A few weeks since, a captain, accompanied by a young soldier, apparently about seventeen years of age, arrived in this city, in charge of some rebel prisoners. During their stay in the city, the young soldier alluded to had occasion to visit head-quarters, and at once attracted the attention of Colonel Mundy, as being exceedingly sprightly, and possessed of more than ordinary intelligence. Being in need of such a young man at Barracks No. 1, the Colonel detailed him for service in that institution. He soon won the esteem of his superior officers, and became a general favorite with all connected with the barracks. A few days ago, however, the startling secret was disclosed that the supposed young man was a young lady, and the fact was established beyond doubt, by a soldier who was raised in the same town, with her, and knew her parents. She "acknowledged the corn," and begged to be retained in the position to which she had been assigned; having been in the service ten months, she desired to serve during the war. Her wish was accordingly granted, and she is still at her post. We learned the facts above stated, and took occasion to visit the barracks, and was introduced to "Frank Martin," (her assumed name,) and gleaned the following incidents connected with her extraordinary career during the past ten months: Frank was born near Bristol, Penn., and her parents reside in Alleghany City, where she was raised. They are highly respectable people, and in very good circumstances. She was sent to the convent in Wheeling, Va., at twelve years of age, where she remained until the breaking out of the war, having acquired a superior education, and all the accomplishments of modern days. She visited home after leaving the convent; and, after taking leave of her parents, proceeded to this city, in July last, with the design of enlisting in the 2d East Tennessee Cavalry, which she accomplished, and accompanied the Army of the Cumberland to Nashville. She was in the thickest of the fight at Murfreesboro, and was severely wounded in the shoulder, but fought gallantly, and waded Stone River into Murfreesboro, on the memorable Sunday on which our forces were driven back. She had her wound dressed, and here her sex was disclosed, and General Rosecrans made acquainted with the fact. She was accordingly mustered out of service, notwithstanding her earnest entreaty to be allowed to serve the cause she loved so well. The General was very favorably impressed with her daring bravery, and superintended the arrangements for her safe transmission to her parents. She left the Army of the Cumberland, resolved to enlist again in the first regiment she met. NEGRO SERMON DELIVERED AT TRIUNE, TENN. Camp near Triune, Tenn., _May 16, 1863_.