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30 other a force of one dyne, then each contains unit of magnetic matter. This definition again means that we have arbitrarily assumed the ethereal coefficient of air to be unity.

When making the experiment with electric charges we found that by filling the space between the active charges with a substance such as oil or glass, the force was diminished. No such effect is observable with magnets. We may put them under oil or water, or we may put a sheet of glass between them, and we shall find precisely the same force. If, however, we immerse them in liquid oxygen there will be a decrease of force, and if such a thing as an iron atmosphere were possible, the decrease in such an atmosphere would be very great. We may therefore say that the ethereal coefficient for magnetic forces is unity for air, oil, wood and any so-called non-magnetic substance; and smaller than unity for magnetic substances such as iron, nickel and cobalt. If we try the experiment with a plate of bismuth we shall find a slight increase of the force, showing that the magnetic ethereal coefficient for bismuth is a shade greater than unity, the value assumed for air. For iron it is enormously smaller. We may say that iron is more permeable to