Make distinct solutions of common salt, nitre, and alum; set them in
three saucers in any warm place, and let part of the water dry away or
evaporate; then remove them to a warm room. The particles of the salts
in each saucer will begin to attract each other, and form crystals, but
not all of the same figure: the common salt will yield crystals with
six square and equal faces, or sides; the nitre, six-sided crystals;
and the alum, eight-sided crystals; and if these crystals be dissolved
over and over again, they will always appear in the same forms.