What did chemists recognize/claim by the mid 1800s?

Motivation cannot be developed with practice; a person either possesses it or they do not.

The empirical formula of a compound is ch2o and its mass is 120 amu/molecule, what is its formula?

Which best describes the present-day universe? opaque, expanding very slowly, stars produce heavy elements transparent, expanding at an accelerated rate, stars produce heavy elements opaque, expanding at an accelerated rate, stars produce only hydrogen and helium transparent, expanding very slowly, stars produce only hydrogen and helium