See page 12. Hg is either a monad or a dyad.
"An Introduction to Chemical Science"
R.P. Williams
Pythagoras likewise inculcated the famous numerical system of the monad, dyad, and triad; and by means of his sacred quaternary, elucidated the formation of the world, the arcana of nature, and the principles both of music and morals.
"Knickerbocker's History of New York, Complete"
Washington Irving