Januarius was a bishop of Benevento killed in the persecutions of the early fourth century, and his relics were brought to Naples, where he became patron of the city. Two sealed glass ampoules held in a silver reliquary contain a dark solid substance identified by tradition as his blood, collected at his death. Three times a year, on the Saturday before the first Sunday of May, on his feast day in September and in December, the reliquary is brought out and held up before the congregation, and the contents usually become liquid within minutes or hours.
The ceremony is not a quiet one. A group of women known as the relatives of San Gennaro pray loudly and sometimes scold the saint until it happens, and the announcement is made from the altar and greeted with applause and a wave of handkerchiefs. Failures are recorded and read as warnings, and Neapolitan memory attaches them to particular disasters, which is exactly the kind of retrospective fit that makes the pattern hard to test.
The Catholic Church permits the devotion and has never issued a ruling on its cause, a position it maintains carefully. Chemists have shown that a thixotropic gel, one that liquefies when shaken and sets again when left alone, can be made from materials available in the fourteenth century and behaves the same way, published in Nature in 1991. The custodians have not permitted the ampoules to be opened or sampled, so the contents have never been analysed directly, and the question stands where it has always stood.