… is from page 202 of Lord Acton’s late-1890s lecture “The Influence of America,” as this lecture appears in Essays in the History of Liberty: Selected Essays of Lord Acton, Vol. 1 (J. Rufus Fears, ed., 1985; on-line access to this essay is available free of charge here):
Who are a free people? Not those over whom government is reasonably and equitably exercised; but those who live under a government so constitutionally checked and controlled that proper provision is made against its being otherwise exercised.