It appears, however, that the set of crystals with regard to the line of magnetic force does not depend solely upon their density in particular directions. Professor Matteucci, of Pisa, has proved that the diamagnetic force is inversely as the conducting power of substances for electricity, that the conducting power is a maximum in the planes of principal cleavage, and that a needle of crystallized bismuth, in which the planes of cleavage are parallel to its length, places itself equatorially with more force when these planes are vertical, or at right angles to the force, than when they are horizontal or parallel to it. Experiments had hitherto been made only with diamagnetic or slightly paramagnetic bodies, which induced M. le Roux to try the effect of magnetism on pulverized iron compressed by the hydraulic press, which reduced the grains of iron to lamellæ equivalent to planes of cleavage. Cubes of this substance, suspended by a thread over a horseshoe magnet, oscillated for a longer time when the lamellæ were perpendicular than when they were horizontal; that is, the force was stronger when the lamellæ were equatorial than when they were axial, exactly the same result as in Professor Matteucci’s experiment with the needle of bismuth. Thus the vertical position of the cleavages, which increases the diamagnetism of the bismuth, increases also the paramagnetism of the iron. M. le Roux observes that these results are independent of the influence of the currents of electricity induced in the oscillating body, for the fundamental character of the phenomena of Arago’s discovery of rotation by induction is, that the oscillations diminish rapidly in extent without any sensible diminution in their duration, while in his experiments the time of the oscillations varied. He concludes that the arrangement of the molecules must be intimately connected with paramagnetism or diamagnetism itself, since the effect of that arrangement is equally sensible in bismuth and iron, although the diamagnetism of the former is 25,000 times weaker than the paramagnetism of the latter.
The diamagnetism of conducting substances and metals, such as gold, silver, and copper, is augmented by division. Compression has also a great effect on magnetic action. For example, a bar of soft iron sets with its longest dimensions from pole to pole of a magnet, but a bar of compressed carbonate of iron-dust, whose shortest dimensions coincide with the line of pressure, sets equatorially. A bar of bismuth whose plane of principal cleavage is parallel to its length sets equatorially, but a bar of compressed bismuth dust, whose shortest dimensions coincide with the line of pressure, or a bar of bismuth whose principal planes of cleavage are transverse to its length, sets with its length axially. The antithesis is perfect whether the bars are under the influence of a magnet or electro-magnet. For since the diamagnetic force is inversely as the conducting power of a body for electricity, and that the latter is a maximum in the direction of the planes of principal cleavage, therefore when these planes are parallel to the axis of the bismuth bar it sets equatorially; but as the conducting power is augmented when the bismuth dust is compressed in the direction of the force, the diamagnetic power is diminished, and the bar sets axially. Again, since the paramagnetic force augments with the conducting power, the action of the magnet on the iron is antithetic to that on the bismuth.
The action of an electro-magnet on copper is strongly contrasted with that which it exerts on iron or bismuth. For when a copper bar suspended by a thread revolves before its pole, it is brought to a dead halt as soon as the electric current acts upon it, and maintains its position with considerable tenacity, for it does not return when pushed out of it, but keeps its new place with stiffness; however, as soon as the electric current ceases, there is a strong revulsion, the bar revolving the contrary way. Even when swinging with considerable force it may be caught and retained in any position at pleasure, but there is no revulsion when it is arrested either in the axial or equatorial position; at any angle between these two, but especially midway, the electricity will make it move towards the axis, but it is arrested before it comes to it. The action depends much on the form and dimensions of the bar and the magnetic pole, which ought to be flat. The phenomena are due to the high electro-conducting power of the copper, and are met with in some of the other pure metals, though in a far inferior degree.
Great magnetic power is requisite for all these experiments. Dr. Faraday employed a magnet that could sustain a weight of 450 lbs. at each pole, and the poles were either pointed or flat surfaces at pleasure, as the kind of experiment required.
Heat strongly affects the magnetic properties of bodies. Dr. Faraday found that, when the temperature of nickel is increased, its magnetic force diminishes; when that of iron is increased its magnetic force remains the same, while that of cobalt increases; which seems to indicate that there is a temperature at which the magnetic force is a maximum, above and below which it diminishes. Nickel loses its magnetism at the temperature of boiling oil, iron at a red heat, and cobalt near the temperature at which copper melts. Calcareous spar retains its magnetic character at a very high temperature; but the same substance when it contains iron, and also oxide of iron, loses it entirely at a dull red heat. A crystal of the ferrocarbonate of lime was absolutely reversed by change of temperature, for at a low heat the optic axis pointed axially, and at a high temperature equatorially. With the exception of these substances, magnecrystals, whether paramagnetic or diamagnetic, are generally all affected alike by heat. The difference between the forces in any two different directions, as for instance the greatest and least principal axes, diminishes as the temperature is raised, increases as the temperature is lowered, and is constant for a given temperature. No unmixed or pure substance has as yet passed by heat from the paramagnetic to the diamagnetic state. No simple magnecrystal has shown any inversion of this kind, nor have any of the chief axes of power changed their characters or relations to one another.
