The old days being roughly 1600 - 1850, the time when chemistry and the nature of the elements was being investigated empirically.
For example, hydrogen gas was discovered by Robert Boyle in 1671 after pouring acids over iron filings. It was recognized by Henry Cavendish in 1766.
How did these people trap the gas? What kind of equipment and setup allows this? And crucially, how pure could it be without mixing with normal ambient air?