The Scientific Approach
The scientifically possible cause for the strange fire deaths, proposed at least as early as 1783, was the idea that the victims’ own body fats were enough to feed the fire consuming them, if their clothes acted as a wick for the flame.
In candles, the wick itself is not what produces a flame, even though it is what is first lit; shortly after the wick is lit, the wax below the flame becomes fluid enough that it gets sucked up through the porous wick to where the flame is... and from then on, the wick only burns when the wax gets too low to be carried up to the flame. When the wick burns low enough again, the liquid wax is once again drawn up and used as fuel for the flame. More than once, it was proposed that if a person’s clothes caught fire, then the cloth could soak up body fats from the tissues it contained, burning the body first, and burning the clothing second. If true, this could explain the selective nature of the fires in these strange deaths; the major parts of the bodies consumed would generally be the parts covered by clothing that could absorb liquid fat... and most of these fats would be in the victims’ torsos, the very part most commonly reduced to ash.
Protests against this idea were quick to be made by the both supporters of the internal combustion theory and supporters of the preternatural combustibility theory. The main arguments made against the “candle effect” (as it came to be known) was that such a fire would never be hot enough to break down a body (and certainly not the bones!); that said fire would burn slow enough that any fool, even drunk, would have a reasonable chance to extinguish it; and lastly, that plenty of people died of burns caused by their clothing catching fire, but very few turned up as a pile of ashes... so the flames in the strange fire deaths had to be something very different from the sort of common flames that burning clothes provided.
But as the 19th century proceeded, so did the scientific understanding of burning and combustion... and with it, new answers for some of the questions surrounding the strange fire deaths, as well as evidence that questioned the proposed criteria for both internal combustion and preternatural combustibility.
The first problem was that as more and more cases of these strange fire deaths were found, largely due to an increased number of people specifically looking for them, it became clear that victims didn’t necessarily have to be women, old, or drinkers... in short, most of Lair’s proposed criteria about who victims would be was actually wrong.
Another problem was that new evidence brought into question Rolli’s assumption for a sudden, violent, combustion in the strange fire deaths. In 1888, an old soldier was found incinerated in a hayloft in Aberdeen, Scotland. Despite being converted to ash, the features of his calm, sleeping face were still clearly visible; this led the investigating physician, Dr. J. Mackenzie Booth, to look into some of the earlier deaths of this sort, and he reached the conclusion that in almost all cases the victim had not resisted the fire that consumed them. This fact had been noticed before, and was interpreted by Rolli in his study of the deaths as proof that the strange fires had consumed their victims almost instantly. Dr. Booth reached a very different conclusion from the same evidence: he felt the reason the victims didn’t fight the fires was that the victims were dead before their bodies began to burn. These deaths were likely due to suffocation, often helped along by being too drunk to respond to the fire that killed the victims... and the fires could take their own sweet time destroying the victims’ remains.
Other criteria started to be questioned as well; for example, since these deaths now appeared to always have some form of external fire involved, the observation that they occurred more in the winter than the summer became simple to explain... more people spend time near fire in the winter than in the summer. In addition, since women’s work in homes and businesses typically involved using fires to cook, the reason more women seemed to have suffered from these strange deaths could be easily explained as due to this increased exposure to fire.
The next problem for theorists of spontaneous combustion and preternatural combustibility is that, during the 19th century, several of these strange combustion deaths were claimed to be witnessed in progress, and the victims were quickly investigated in regard to testing Rolli and Lair’s proposed criteria... and, not surprisingly, the criteria was brought into question. The most famous of these events was that witnessed on May 12th, 1890, by Dr. B.H. Hartwell of Ayer, MA., in the United States. Called to the location of a deceased -- but still burning -- woman, he saw the body in full combustion, flames licking upwards of fifteen inches from it. Dirt was shoveled onto the body to stifle the flames. The woman, 49 years of age, of good health, active lifestyle, and not prone to drink, had been burning a pile of roots when somehow she had accidentally set her clothes aflame. The event happened after a rainfall, and the fire did not consume the leaves or other matter under the woman’s body... so, just by setting her clothes on fire, she had not only been killed but also set well and truly aflame, burning of just what fuel her own body supplied.
Dr. Hartwell clearly stated that he felt the fire was caused by a wick effect of the woman’s body fat being sopped up and burned through the clothes, and even mentioned another instance of which he knew wherein an elderly woman who fell during the night with an oil lamp set the straw matting of her floor alight; she was found burned to death in the morning, but the fire that had killed her had extinguished itself after using all the oxygen in the room, leading to very little damage otherwise. Yet, given these observations, Hartwell still found it hard to believe that a normal human body could simply burn as he witnessed... so his final explanation fell into the category of preternatural combustibility, with his belief that some people just have fat which is far more flammable than normal human fat.
Despite this continuing disbelief by professionals that human bodies could just plain burn, the evidence was clear... and both professional and public belief on the matter of the strange fire deaths were slowly tipping away from the ideas of spontaneous combustion and preternatural combustibility, towards the real possibility of the 'candle effect' being the most plausible answer.