Spontaneous Human Combustion
In Charles Dickens' Bleak House, the papers that will uncover Lady Dedlock's hidden past are fortuitously destroyed when a drunken Krook spontaneously combusts, thus destroying himself and the papers in flames. Facing criticism of this plot device, Dickens wrote in the preface to the novel: "The possibility of what is called spontaneous combustion has been denied since the death of Mr. Krook … I have no need to observe that I do not wilfully or negligently mislead my readers and that before I wrote that description I took pains to investigate the subject." The idea of human beings just bursting into flames has obviously been around for some time, Dickens quotes two cases that were over 100 years before he was writing to justify its inclusion in his novel. His inclusion set off a trope that survives today.
Most of the assumptions from then on were that heavy drinking could cause people to just burst into flames, likely due to the flammability of alcohol vapour. This was backed up by reports from police and fire investigators who were perplexed by partially burned corpses near unburned rugs or furniture. They were completely baffled as to how a body could burn down to ashes except for a leg or a foot, while the rest of the room avoids being consumed by the flames. In many cases, no obvious cause of ignition can be found other than maybe a cigarette or coal fire, but in all cases, the mystery surrounds why the body has burned away yet the furniture has not. The conclusion was that the body had somehow burned from the inside out.
The answer to the riddle was found by Dr. John de Haan of the California Criminalistic Institute. He wrapped a dead pig (which has a similar fat content to a human body) in a blanket, placed it inside a mockup of a room and started a small fire on it with a little petrol. The animal took over 5 hours to burn away but due the wick effect of the blanket and the fat in the animal, it burned slowly and with low external heat. The surrounding room, though showing signs of scorching, did not burn. Almost all cases of SHC are of someone who is elderly or infirm or unable to stop a small fire on their person from perhaps a cigarette or candle. Possibly because they are very drunk or maybe even already dead. There is no evidence or plausible method for a body to create sufficient heat to cause it to burn without an external source, if that external source starts a fire in an incapacitated body, in a low oxygen environment, then it will likely smolder away for hours until only ash is left. The remaining room will not burn and as there is insufficient fat at the extremities, then that is often all that is left to discover.