There is no substance to be found in nature existing independently of magnetic power. But it influences bodies in different ways: one set acting with relation to magnetism, like iron, and arranging themselves along the line of magnetic force,—these are called magnetic bodies; another set, of which bismuth may be taken as the representative, always placing themselves at right angles to this line,—these are called diamagnetic bodies.[186] This is strikingly shown by means of powerful electro-magnets; but the magnetism of the earth is sufficient, under proper care, to exhibit the phenomena.
Every substance in nature is in one or other of these conditions. The rocks, forming the crust of the earth, and the minerals which are discovered in them; the surface soil, which is by nature prepared as the fitting habitation of the vegetable world, and every tree, shrub, and herb which finds root therein, with their carbonaceous matter, in all its states of wood, leaf, flower, and fruit; the animal kingdom, from the lowest monad through the entire series up to man,—have, all of them, distinct magnetic or diamagnetic relations.
“It is a curious sight,” says Dr. Faraday, “to see a piece of wood or of beef, or an apple, or a bottle of water repelled by a magnet, or, taking the leaf of a tree, and hanging it up between the poles, to observe it take an equatorial position. Whether any similar effects occur in nature among the myriads of forms which, upon all parts of its surface, are surrounded by air, and are subject to the action of lines of magnetic force, is a question which can only be answered by future observation.”[187]
At present, the bodies which are known to exhibit decided ferro-magnetic properties are the following, which stand arranged in the order of their intensity:—
- Iron, Nickel, Cobalt, Manganese,
- Chromium, Cerium, Titanium,
- Palladium, Platinum, Osmium.
It is interesting to know that there are evidences that two bodies which, when separate, are not magnetic, as iron is, become so when combined. Copper and zinc are both of the diamagnetic class, but many kinds of brass are discovered to be magnetic.
The salts of the above metals are, to a greater or less extent, ferro-magnetic, but they may be rendered neutral by water, which is a diamagnetic body, being repelled by the magnet. It will be unnecessary, here, to enumerate the class of bodies which are diamagnetic; indeed, all not included in the preceding list may be considered as belonging to that class, with the exception of gases and vapours, which appear to exist, relatively to each other, sometimes in the one, and sometimes in the other condition.[188]
To endeavour to reduce our knowledge of these facts to some practical explanation, we must bear in mind that particular spaces around the north and south geographical poles of the earth, are regarded as circles to which all the magnetic lines of force converge. Under circumstances which should prevent any interference with what is called ferro-magnetic action, all bodies coming under that class would arrange themselves according to the laws which would regulate the disposition of an infinite number of magnets, free to move within the sphere of each other’s influence. The north and south pole of one magnetic body would attach itself to the south and north pole of another, until we had a line of magnets of any extent; the two ends being in opposite states, like the magnetic points of convergence of the earth.
Every body, not ferro-magnetic, places itself across such a line of magnetic force as we have conceived; and if the earth were made up of separate layers of ferro-magnetic and diamagnetic bodies, the result would be the formation of bands at right angles to each other. This is not the case, by reason of the intermingling of the two classes of substances. Out of the known chemical elements we find only about ten which are actively ferro-magnetic; the others combining with these give rise to either a weaker state, a neutral condition, or the balance of action is turned to the diamagnetic side. Sulphate of iron, for instance, is a magnetic salt; but in solution, water being diamagnetic, it loses its property. The yellow prussiate of potash dissolved in water is a diamagnetic body; but the red prussiate, which contains an atom less of potassium, is magnetic: but in the solid state they are both diamagnetic.[189]
From this it would appear that the chemical composition of a body regulated its relation to magnetism. The following facts will show, however, that the molecular structure is more particularly concerned in determining the molecular condition of substances.