<P> If two elements form more than one compound between them, then the ratios of the masses of the second element which combine with a fixed mass of the first element will be ratios of small whole numbers . </P> <P> For example, Dalton knew that the element carbon forms two oxides by combining with oxygen in different proportions . A fixed mass of carbon, say 100 grams, may react with 133 grams of oxygen to produce one oxide, or with 266 grams of oxygen to produce the other . The ratio of the masses of oxygen that can react with 100 grams of carbon is 266: 133 = 2: 1, a ratio of small whole numbers . Dalton interpreted this result in his atomic theory by proposing (correctly in this case) that the two oxides have one and two oxygen atoms respectively for each carbon atom . In modern notation the first is CO (carbon monoxide) and the second is CO (carbon dioxide). </P> <P> John Dalton first expressed this observation in 1804 . A few years previously, the French chemist Joseph Proust had proposed the law of definite proportions, which expressed that the elements combined to form compounds in certain well - defined proportions, rather than mixing in just any proportion; and Antoine Lavoisier proved the law of conservation of mass, which helped out Dalton . Careful study of the actual numerical values of these proportions led Dalton to propose his law of multiple proportions . This was an important step toward the atomic theory that he would propose later that year, and it laid the basis for chemical formulas for compounds . </P> <P> Another example of the law can be seen by comparing ethane (C H) with propane (C H). The weight of hydrogen which combines with 1 g carbon is 0.252 g in ethane and 0.224 g in propane . The ratio of those weights is 1.125, which can be expressed as the ratio of two small numbers 9: 8 . </P>

Who said different elements combine together to form compounds