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.