Visible and Invisible Portraits
These are performed with French chalk, a natural production of the earth, (sold in most oil-shops,) of a greasy, but extraordinary nature. It is made use of to draw portraits upon looking-glasses; which may be made visible and invisible, alternately, by breathing on and wiping off, and they will so continue for many months. The lines will appear very distinct where the glass is strongly breathed on, and disappear entirely when it is wiped dry again.