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.