John Dalton was an English chemist who developed the idea of atomic theory of chemical elements.  Daltons atomic theory of chemical elements assumed that each element had unique atoms associated with and specific to that atom.  This was in opposition to Lavoisiers definition of elements which was that elements are substances that chemists could not break down further into simpler parts.  Daltons idea also differed from the idea of corpuscular theory of matter, which believed that all atoms were the same, and had been a supported theory since the 17th century.  To help support his idea, Dalton worked on defining the relative weights of atoms in chemicals in his work New System of Chemical Philosophy, published in 1808.  His text showed calculations  to determine the relative atomic weights of Lavoisiers different elements based on experimental data pertaining to the relative amounts of different elements in chemical combinations.  Dalton argued that elements would combine in the simplest form possible.  Water was known to be a combination of hydrogen and oxygen, thus Dalton believed water to be a binary compound containing one hydrogen and one oxygen.
Dalton was able to accurately compute the relative quantity of gases in atmospheric air.  He used the specific gravity of azotic (nitrogen), oxygenous, carbonic acid (carbon dioxide), and hydrogenous gases as well as aqueous vapor determined by Lavoisier and Davy to determine the proportional weights of each as a percent of a whole volume of atmospheric air. Dalton determined that atmospheric air contains 75.55% azotic gas, 23.32% oxygenous gas, 1.03% aqueous vapor, and 0.10% carbonic acid gas.