According to the the generally accepted scientific understanding of the 18th century, all combustion was attributable to phlogiston, a colorless, odorless, tasteless, massless substance. At the time, the scientific community believed that all flammable materials contained a certain amount of phlogiston. The act of burning the materials was in fact merely releasing the phlogiston into the air. Once burned, the materials were held to be in their "true" form, the 'calx'. Thus the act of burning was known as 'dephlogisticating' a substance.

When new experiments revealed inconsistencies with this theory (such as the fact that Magnesium gains mass when burned), scientists came up with wild theories about phlogiston having negative weight, or being lighter than air. Finally, the alternative theory of oxidation was derived, and has held to this day.

Reading about 18th century scientists' fanciful and now seemingly foolish theories about undetectable substances like phlogiston feels eerily reminiscent of my freshman physics class, where I was taught that a force is a massless effect that is detectable only by its effect on other objects, and that this effect is felt across infinite distances. It's just difficult to claim that, "OK, OK, all those other crazy theories about undetectable particles were stupid, but this one, this one is true."

Of course, a modern alternative is String Theory, which suggests that our universe actually has nine spatial dimensions, and that what we call 'force' is really just vibrations in alternate dimensions. In some ways, that theory actually sounds more plausible (it doesn't assign force a mystical, unknowable cause), but of course it has believability problems of its own.

I don't mean to suggest that an advanced physics theory needs to sound plausible to a layman to be correct. It just looks kind of silly when placed beside other similarly fantastical theories that were later proven quite false.