John Ruskin (1873). “The Eagle's Nest: Ten Lectures on the Relation of Natural Science to Art, Given Before the University of Oxford in Lent Term, 1872”, p.80
Marie Curie's letter to Eve Curie Labouisse (July 1929), as quoted in Eve Curie Labouisse "Madame Curie: A Biography" translated by Vincent Sheean (p. 341), 1937.