes most valuable and interefting difcoveries, Ff2 towards See my Effay on Magnetifm. towards which, perhaps, the previous steps are already trodden." From it's action on the compass in all parts of the world, it is plain, that it's influence is univerfal. From our knowledge of this, we are naturally led to fuppofe, that there may be other invifible agents exerting their influence on us, and on our globe. Let the modern philofopher, who denies the existence of a God, because he cannot perceive him with his corporeal eyes, tell you what magnetifm is, and how it exifts. Let him, who will underftand every thing that exifts, before he allows of it's exiftence, firft employ himself here; ́and when he has given the world a proof of his powers, let him attempt an higher fubject. The loadstone, leading-flone, or natural magnet, is an iron ore or ferruginous ftone, found in the bowels of the earth, generally in iron mines, of all forms and fizes, and of various colours. It is endowed with the property of attracting iron; and of both pointing itfelf, and alfo enabling a needle, touched upon it, and duly poised, to point towards the poles of the world. Loadftones are in general very hard and brittle, and for the most part more vigorous in proportion to their degree of hardness. Confiderable portions of iron may be extracted from them. Newman fays, that they are almost totally foluble in fpirit of nitre, and partially in the vitriolic and marine acids. Mr. Kirwan fays, that the magnet feems to contain a fmall quantity of fulphur, is often contaminated with a mixture of quartz and argill; it is poffible, it may contain nickel, for this, when purified to a certain degree, acquires the proper ties * Condorcet and many of his school, have laughed at mankind for believing in an invifible Being. ties of a magnet; but it's conftitution has not as yet been properly examined.* Artificial magnets, which are made of steel, are now generally ufed in preference to the natural magnet; not only as they may be procured with greater eafe, but because they are far fuperior to the natural magnet in ftrength, and communicate the magnetic virtue more powerfully, and may be varied in their form more eafily, fo that the na-. tural magnet is now very little cfteemed, except as a curiofity. The power of attracting iron, &c. poffeffed by the loadstone, which is alfo communicable to iron and steel, is called MAGNETISM. It has been fuppofed, that iron and the loadstone were, the only two bodies which could be rendered magnetical; but it now appears, that nickel, when purified from iron, becomes more inftead of lefs magnetic, and acquires, what iron does not, the properties of a magnet.† A rod or bar, of iron or fteel, to which a permanent polarity has been communicated, is called a magnet. The points in a magnet which feem to poffefs the greatest power, or in which the virtue feems to be concentrated, are termed the poles of a magnet. The magnetical meridian is a vertical circle in the heavens, which interfects the horizon in the points to which the magnetical needle, when at reft, is directed. The axis of a magnet is a right line, which paffes from one pole to the other. The equator of a magnet is a line perpendicular to the axis of the magnet, and exactly between the two poles. Ff3 Kirwan's Elements of Mineralogy, p. 271. The + Ibid. p. 369. The distinguishing and characteristic proper ties of a magnet, are, First, It's attractive and repulfive powers. Secondly, The force by which it places itself, when fufpended freely, in a certain direction towards the poles of the earth. Thirdly, It's dip or inclination towards a point below the horizon. Fourthly, The property which it poffeffes of communicating the foregoing powers to iron or steel. OF THE TENDENCY OF IRON AND A MAGNET TO APPROACH EACH OTHER, This curious property of the magnet was that by which it was first discovered, and by which it engaged the attention of the curious. Every fubftance that contains iron, is more or lefs attracted by the magnet. And fo univerfally is this metal diffeminated, that there are very few fubftances that are not in fome degree capable of being attracted by the magnet. You will find it in animals, vegetables, minerals, and even in the air. Iron is attracted with different degrees of force, according to the different ftates of it's exiftence; but it never becomes quite infenfible to the magnetic power, Even the pureft calx, or the completeft folution ever made of the metal, when accurately examined, is found to be in fome degree obedient to the magnet. TO ASCERTAIN WHETHER A BODY HAS ANY IRON, OR IS CAPABLE OF BEING ATTRACTED BY THE MAGNET. If the given body contains evidently a large quantity of iron, on bringing a magnet in contact • Cavallo on Magnetiẩm, p. 66. tact therewith, you will find them adhere so strongly as to require a certain degree of force to feparate them. If the body be not fenfibly attracted by the magnet in this way, then you may float it by a piece of wood or cork on water; in this fituation it is more easily acted on, and confequently small quantities of iron are readily difcovered. The magnet fhould be prefented fideways to the body, and when it is at reft, it is fometimes neceffary to bring the magnet within one tenth of an inch diftance from the swimming body in order to perceive the attraction. The A ftill fmaller degree of attraction may be discovered by placing the given body upon quickfilver, and then prefenting a magnet to it. veffel, in which the quickfilver is contained, fhould be at leaft fix inches in diameter, otherwife the curvature of the fluid will be perpetually carrying the body towards the fides of the veffel. The quickfilver fhould be pure, and occafionally cleared by paffing it through a funnel of clean writing paper; the smaller the aperture of the funnel, the better it will answer the purpose. The air fhould be agitated as little as poffible. Attending to these precautions, you will feldom fail to difcover whether a body contains any ferruginous particles. I place a piece of iron on a cork, and put the cork into a bafon of water. I prefent a magnet to it, and it is attracted thereby, and follows the magnet, fo that I can move it without touching, wherever I pleafe. On this principle, many in genious and entertaining pieces of mechanifm have been contrived. The tendency between the magnet and the iron is reciprocal; for, if the magnet be put on the cork, it will follow the iron in the fame manner, as this followed the magnet. And this attraction takes place, although a piece of paper, glafs, Ff4 brafs, |