When you burn steel wool, it gets heavier

Anti-phlogiston at work, clearly!

Back in the day (primarily the 1700s), one of the prevailing theories of chemistry was that as materials burned, they released phlogiston, a gas that was somehow stored in the material (the wood, paper, whatever). That release of phlogiston made the material lighter.

It makes sense, right.

Sure, until you look at something like what you see above. The burning metal gets heavier as it burns. Maybe phlogiston has negative mass...or there's anti-phlogiston...or phlogiston just doesn't exist.

Along came Antoine Lavoisier in the 1770s and the discovery of oxygen. Now we know that the metallic oxide has metal AND oxygen, so it has more mass, more stuff, more weight.

But that took a looooong while to figure out.