THE ALABAMA INSURRECTION. – We learn through a private source that the evidence of the insurrectionary plot in Alabama, which we alluded to some days ago, was extorted from a slave by the application of eight hundred lashes to his back. After this gentle manifestation his memory was entirely restored and he was able to verify every suspicion of his tormentors. It was evidence obtained by this process on which the preachers were hung in Texas, and on which most of the cruel punishments indicted on Northern travelers in the South are grounded. It is a well known fact, that the States in which these ingenious methods of acquiring knowledge are in vogue, are the most ardent in their devotion to the Democratic Party; and it is marvelous, if we revert to the history of the reign of the late King BOMBA, to witness the similarity of means and instruments used in keeping up an intense Democracy and an intense Despotism.