Taken from Wikipedia, the big tome of knowledge that must not be trusted at once
The process of manufacturing the fluid was a very carefully guarded military secret — so secret, in fact, that today we still do not know how it was made. Various sources speculate that its constituents may have included sulfur, quicklime, and liquid petroleum. It is not clear if the mixture was ignited by a flame as the mixture emerged from the syringe, or that it ignited spontaneously when it came into contact with water. If the latter is the case, it is possible that the ingredient responsible was calcium phosphide, made by heating lime, bones and charcoal. On contact with water, calcium phosphide releases phosphine, which ignites spontaneously.
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