the number of chemical elements in the Periodic Table;

Argon,
Hydrogen,
Zinc,
Iron,
Potassium,
Nickel,
Lead,
Mercury,
Phospherus,
Gold,
Cerium,
Chromium,
Sulfur,
Titanium,
Arsenic,
Nitrogen,
Tin,
Uranium,
Silver,
Vanadium,
Carbon.

See Primo Levi, The Periodic Table ,first published 1975, 21 autobiographical stories/commentaries by Levi, chemist and Italian Jew who survived the concentration camps.