W. Scheele discovered "fire air" (oxygen) in 1771 by heating mercuric oxide with nitrates. How did Lavoisier know Scheele had isolated a particular elemental gas?
How did Cavendish recognize hydrogen as discrete substance, after Boyle's initial isolation of it?
I hope I'm not looking at this wrong, but given that atomic theory only entered mainstream science in the 19th century, how did these great men know they had isolated "elements" and discrete gases.