Not So Spontaneous Combustion
Over the next fifty years or so, many more cases of these strange deaths -- historical and contemporary -- were documented and added to the picture. Rolli’s theoretical ideas of internal combustion suggested the possibility that this strange form of death could be caught before it happened, if the right set of symptoms were observed. Of course, what these symptoms would be was a matter of guesswork, but from this guesswork came many reports of strange medical conditions presumed to be somehow related to the combustion deaths... events such as people injured or dying by vomiting or excreting flames, or reports that people in the 9th century sometimes cut off a hand or foot to prevent an invisible fire from spreading through themselves.
While some of the learned men took these newly reported oddities as absolute proof of the internal nature of the fires causing the strange deaths, others started to protest that perhaps the evidence in the deaths had been read wrong to start with. It was noted that in nearly all known deaths that fit Rolli’s criteria for an internal combustion fire, there was actually an obvious external source of fire available -- a candle, a hearth, a pipe -- and that the victims were likely setting themselves aflame with these. Still, even these protests were hedged by an admittance that, under normal circumstances, an external source of fire would never be able to cause the extreme and controlled damage done by the fires in the deaths being examined.
Thus a new theory started to form to explain the bizarre fire deaths. This new idea was that normal human bodies would indeed be incapable of being burned to the extent of the reported combustion deaths... but the people being burned to ashes didn’t have normal human bodies. These people had, it was asserted, made themselves more flammable than normal, so that a small flame could easily set them violently aflame and consume them in a short time. This proposed condition, called preternatural combustibility, was most often believed to be caused by an old culprit... alcoholism. When a person drank an abnormally large amount of alcohol for an abnormally long amount of time, it was believed that their body’s tissues could become more and more saturated with the flammable fluids which, in turn, made even the normally unburnable bones of these people become flammable.
The definitive statement of the theory of preternatural combustibility came in 1800 when Pierre-aimi Lair published his study of the strange fire deaths, entitled “On the Combustion of the Human Body, produced by the long and immoderate Use of Spirituous Liquors”. As the title shows, Lair was absolutely convinced that these deaths were caused by alcohol; and he hoped that by publishing his study, he might convince many people to alter their habits in regard to drinking themselves stupid.
Lair studied fifteen cases of these combustion deaths, and stated that his information might not be perfect; yet he came up with very definitive -- and incorrect -- criteria for preternatural combustibility:

Lair himself admitted that these criteria were likely not written in stone... they simply applied well to most of the fifteen cases he had considered. Several of the cases he looked at did not fit his criteria; but just as the death of John Hitchell was ignored by supporters of Rolli’s theory of internal combustion, so the cases that did not fit Lair’s criteria were quietly ignored. Soon, Lair’s not-so-perfect list of criteria was being treated as unquestionably correct by supporters of preternatural combustibility.
Over the next one hundred years or so, preternatural combustibility as the cause for the strange fire deaths was to be accepted and argued for as much as the theory of internal combustion, by now renamed “spontaneous human combustion”; and soon this debate was to become a public one.