MEDIUM, in philosophy, that space or region through which a body in motion passes to any point; thus air, is 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 consilience in the parts of the medium, whereby the motion of bodies in it is 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.
Subtile or ætherial MEDIUM. See ÆTHER.