“With utmost secrecy it was confided to me that the Prince of San Severo has made a certain substance similar to the blood of San Gennaro, and which in accordance with changes in the air appears to have the same effects”. So the Apostolic Nuncio Lucio Gualtieri wrote in a letter of 18 May 1751. Already tarnished by the recent publication of the Lettera Apologetica and his affiliation to Masonry, Raimondo di Sangro’s reputation with the Church was permanently compromised by of this experiment, which seemed to implicitly question the miracle of the liquefaction of the blood of the patron saint of Naples.

In reality, the Prince, as an indefatigable experimenter, meant to check – as his contemporary de Lalande well understood – “a pure hypothesis regarding Physics”, i.e. that a substance could in specific circumstances dissolve and solidify again. This, however, does not imply that he considered that the phenomenon of liquefaction which took place inside the real ampoules with the blood of San Gennaro did so thanks to the same causes and according to the same method as reproduced in the laboratory. In any case, the system devised by the Prince was much more complex than the concise report which the Apostolic Nuncio made on it.

De Lalande notes that “he had a monstrance or display case similar to that of San Gennaro’s built, with two ampoules of the same form, full of an amalgam of gold and mercury mixed with vermillion, the same colour as coagulated blood. To make this amalgam fluid there is in the space around the edge […] a reservoir of fluid mercury with a valve, which, when the case is overturned, opens and lets the mercury enter the ampoule. At this point, the amalgam becomes liquid and imitates liquefaction. But this is a pure hypothesis of physics, suited to explain an effect. It is the mark of a great physicist to want to explain and imitate everything”.

Another eighteenth-century traveller describes the composition of the substance and the experiment in still greater detail, explaining also how liquefaction did not take place mechanically every time the ampoule was turned, but could even be partial, just as happened with the blood of San Gennaro. And, commenting on the ingenious system created by the Prince, concludes: “All I can say is that it worked perfectly”.