Although the relations between the force of magnetism and the atoms of matter do not exhibit such brilliant phenomena as light does, they are nevertheless most interesting and wonderful. Mr. Faraday discovered that all substances, whether solid, liquid, or aëriform, are either magnetic like iron, or diamagnetic like bismuth, the latter being by far the most numerous. Thus if a bar of iron be freely suspended between the poles of an extremely powerful magnet or electro-magnet, it will be attracted by both poles and will rest or sit axially, that is, with its length between the poles or in the line of magnetic force; whereas an equal and similar bar of bismuth so suspended will be repelled by both poles and will rest or sit equatorially, that is with its length perpendicular to the line of magnetic force. Magnetism and diamagnetism are both dual forces, but they are in complete antithesis to one another, which is strikingly illustrated by their action on crystalline matter. A sphere of amorphous substance freely suspended under magnetic influence is indifferent, that is to say it has no tendency to set one way more than another; but a sphere cut out of a crystal whether magnetic or diamagnetic, is more powerfully attracted or repelled in one direction than in any other, which shows a connection between the magnetic forces and crystalline structure. Crystals of carbonate of iron and carbonate of lime are isomorphous, that is, they have exactly the same crystalline form, but the carbonate of iron being highly magnetic is most powerfully attracted in the direction of its greatest optical axis which therefore sets axially, that is, in the line of magnetic force; while the principal optic axis of the carbonate of lime, which is diamagnetic, is most powerfully repelled and therefore sets equatorially. In both cases the antithetic forces follow the same law of decrease in intensity from the greatest optical axis to the least. A bar of soft iron sets with its longest dimensions axially, but a bar of highly compressed iron-dust, whose shortest dimensions coincide with the line of pressure, sets equatorially, because it is most powerfully attracted in the line of greatest density. A bar of bismuth sets equatorially, but a bar of highly compressed bismuth dust, whose shortest dimensions coincide with the line of pressure, sets with its length axially, because it is most strongly repelled in the direction of its greatest density. Hence the action of magnets upon matter is most powerful in the line of maximum density, the force being attractive or repulsive according to the kind of magnetism possessed by the atoms. It follows therefore that the density is greatest in the line of the principal optical axis, and gradually decreases to the least optical axis, where it is a minimum.