Of the Origin and Progress of Language (Edinburgh and London: J. Balfour and T. Cadell, 2nd ed., 1774), Vol. I, Book II, Ch. II, pp. 224-225 https://archive.org/stream/originandprogre01conggoog#page/n251/mode/2up.
James Burnett, Lord Monboddo: Frasi in inglese
Letter to Richard Price (Sept. 15, 1780) as quoted by William Angus Knight, Lord Monboddo and Some of His Contemporaries https://books.google.com/books?id=GAEQAAAAYAAJ (1900).