 Although primarily a theoretical and conceptual posit, the law of constant proportions became an important tool for chemical analysis. For example, chemists had come to understand that atmospheric air is composed of both nitrogen and oxygen and is not an element. But was air a genuine compound of these elements or some looser mixture of nitrogen and oxygen, that could vary at different times and in different places? The law of constant proportions gave a criterion for distinguishing compounds from genuine mixtures. If air was a compound, then it would always have the same proportion of nitrogen and oxygen and it should further be distinguishable from other compounds of nitrogen and oxygen such as nitrous oxide. If air was not a genuine compound, then it would be an example of a solution, a homogenous mixture of oxygen and nitrogen that could vary in proportions.