Drennon's answer is incorrect. Montalembert's writings had nothing to do with Tyrannicide. John Stuart Mill's reference is to two trials that took place back-to-back in England in 1858. Paraphrasing from the legal accounts:
Queen versus Truelove. Indictment found at the Central Criminal Court
and removed into the Court of Queen's Bench by certiorari, for
publishing a libel on His Imperial Majesty the Emperor of the French,
and for inciting to assassination. Tried June 1858. Verdict: not
Queen versus Tchorzewski. In the Court of the Queen's Bench,
Stanislaus Tchorzewski, a bookseller of Green Street, was charged on
an indictment for publishing a pamphlet entitled “Lettre an Parlement
et a la Presse,” and signed “ Le Comité de la Commune Révolutionnaire,
Félix Pyat, Besson, A. Talandisr,” justifying the Orsini assassination
attempt, and inciting to the assassination of the Emperor of the
French. Verdict: not guilty.
The Orsini plot was a conspiracy to assassinate the Emperor of France in January of 1858.