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?
  • $\begingroup$ This may perhaps be a better fit for/more 'on-topic' at history of science and mathematics rather than chemistry.SE. $\endgroup$
    – NotEvans.
    May 21 '17 at 16:20
  • $\begingroup$ How does migration work. $\endgroup$ May 21 '17 at 16:31
  • $\begingroup$ I'm sure the whole story is convoluted. But in a nutshell early chemists used reactions for a lot of the simple inorganic compounds to figure out the elemental ratio. $\endgroup$
    – MaxW
    May 21 '17 at 18:02
  • $\begingroup$ The book "Crucibles: the story of Chemistry" by Jaffe explains this in thorough and interesting detail. I don't remember the details, but generally they measured volumes of gases evolved from different reactions and found correlations/commonalities between the reactions. They found that the volume of the gases was related to the amount of substance reacted in whole-number ratios, etc. $\endgroup$
    – khaverim
    May 21 '17 at 18:57
  • $\begingroup$ For migration flag your post for moderator attention and explain that you want it migrated. I have not checked thoroughly, but I think it's likely that this is also a duplicate over at HSM, you probably aren't the first person to have been curious about this. $\endgroup$
    – orthocresol
    May 21 '17 at 18:57