"... when Giuseppe Mazzini, an Italian exile in London, became convinced that the British government was opening his mail. Mazzini, a revolutionary who’d been thrown in jail in Genoa, imprisoned in Savona, sentenced to death in absentia, and arrested in Paris, was plotting the unification of the kingdoms of Italy and the founding of an Italian republic. He suspected that, in London, he’d been the victim of what he called 'post-office espionage': he believed that the Home Secretary, Sir James Graham, had ordered his mail to be opened, at the request of the Austrian Ambassador, who, like many people, feared what Mazzini hoped—that an insurrection in Italy would spark a series of revolutions across Europe. Mazzini knew how to find out: he put poppy seeds, strands of hair, and grains of sand into envelopes, sealed the envelopes with wax, and sent them, by post, to himself. When the letters arrived—still sealed—they contained no poppy seeds, no hair, and no grains of sand. Mazzini then had his friend Thomas Duncombe, a Member of Parliament, submit a petition to the House of Commons. Duncombe wanted to know if Graham really had ordered the opening of Mazzini’s mail. Was the British government in the business of prying into people’s private correspondence? Graham said the answer to that question was a secret."
So begins "The Prism: Privacy in an age of publicity," by Jill LePore at The New Yorker.