Mr. S. L. MITCHILL declared his wish that the proposition of the gentleman from Pennsylvania (Mr. BARD) should be considered merely as a subject of political economy. In the remarks which he proposed to offer upon it, he should, therefore, confine himself to that object. He would, therefore, say nothing on the immorality of a trade which deprived a large portion of the human species of their rights. He should pass over, in silence, every thing that might be urged to exhibit it as impious and irreligious; and he would not utter a word on its repugnance to the principles of our equal jurisprudence, and the spirit of our free Government. The slavery of a portion of our species was a copious theme, when viewed in either of these aspects; but, on the present occasion, he was willing to waive them all. The proposition was to be considered only in its commercial, economical, and fiscal relations; and on each of these it would be proper to make a few observations. It was much to be regretted that the severe and pointed statute against the slave trade had been so little regarded. In defiance of its forbiddance and its penalties, it was well known that citizens and vessels of the United States were still engaged in that traffic. During the present session, memorials had been presented to Congress praying for exoneration from the exportation bonds, which had been given to one of the collectors of the customs, to ensure the landing of a cargo of New England rum in Africa, which it was not pretended to be denied was bartered away for slaves. These voyages were said to be carried on under the flag of a foreign nation; and the common practice, as was alleged, was, to go to the island of St. Croix and procure Danish papers and colors. Under this cover, the voyages were performed. To prevent the confiscation of the vessels under the law, on conviction of being engaged in the slave trade, it had been customary to sell that article of property in a foreign port.