MEDIUM, in Philosophy, that space or region through which a body in motion passes to any point: thus ether 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 reflurance 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 æreal medium, wherein we live and breathe, there is another more universal one, which he calls an æthereal 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 ÆTHER, ELECTRICITY, FIRE, &c.