I think so long as the solid is kept with water on top of it ("not less
than half its own weight of water" according to my old lab safety manual)
there is little danger.
However, as with all potentially hazardous substances, unless you have a
use for it, proper disposal would be wise.
Alex Nanson mentioned Roscoe and Schorlemmer in a message I read today,
bringing back memories. Although I'm NOT old enough to have been taught by
them, they were from my old college, and the undergraduate labs in which I
started my degree were named after them (and I don't think had changed at
all since their time there). Also the lab where I learnt physics had a
small plaque on the bench where I sat which said 'Rutherford first split
the atom here' and gave a date.
Peter Marshall
On Fixing Shadows and elsewhere:
http://www.people.virginia.edu/~ds8s
Family Pictures, German Indications, London demonstrations &
The Buildings of London etc: http://www.spelthorne.ac.uk/pm/