Ralph Waldo Emerson, Robert Ernest Spiller, Alfred Riggs Ferguson, Joseph Slater, Jean Ferguson Carr (1971). “The Collected Works of Ralph Waldo Emerson: Nature, addresses, and lectures”, p.53, Harvard University Press
Justus von Liebig (1859). “Familiar Letters on Chemistry, in its relation to Physiology, Dietetics, Agriculture, Commerce and Political Economy: Edited by John Blyth”, p.21