CABINET NOIR  , the name given in France to the office where the letters of suspected persons were opened and read by public officials before being forwarded to their destination . This practice had been in vogue since the establishment of posts, and was frequently used by the ministers of Louis XIII. and Louis XIV.; but it was not until the reign of Louis XV. that a separate office for this purpose was created . This was called the cabinet du secret des posies, or more popularly the cabinet noir . Although declaimed against at the time. of the Revolution, it was used both by the revolutionary leaders and by Napoleon . The cabinet noir has now disappeared, but the right to open letters in cases of emergency appears still to be retained by the French government; and a similar right is occasionally exercised in England under the direction of a secretary of state, and, indeed, in all civilized countries . In England this power was frequently employed during the 18th century and was confirmed by the Post Office Act of 1837; its most notorious use being, perhaps, the opening of Mazzini's letters in 1844 .