A FUGITIVE SLAVE CASE IN WASHINGTON. – On last Monday evening a colored woman, named AGNES ROBINSON, and her child MARY, were arrested in this city on a warrant issued by Judge MAERRICK, of the Circuit Court, on proof furnished from Washington County, Maryland, and directed to Marshal SELDEN, as the act of September, 1850, requires, representing that the said woman had escaped from servitude in said county some years since, and had been living in Washington City. Mr. DAVID WITMER was the claimant, and on his behalf it was argued by JOHN H. McCUTCHEN, and for the defendant WALTER D. DAVIDGE, in chambers before Judge MERRICK. The forms of the Fugitive Slave act were complied with and Judge MERRICK remanded the woman back to Hagerstown, Md., where the question of their right to freedom must be determined, the Judge having, of course, no power to decide that point. – Washington States, 24th.