in Logic, the mean or middle term of a syllogism, being an argument, reason, or consideration, for which we affirm or deny any thing; or, it is the cause why the greater extreme is affirmed or denied of the less in the conclusion.

in Arithmetic, or arithmetical medium or mean, called in the schools medium rei; that which is equally distant from each extreme, or which exceeds the lesser extreme as much as it is exceeded by the greater, in respect of quantity, not of proportion; thus 9 is a medium betwixt 6 and 12.

Geometrical MEDIUM, called in the schools medium perfonæ, is that where the same ratio is preserved between the first and second as between the second and third terms, or that which exceeds in the same ratio or quota of itself, as it is exceeded: thus 6 is a geometrical medium between 4 and 9.

in Philosophy, that space or region through which a body in motion passes to any point: thus aether is supposed to be the medium through which the heavenly bodies move; air, the medium wherein bodies move near our earth; water, the medium wherein fishes live and move; and glass is also a medium of light, as it affords it a free passage. That density or consistency in the parts of the medium, whereby the motion of bodies in it is retarded, is called the resistence of the medium; which, together with the force of gravity, is the cause of the cessation of the motion of projectiles.

Subtle or Ethereal MEDIUM. Sir Isaac Newton considers it probable, that, beside the particular aerial medium, wherein we live and breathe, there is another more universal one, which he calls an ethereal medium; vastly more rare, subtle, elastic, and active, than air, and by that means freely permeating the pores and interstices of all other mediums, and diffusing itself through the whole creation; and by the intervention hereof he thinks it is that most of the great phenomena of nature are effected. See AETHER, ELECTRICITY, FIRE, &c.

in optics, any substance through which light is transmitted.