The Study of Spontaneous Combustion Begins
Though the idea had probably been bouncing around for awhile, the basic concept and arguments for humans just bursting into flames was not presented as a formal topic to learned men until 1745. In that year an article was published in the Philosophical Transactions of London, England, written by a Mr. Paul Rolli, which presented three unusual deaths by fire; those of the Countess de Bandi Cesanate in 1731, Grace Pett in 1744, and John Hitchell in 1613. In his article, Rolli noted the similarities of the three cases (which, roughly speaking, was that all three died of fires from unknown and unexplained origins); then he laid out his arguments for the possibility of internal combustion of the human body as the cause of all three deaths. The arguments he put forward were to last for nearly two-hundred years, becoming the classic criteria for proving an abnormal internal source for the strange fire deaths. Rolli’s criteria, as he saw it in the cases he presented, were:

Rolli’s arguments were especially compelling in the case of the Countess de Bandi’s death. The Countess’ remains had been found on the floor of her room, halfway between her bed and the window, and consisted of a pile of ashes, two unburned lower legs, and a calcined section of her skull lying on the ground between the legs. To Rolli’s imagination, this suggested but one possibility... that the Countess had been caught completely off guard by the combustion while walking to the window, and that it had started in her lower abdomen and burned so fiercely that her body was reduced to ashes as she was still standing; after which, her skull fell straight down to land between the unburned remains of her lower legs.
Of course, claiming a fire from within a person’s body as the cause of death does beg one more question: how and why would they catch fire? Rolli’s theory as to what caused this spectacular state was that “effluvia” ? gases and waste ? within the Countess’ stomach and intestines combined with vapors from various liquors and her bodily fat to become a substance that could be easily ignited during rest by motions within the body caused by deep breathing. The victim of another case that Rolli studied, Grace Pett, had been a drinker; so Rolli argued that the alcohol already in her body’s tissues added to the same chemical effect that had claimed the Countess. Oddly, the strange death of John Hitchell, Rolli’s third example, didn’t fit Rolli’s criteria for internal combustion, nor did this case closely resemble the deaths of the Countess de Bandi or Grace Pett in any way other than Hitchell also burned to death... but no one seems to have noticed these discrepancies, possibly because Rolli’s idea of how these deaths occurred largely reflected what many people already believed. Rolli’s proposal of spontaneous internal combustion caught the attention and imagination of the learned men of Europe, and the search for more cases of this unusual demise began.
This initial search was very general in nature; just about any incident involving an unusual fire was considered as possible evidence. Rolli himself was sure that the three fire deaths he described were likely closely related to the then popular reports of luminous humans... people whose bodies were said to glow or spark under particular circumstances. Rolli felt that these unusual people were simply luckier than the ones who were being found incinerated!