It appears that, as the molecules of crystals and compressed bodies affect magnetism, so magnetism acts upon the molecules of matter, for torsion diminishes the magnetic force, and the elasticity of iron and steel is altered by magnetism. M. Matteucci has found that the mechanical compression of glass alters the rotatory power of a polarized ray of light transmitted through it, and that a change takes place in the temper of glass under the influence of powerful magnetism.
Even from the limited view of the powers of nature which precedes, it is evident that the progress of science based upon experiment tends to show that the various forces of light, heat, motion, chemical affinity, electricity, and magnetism will ultimately be traced to one common origin; that they are so directly related, and mutually dependent, that they are convertible, motion producing heat, and heat motion; chemical affinity producing electricity, and electricity chemical action, &c., each mediately or immediately producing the other. These forces are transmitted through substances; they act upon matter, causing changes in the molecular structure of bodies either momentary or permanent, and reciprocally the changes indicate the action of these forces. Matter and force are only known to us as manifestations of Almighty power: we are assured that we can neither create nor destroy them—that their amount is the same now as in the beginning. In chemical attraction the powers with which a molecule of matter is endowed, and which give rise to various qualities, never change; even when passing through a thousand combinations, the molecule and its power are ever the same.
Machinery does not create force; it only enables us to turn the forces of nature to the best advantage; it is by the force of wind or falling water that our corn is ground, and the steam engine owes its power to the force of heat and chemical action. As force cannot be created, neither can it be annihilated. It may be dispersed in various directions, and subdivided so as to become evanescent to our perceptions; it may be balanced so as to be in abeyance, or become potential as in static electricity; but the instant the impediment is removed the force is manifested by motion; it may also be turned into heat by friction, but it is never lost. Every motion we make, every breath, every word we utter, is a force that produces pulsations which are communicated to continually increasing particles of air, and conveyed through countless channels so as to become indeed imperceptible to our senses, yet they are demonstrated to exist as witnesses of the words we have spoken or the actions we have performed, by analysis, that all-powerful instrument of human reason.[[19]]
A body acquires heat in the exact proportion that the adjacent substances become cold, and when heat is absorbed by a body it becomes an expansive force at the expense of those around that contract, but it is not lost. In chemical action at a distance the principle of the conservation of force is maintained, for a chemical action may be produced miles away from an electro-magnet, perfectly equivalent to the dominant chemical action in the battery. The two electricities are developed in equal proportions, which may be combined so as to produce many changes in their respective relations, yet the sum of the force of one kind can never be made in the smallest degree either to exceed or to come short of the sum of the other. Experimental research proves that the conservation of force is an unalterable law of nature—“a principle in physics as large and sure as that of the indestructibility of matter or the invariability of gravity. No hypothesis should be admitted, nor any assertion of a fact credited, that denies this principle. No view should be inconsistent or incompatible with it. Many of our hypotheses in the present state of science may not comprehend it, and may be unable to suggest its consequences, but none should oppose or contradict it.”
Having thus expressed his conviction of the truth of this great principle, Dr. Faraday considers the case of gravity, and concludes that “the definition of gravity as an attractive force between the particles of matter varying inversely as the square of the distance, while it stands as a full definition of the power, is inconsistent with the principle of the conservation of force.” For while in this definition the principle is maintained of the constancy of the force at the same distance, it implies a creation of force to an enormous amount when the distance is diminished, and an equal amount annihilated when the distance is increased,—“an effect,” he says, “which is equal in its infinity and its consequences with creation, and only within the power of Him who creates.” He continues, “It will not be imagined for a moment that I am opposed to what may be called the law of gravitating action, that is, the law by which all the known effects of gravity are governed; what I am considering is the definition of the force of gravitation. That the result of one exercise of a power may be inversely as the square of the distance, I believe and admit; and I know that it is so in the case of gravity, and has been verified to an extent that could hardly have been within the conception of Newton himself when he gave utterance to the law; but that the totality of a force can be employed according to that law I do not believe either in relation to gravitation, or electricity, or magnetism, or any other supposed form of power. That there should be a power of gravitation existing by itself, having no relation to the other natural powers, and no respect to the law of the conservation of force, is as little likely as that there should be a principle of levity as well as gravity. Gravity may be only the residual part of the other forces of nature, as Mossotti has tried to show; but that it should fall out from the law of all other forces, and should be outside the reach either of farther experiment or philosophical conclusions, is not probable. So we must strive to learn more of this outstanding power, and endeavour to avoid any definition of it which is incompatible with the principles of force generally, for all the phenomena of nature lead us to believe that the great and governing law is one. Thus gravitation can only be considered as part of a more general force whose law has yet to be discovered.”