§ 396. For a long time the only known means of estimating the intensity of the magnetic forces, was to ascertain the weight which might be suspended from a magnet by means of a piece of soft iron. This of course is a very inaccurate operation; but Coulomb suggested two methods by which this important point may be determined with the greatest precision. The first of these is the torsion balance, the operation of which has been already described, and the second, the method of oscillations. This consists in counting the number of oscillations made by a magnetic needle in resuming its original position, under the influence of any magnetic force, when disturbed from its position of equilibrium. The movements of the needle are subject to the same laws which govern the oscillations of a pendulum acted on by the force of gravity; and it is a necessary consequence of these laws that the intensity of the force producing the oscillations is proportional to the square of the number of oscillations performed in a given time.

By these methods it has been determined that the forces, both of magnetical attraction and repulsion, are in inverse proportion to the squares of the distances of the active bodies.

$397. The earth, thus regarded as a magnet, is capable of acting by induction upon other bodies; and if in these latitudes we hold a bar of pure soft iron in the direction of the magnetic dip, the lower end will have the properties of the marked pole of a magnet, and the upper those of an unmarked pole, as may easily be ascertained by approaching them with a magnetic needle. That this state of polarity is only temporary, may be proved by reversing the bar, when the poles will be found in the same position as before. A bar of steel, or of hard iron, will become permanently magnetie by remaining long in the same position, and it commonly happens that the fire-irons, poker, and tongs, which usually incline against the grate, in a position not very different from that of the dip, become magnets.

§ 398. The communication of magnetism to a steel bar by this influence may be greatly expedited by causing it to vibrate strongly, as by striking it, when in the proper position, upon the end with a hammer. It may also be greatly increased by the inductive influence of other masses of iron in contact with it; notwithstanding the iron itself, which thus adds to the effect, derives its power also from the same source, namely, the magnetism of the earth. Thus a steel bar acquires a feeble magnetism by being hammered vertically when resting upon stone or pewter, but receives a considerable

accession of power when subjected to the same degree of hammering while placed upon a parlour poker in a similar position. The power of a magnet is, in fact, always increased by the reaction of another body, in which it induces magnetism; and this property is made useful in arming (as it is termed) a natural loadstone, and thereby increasing its power. Two plates of very soft iron are provided, equal in breadth to the surfaces of the stone which include the poles, and a little longer; so that when applied to them, a portion of each plate projects beyond the loadstone. The armature is fixed on very firmly by wires, or an external case of any metal not susceptible of magnetism. The power of the stone is found to be thus very much augmented.

§ 399. As the mechanical vibration of the particles of a steel bar facilitates its reception of magnetic polarity, so will the diminution of its coercive power by the same means promote its return to the neutral state, and highly-charged magnets quickly lose their virtue by any concussion of their particles. A fall on the floor, rubbing with coarse powder for the purpose of polishing, and grinding, will all prove more or less injurious to their powers.

$ 400. The only substance, besides iron and its compounds, which has been found susceptible of magnetic polarity, of the kind which we have just been considering, is the metal, nickel; but there has been some difficulty in determining this point, from the small quantity of iron which is capable of conferring attractive powers upon other metals with which it may be alloyed. This in some instances is so small as to escape the detection of chemical analysis.

§ 401. The influence of heat upon magnetism is very remarkable. If a steel bar be heated to redness, and placed under magnetic induction, and then suddenly cooled, it will be found to have become strongly and permanently magnetic in a very short time. On the other hand, the application of heat to a magnet is invariably attended by a dissipation of its power. It is sensibly affected by the heat of boiling water; and a red heat totally destroys its magnetism. If the temperature, however, be raised only to 100, it loses much of its power during the operation, but recovers it again on becoming cool; but after it has been heated to redness, no part of its magnetism returns. The power of a magnet, moreover, increases as it becomes colder, to the utmost limit to which refrigeration can be carried.

At a white heat, iron appears to be totally insusceptible of any magnetic impression whatever, and a white-hot ball of iron may be brought close to a delicate magnetic needle, without at all disturbing its state of repose. Nickel loses its magnetic influence at a much lower temperature, and becomes totally indifferent when heated to 630° Fahrenheit.

§ 402. The phenomena of magnetism have been connected together by two hypotheses of exactly the same nature as those which have been applied to the analogous phenomena of electricity. The first, or that of pinus, refers them to the agency of a peculiar fluid, having properties very similar to those of the electric fluid in the hypothesis of Franklin, but which act only upon ferruginous bodies and Nickel. The particles of this fluid repel one another with a force which decreases as the distance increases, and they are attracted by the particles of iron with a force varying according to the same law. This hypothesis further requires the supposition that the particles of iron repel one another according to the same law.

