When we first came across pockets of gases underground, what did the process of us actually recognizing these things look like? Did we know what to look for?
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Start with air. Stick a candle in a dish containing water, light it, then put a jar over the candle. The candle burns for a bit, then it goes out. As that happens, some of the water in the dish gets sucked up into the jar. Repeat a few times, and you will see that the part of the air that is used up when the candle burns is always about 20% by volume. Put a mouse in the jar with the burning candle and it dies when the candle does. So you have determined that ~20% of the air around us is used by both combustion and respiration.
Scientists had discovered that blowing air from the lungs through limewater turned it cloudy (carbon dioxide reacts to form calcium carbonate). The gas left after burning a candle did the same, as did the gas evolved from vinegar and soda bicarbonate. So all those things seemed to be the same gas (carbon dioxide).
Using limewater to remove carbon dioxide from air with oxygen also removed left something that made up about 80% of normal air. This took much longer to isolate and characterize, because nitrogen gas is not very reactive. It didn’t support combustion or respiration.
Gases evolved from chemical reactions were compared with known gases – some exploded (hydrogen), some were colored and noxious (chlorine).
The history of chemistry is full of simple experiments performed with available materials, comparing results to understand the natural world around them.
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