Neither is it maintainable that the medium of space cannot be the medium of vision, because "from being all-pervading, it should excite vision through all kinds of bodies, as through a block of rock crystal, but does not through so thin a substance as a leaf of blotting-paper." By clairvoyance it is proved that the visual continuity is maintained through stone walls; and by reason of the _visual and auditory_ medium being the same, that is, medium of space, the "hearing" through stone walls, makes the "seeing" possible. The bell must be connected mediately with the auditory sense, as is the object with the visual sense; and through stone walls there is nothing continuous but the medium of space. Sound is no more a transmissible object than colour; neither belongs to the external object. In all such cases of sensations which are different, although the promoting means are the same for all the senses, that the organs of sense may not be equally susceptible, or capable of being put into functional service by the same degree of cerebral pressure, should be held in mind, or else it might be asked why all the senses are not excited at the same time. TRANSPARENCY. A transparent body, is one through which the visual line is uninterruptedly continuous from an object to the sense. The materials for glass-making are opaque, and the natural opacity of their elementary atoms is unalterable. Hence in some novel arrangement of the atoms towards promoting the direct continuity of the medium of space through them, consists the object of vitrifying and principle of transparency. OPACITY.