The business of the convention being over, Mr. Lincoln, in response to repeated calls, came forward and delivered a speech of such earnestness and power that no one who heard it will ever forget the effect it produced. In referring to his speech some years ago I used the following rather graphic language: "I have heard or read all of Mr. Lincoln's great speeches, and I give it as my opinion that the Bloomington speech was the grand effort of his life. Heretofore he had simply argued the slavery question on grounds of policy--the statesman's grounds--never reaching the question of the radical and the eternal right. Now he was newly baptized and freshly born; he had the fervor of a new convert... His speech was full of fire and energy and force; it was logic; it was pathos; it was enthusiasm; it was justice, equity, truth , and right set ablaze by the divine fires of a soul maddened by the wrong; it was hard, heavy, knotty, gnarly, backed with wrath. I attempted for about fifteen minutes as was usual with me to take notes, but at the end of that time I threw pen and paper away and lived only in the inspiration of the hour. If Mr. Lincoln was six feet, four inches high usually, at Bloomington that day he was seven feet, and inspired at that..."
-Thomas Herndon, Lincoln's law partner and biographer, quoted in Elwell Crissey, Lincoln's Lost Speech (New York: Hawthorn Books, 1967)
On the evening of May 29, 1856, at at the Illinois Anti-Slavery Extension convention in Bloomington, Illinois, Abraham Lincoln gave a speech so powerful that no one in the audience wrote it down. The "Lost Speech" has become one of the great mysteries of American history: exactly what did the future president say at Major's Hall that night to inspire such praise as Herndon's? It's a mystery that's been debated for over a century.
While there have been several attempts to reconstruct the speech, most notably that of Henry Clay Whitney, who was at Major's Hall, but did not publish his reconstruction until 1896, in McClure's Magazine. Even if his reconstruction is close to Lincoln's words, it was Lincoln's forceful and emotional delivery which caused so many to remember the speech.
Wilson Tucker, a science fiction writer who lived in the Bloomington area, wrote a novel, The Lincoln Hunters (New York: Ace Books, 1958), based on that mystery. A team of time travelers from a dystopian world six hundred years in the future go back to 1856 in order to record the Lost Speech. It's not great writing, but it's quite good; the brilliance of the concept makes up for the less-than-stellar prose. The book, while out of print, is available. Amazon Marketplace has copies for one cent (plus $3.99 shipping and handling).
Tucker won't tell you what Lincoln said. It's still a mystery.