Do you know about Lomonosov's and Gay Lussac's contribution in understanding of chemical reactions and Avogadro's contribution?
These three chemists contributed to a great extent to the understanding of absolute formulas of substances. Lomonosov earlier was the first to come up to explain four types of chemical reactions in mid of 1700s and was the one to explain law of mass conservation during chemical reactions. Gay-Lussac later in early 1800s mentioned that gases react with each other in certain proportions. "For example, at the same temperature and pressure, two volumes of hydrogen react with one volume of oxygen and form two volumes of water.".
"Later, Amedeo Avogadro proposed that equal volumes of gases have the same number of particles if measured at the same temperature and pressure. The difficulty of explaining how one volume of oxygen could form two volumes of water without violating the current theory that atoms were indivisible was not resolved until the 1850s when Avogadro's explanation that molecules of gases, such as hydrogen and oxygen, existed as diatomic molecules (molecules with two atoms joined together) was finally accepted. If each oxygen molecule was composed of two oxygen atoms, then it was the molecule and not the atom that split apart to form two volumes of water."
"J. J. Berzelius did a great deal of experimental work in establishing atomic weights and he published his list of the weights of 54 elements along Unlike Dalton's atomic weights, the weights published by Berzelius match quite well the atomic weights used today." 
Mendeleev later came up with periodic classification based on his systematic understanding and analysis of the then known elements and classified them in the periodic table. This later led to coming up with the long form of periodic table which is currently in use.