In May 1753, in the «Novelle Letterarie» of Florence, a famous magazine directed by Giovanni Lami, Raimondo di Sangro sent the first of seven letters to Giovanni Giraldi of the Accademia della Crusca. The letters, translated into French, were then put together by the author in a volume addressed to the physicist Jean-Antoine Nollet, member of the Academy of Sciences in Paris. The subject of these papers was a “marvellous discovery” which the Prince of Sansevero had made in November 1752: the perpetual lamp.

While “engaged in a chemical experiment”, he had accidentally found a substance which, once ignited, had continued to burn for more than three months without the slightest “diminution”, i.e. without any reduction in weight. Torn between his desire of seeing his genius recognised by the scientific community and a natural tendency towards reserve and secrecy, di Sangro made it known that the combustible material was partly made from a substance gleaned from the bone of a human cranium (“the bones of the noblest animal on the Earth”), and in part from substances which he was careful not to specify. He underlined that the matter he had discovered was no simple phosphorus, giving off “a beautiful and lively flame”, even though “rather smaller than those made by wax or oil lamps”.

The perpetual light of the Prince of Sansevero was, and still is, much discussed, and the mystery will presumably remain unsolved. It should however be noted that, despite the fact that Raimondo di Sangro’s light – like a lot of his fabulous discoveries – is highly symbolic and intentionally alludes to esoteric meanings, in his letters to Giraldi and Nollet, he sets out a procedure in line with the methods of experimental science of the period, citing respected physicists (such as Hermann Boerhaave and Petrus van Musschenbroek) and referring to theoretical models widely accepted at the time.

The emotion of discovery, a structured hypothesis for an explanation, repetition of the experiments – all this emerges from the letters of the Prince of Sansevero. He wanted two perpetual lamps to illuminate the Veiled Christ, once it had been moved down to the Underground Chamber of the Sansevero Chapel. The Chamber, however, was never finished, and nothing became of the two lamps. In 1756 di Sangro came back to the idea of the “glorious light”. “Since, therefore, it cannot be doubted that this is a true light, similar to our candles or lamps, and has burned three months and some days without any reduction in the material used for fuel, it can rightly be called perpetual, much more so than those imaginary lights which can sometimes be found in the ancient tombs […] and any other light which does not have the same properties as mine, i.e. all the qualities of other natural flames, does not deserve to be called eternal”.