In some glass-houses the workmen show glass which has been cooled in
the open air; on this they let fall leaden bullets without breaking the
glass. They afterwards desire you to let a few grains of sand fall upon
the glass, by which it is broken into a thousand pieces. The reason
of this is, that the lead does not scratch the surface of the glass;
whereas the sand, being sharp and angular, scratches it sufficiently to
produce the above effect.
Sound and Light.