From his atomic theory, John Dalton deduced a hypothesis about the composition of compounds formed by the same elements: Carbon and oxygen form two compounds, namely carbon dioxide and carbon monoxide:
For a C atom there are two O atoms in the carbon dioxide, but there is only one in carbon monoxide. So, for a given arbon atom(the same mass of C!), there must be two times more oxygen atoms (double mass!) in carbon dioxide than in the carbon monoxide. For other compounds formed by the same elements, similar reasoning should be valid, for example: There are three nitrogen and oxygen compounds, namely nitrogen monoxide, dinitrogen monoxide and nitrogen dioxide :
For two N atoms there will be two O atoms in the nitrogen monoxide (take two molecules), one in a dinitrogen monoxide (take one molecule) and four in nitrogen dioxide (take two molecules). So, for the same mass of N, it must be that the masses of O are in the ratio 4: 2: 1!
When two compounds are formed by the same elements, then: The ratio of the masses of the same element of these two compounds which bind to equal masses of the other element must be simple.
To verify the law of Dalton, the compounds in question are decomposed and elements are weighed separately, eg
Ratio of oxygen masses: 2,28 : 1,14 : 0,571 that means: 4 : 2 : 1