 The microstructuralist can extend this approach from elements to compounds. Compounds are identified principally by their constituent elements. Thus carbon dioxide is that compound of carbon and oxygen with the molar proportion 1:2. In more detail, molecules of carbon dioxide consist of two oxygen atoms and a single carbon atom. The practice of identifying a chemical compound only by its composing elements was the norm in chemistry, until the discovery of isomerism by Friedrich Woehler in 1827. Isomers are distinct compounds that nonetheless share the same constituent elements in the same proportion. Thus fulminic acid and cyanic acid may both be expressed in terms of constituents in the empirical formula CHNO, but their distinct chemical and physical properties identify them as different substances. The explanation of isomerism (and many other facts in organic chemistry), provided first by August Kekulé and Archibald Scott Couper, is the fact that certain compounds exist in molecular form and these molecules have internal structure. Thus molecules of isomers have the same atoms in different spatial arrangements, e.g., CHNO may be arranged as H–O–C≡N (cyanic acid) or as H–C=N–O (fulminic acid). Isomerism means that specifying the chemical composition alone is not sufficient for classification. The microstructure of a compound concerns not just the elemental atoms in its molecules, but also their spatial arrangement.