Potter Stewart would say “I know it when I see it,” but would he? The line between a solid and an extremely viscous liquid isn’t necessarily clear.
People sometimes say that glass is a liquid, citing as evidence the fact that the windows in medieval churches are thicker at the bottom, which suggests that the glass in the windows has been gradually flowing down over the centuries. But that appears to be a misunderstanding. From the venerable Usenet Physics FAQ list:
It is sometimes said that glass in very old churches is thicker at the bottom than at the top because glass is a liquid, and so over several centuries it has flowed towards the bottom. This is not true. In Mediaeval times panes of glass were often made by the Crown glass process. A lump of molten glass was rolled, blown, expanded, flattened and finally spun into a disc before being cut into panes. The sheets were thicker towards the edge of the disc and were usually installed with the heavier side at the bottom. Other techniques of forming glass panes have been used but it is only the relatively recent float glass processes which have produced good quality flat sheets of glass.
The FAQ entry contains a bunch of references, including this AJP article, for those who want to geek out on this subject.
I was reminded of this when a friend pointed me to the pitch drop experiment at the University of Queensland. Someone put some pitch, which has a viscosity 1011 times that of water, in a funnel in 1930, and they’ve been letting it drip out ever since, at a rate of one drop every 10 years or so.
The next drop is due soon. You can hope to catch it on a live video feed at the site.
Here’s a picture of the experiment:
And here’s what happens when you hit the pitch with a hammer:
I think you could be forgiven for mistaking this stuff for a solid.