Solids and Liquids
The study of chemical reactions involving inelastic substances (non gases) led Dalton in 1808 to his Law of Simple Multiple Proportions and to formulate his atomic theory.
Thanks to this law, and helped by the simple imagery of atoms, a system of relative weights of all elements could be devised. This process culminated at the 1860 Karlsruhe Conference where the atomic weight scale was adopted.
One obvious implication of Dalton's work is that one atomic weight of one element (35.45 grams of Cl) has the same number of atoms as one atomic weight of another element (22.99 grams of Na).
While Dalton found simplicity in solids, at about the same time Gay-Lussac was finding even greater simplicity in gases. By studying reactions that had gases as input and gases as output, he was led to his Law of Combining Volumes.
Not so clear how to combine Gay-Lussac's findings with the atomic theory. One would like to say that equal volumes of gas contain equal numbers of atoms, but that doesn't work out.
To make sense of both Dalton and Gay-Lussac's work one needs the concept of a molecule and a theory of chemical affinity (chemical bond). The result is that equal volumes of gas contain equal numbers of molecules. While Avogadro guessed this correctly in 1811, the rest of the chemical world only became convinced following the work of Cannizzaro in 1858, nearly fifty years later. (Avogadro had died in 1856).
On The Web
It is both instructive and entertaining
to repeat some of the classic chemical experiments.
Unfortunately, it can also be too explosive to be done at home!