The situation was made more complex when chemists realized that elemental composition was not in general sufficient to distinguish substances. Dalton was aware that the same elements sometimes give rise to several compounds; there are several oxides of nitrogen, for example. But given the law of constant proportions, these can be distinguished by specifying the combining proportions, which is what is represented by distinct chemical formulas, for example N2O, NO and N2O3 for different oxides of