1911 Encyclopedia Britannica
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 postes, 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.
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Chisholm, Hugh, General Editor. Entry for 'Cabinet Noir'. 1911 Encyclopedia Britanica. https://www.studylight.org/encyclopedias/eng/bri/c/cabinet-noir.html. 1910.