DEAR SIR; In a speech delivered by you, in the House of Representatives, in March last, I understood you to say that you had been in the valley of the Great Salt Lake, and that you were acquainted, from personal observation, with a large part of the territory of California. Will you be so good as to give me your opinion, and the reasons for entertaining it, of the probability or improbability of the introduction of slave labor into any part of the territory recently acquired by the United States from Mexico; provided such introduction be not prohibited by law. I wish to obtain your opinion in regard to other kinds of labor, as well as agricultural; because, as it seems to me, a most unwarrantable, if not a most disingenuous attempt has been made, to lead the public to believe that no form of slave labor will ever be introduced there, because, possibly, or probably, it may not be introduced for agricultural purposes. A reply at your earliest convenience will much oblige Yours, very truly, HORACE MANN. WASHINGTON, June 10, 1850. HON. HORACE MANN; I received a note from you some days ago, making certain inquiries, but which, up to this time, I have been unable to answer. I desire to take no part in the question now dividing the country; but as you have asked my judgment upon a matter which appears to be a disputed point, I cannot, consistently with the law of courtesy, refuse you an answer. That answer will be in conformity with what I have frequently said, heretofore, in private conversation with gentlemen on this subject. The point of inquiry seems to be, whether slave labor could be profitably employed in Oregon, California, Utah, and New Mexico. If the nature of the climate and resources of these countries are such as to furnish a profitable market for slave labor, it appears to be conceded, on all sides, that it would be introduced, if left free to seek profitable investment, like other capital. The whole point at issue, then, is dependent, as it is conceived, upon the determination of the first point of inquiry. Hence, to that point, only, it is necessary for me to confine my answer.