Fill a large glass tube with water, and throw into it a few particles
of bruised amber; then hold the tube, by a handle for the purpose,
upright in the flame of a lamp, and, as the water becomes warm, it will
be seen that currents, carrying with them the pieces of amber, will
begin to ascend in the centre, and to descend towards the circumference
of the tube. These currents will soon become rapid in their motions,
and continue till the water boils.