Conversation so far:
can two elements form more than one compound
two elements react to form more than one compound, a fixed mass of one element will react with masses of the other element in a ratio of small, whole numbers.
What is an example of two elements forming more than one compound
A fixed mass of carbon, say 100 grams, may react with 133 grams of oxygen to produce CO, or with 266 grams of oxygen to produce CO2.
What is another example of two elements forming more than one compound
comparing ethane- C2H6- with propane -C3H8-: The weight of hydrogen which combines with 1 g carbon is 0.252 g in ethane and 0.224 g in propane.

Question: When was the discovery first expressed
Self-contained rewrite: When was the discovery on how two elements can form more than one compound first expressed