Before the periodic table/atomic theory was nailed down, how did anyone figure out what substances were actually elements?
It seems like it would be extremely easy to mistake a common, stable molecule like salt for something elemental. How did people break down substances to the point of purifying them, and then how did they know they couldn't purify them any further? It seems easy now to theorize what an element of a certain weight would look or act like, but before most of them were known did they have anything to go off of, or was it guesswork?