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Re: mass extinctions and DDT
At 3:39 PM +0100 12/30/01, David Marjanovic wrote:
> >Aren't there some big windows in a chaple in Europe that are now thicker
>at the base because glass is just a really really viscous liquid? Or is
> >that science babble?
Some windows are thicker at the base after a mere 100 years (depending on
the sort of glass AFAIK). Very old windows tend to become white -- the glass
finally crystallizes = freezes into a solid.
Glass does not flow over even geologic time scales, although in
theory it is an extremely viscous liquid. Extremely means that the
predicted relaxation time at room temperature for window glass is
10E34 years, longer than the 15 billion year age of the universe.
(Edgar Dutra Zanotto, "Do Cathedral Glasses flow," American Journal
of Physics, V. 66, 392-395 (May 1998). It is likely that cathedral
window panes are thicker at the bottom than the top because the
glaziers installed it that way.
Glasses do degrade and crystallize over very long times; tektites are
an example. The degradation times depend on their composition.
-- Jeff Hecht