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 was it deduced that these gases are diatomic. Also how were the constitution of various compounds, in terms of both number and structure discerned?