MEDIUM, in Philosophy, that space or region throughout 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 in which bodies move near our earth; water, the medium in which 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, by which the motion of bodies in it becomes retarded, is called the resistance of the medium, which, together with the force of gravity, is the cause of the cessation of the motion of projectiles. Sir Isaac Newton considers it probable that, besides the particular ærial medium in which we live and breathe, there is another more universal one, which he calls an ætheral medium; vastly more rare, subtle, elastic, and active, than air, and freely permeating the pores and interstices of all other mediums, and diffusing itself throughout the whole of creation, and by the intervention of which it is that, according to him, most of the great phenomena of nature are produced.