The principal obstacle to transparency is interposed electric matter. In the earliest stages of glass-making an immense volume of electric matter is got rid of by means of the furnace fire, which becomes sooty smoke while ascending and passing through the furnace funnel; and to prevent all return of the like, it is, that solid oxygen is added to the materials when fused, the interstices of which, in the vitrified mass, secure the direct continuity of the visual medium. Priestley made black wax and brass filings transparent, by only removing all interposed electric matter. The body of a living man, by being de-electrised, has been made transparent. In these instances the transparency is of short continuance, and the opacity is restored by returning electric matter. Fire, in de-electrising gems and crystals, destroys all partial opacity. The clearest water is made cloudy on receiving the charge from the electrifying jar; by uncustomary electric matter, the atmosphere is made foggy, and is transparent again when the electric matter becomes a constituent of rain-water. These instances show, that, electric matter lying in the way of the medium of space and vision, interrupts its regular continuity, consequently, its direct pressure; yet not wholly,--clairvoyance and sound make manifest that the continuity is maintained through the most opaque bodies. The principle bears strongly on the physiology of clairvoyance. THE NERVOUS FLUID. Were there a distinct fluid belonging to the nerves of sensation, and insulated, it could not be affected by external circumstances, nor its cerebral excitement be productive in the least of any knowledge, relative or inferential of external bodies. Were the fluid not insulated, it should be subject to waste like the lachrymal fluid, and must excite the brain differently at different times, even under equal circumstances; which must make it impossible to identify the same body after its removal out of the axis-of-vision direction. A distinct fluid, not insulated, has to be in contact with the line of medium of space which the external object terminates, which adds to the difficulty of waste, in the possibility of the nerves becoming flooded with an abnormal fluid, medium of space. Much more likely is it, that, _the cerebral exciting fluid, of the nerves generally, consists in medium of space_, received from without through the cuticular insertions and orifices of the nerves as streamlets from the great ocean of space, subject to neither ebb nor flow, and liable to change of pressure occasioned by external agency. According to this idea, the object and brain are the terms of the visual line; and medium of space, continuous from the object through the nerves to the brain, is the connecting link.