During times when there was no spectroscopy of any kind, people did make guesses on the molecular constituency and structure.
For example, John Dalton's original atomic hypothesis was that all elements were mono-atomic and that the atoms in compounds have the simplest possible ratios. However, this was proved to be wrong and later it was recognized that oxygen and various other gases were actually diatomic.
- How? How were the constitution of various compounds, in terms of both number and structure discerned? [for example, in this case, the atomicity of gases].
Note: This question here also deals with a similar problem but for the specific case of water. But, my question is about how this is done generally, what are the techniques and experiments and hypotheses.
Edit: This question and answer therein and the current answer to this question are specific to gases and water. Is there anything more generally applicable to organic and inorganic compounds (other than only gases)?