The second hypothesis is exactly similar to the electric hypothesis of Du Fay, and is founded on the supposition of two magnetic fluids residing in the particles of iron, and incapable of quitting them. One of these imparts the northern, and the other the southern polarity, and they have been denominated respectively the boreal and austral fluids. The particles of each attract those of the other, but repel those of the same kind. When in combination with each other these fluids are inactive; each becoming active only when separate. The decomposition of the neutral fluid is effected by the inductive influence of either the one or the other when acting independently. Both hypotheses differ from the corresponding electrical assumptions by the necessary condition that the imponderable fluid cannot travel from particle to particle, but that the magnetic elements, or atomic portions of matter to which they are attached, are separated from each other by extremely small spaces, within which the displacements and motions of the magnetic fluids are restricted.

Either of these general views affords a sufficient basis for mathematical calculations which embrace all the leading facts of the science, but cannot for a moment be supposed to represent the real mechanism of the phenomena, especially since the inseparable connexion which has lately been proved to exist between the electrical and magnetical forces, into the nature of which it will be our future business to inquire.

§ 403. We are now, however, arrived at a point at which a more intimate knowledge of the properties of different kinds of matter, simple and compound, and of the laws of chemical combination and decomposition, is essential to our progress: and we must proceed to examine the character of chemical affinity; the laws which limit its action; and its connexion with the other forces which have already passed under our review.

CHEMICAL AFFINITY.

§ 404. Chemical affinity, or the highest degree of heterogeneous attraction, we have already recognised (§ 22) as the force which causes the dissimilar particles of different kinds of matter to combine together in the most intimate manner; forming compounds differing in all their essential qualities and actions from their constituent ingredients, and constituting, in fact, distinct species of

matter.

The greater the original opposition of properties in bodies, the stronger appears to be this species of attraction between them. Heterogeneous adhesion seems to depend upon a certain similarity of nature. To effect the solution of a metal, another liquid metal, as mercury at ordinary temperature, or tin or zinc at higher degrees of heat, must be employed: inflammable liquids, as alcohol and ether, readily dissolve resins, fats, and other inflammable solids, which are not taken up by water; while water will dissolve salts and acids, which are but little acted upon by the mere adhesive force of the first-named substances. By the force of affinity, on the contrary, metallic bodies combine with non-metallic; acids with alkalies; combustible with non-combustible bodies.

The first questions which seem naturally to suggest themselves upon this statement are, what are the simplest forms of matter? What is the chemical constitution of the familiar forms of matter with which we are surrounded? Are they simple or compound? What are the elements, or first ponderable principles of our globe?

§ 405. Now, in discussing these questions, we have something more formidable to deal with than mere ignorance :—namely, the strong prejudices of erroneous education. It requires no great advancement in classical knowledge to discover that the ancients considered that the four elements, out of which all other things in nature were compounded, were earth, air, fire, and water. The notion originated with a celebrated philosopher and good poet, named Empedocles, who flourished about four centuries before the

Christian era. His philosophical opinions were sung in verse by his contemporaries, and have been incorporated into the poetry of all succeeding ages; and have thus been invested with a charm and an authority well calculated to mislead the judgment, if not submitted to correction.

It is much to be feared that many highly-educated persons, persons, that is, thoroughly imbued with the taste and accomplished in all the literature of the ancients, are content with the meagre guesses made by them respecting the nature of those surrounding objects of paramount interest, upon which their very existence depends. The time of their youth has been devoted to pursuits with which the natural philosophy of the Greeks, or rather their crude speculations upon natural phenomena, are thoroughly interwoven; and in after-life they often want the time, or the inclination, to correct their careless observation of facts, and reckless assumption of abstract principles, by the sober results of the Baconian philosophy. But the error has been disseminated far beyond the sphere in which it originated, and has struck deep root in the general mind; its fibres having become entwined in the very texture of common language. Even our dictionaries of highest authority inform the inquirer, that the word element means "the first or constituent principle of any thing," and that the "four elements usually so called, are earth, fire, air, water, of which our world is composed*." We cannot commence our inquiry into the nature of chemical combination better than by applying the test of experiment to this popular and classical notion of the elements of all things.

Air and water, moreover, are almost necessarily present in all our operations, and it is of the utmost importance that we should become as early as possible acquainted with their properties.

Let us begin with air.

§ 406. The mechanical properties of the atmosphere in which we draw our breath, and on which our vital functions all depend, have been already examined (§ 42, et seq.): it is with its chemical properties that we are now to be engaged. And first, let us inquire whether air be capable of resolution into parts of dissimilar nature and properties by the arts of chemistry.

Now, the only means at our disposal for effecting the decomposition of bodies, compounded under the force of affinity, is a

TODD'S JOHNSON. This illustration of the word element, still holds its place even in the admirable New Dictionary of the English Language, of Mr. Charles Richardson. The French lexicographers have long since ceased to propagate error.