Chemical composition is the proportion by weight of each element present in a chemical compound. The law of definite proportions discovered by J. L. Proust, states that pure compounds have a fixed and invariable composition. A few compounds, termed non-stoichiometric, disobey this law: they have lattice vacancies or extra atoms, and the composition varies within a certain range depending on the formation conditions. The law of multiple proportions, discovered by John Dalton, states that if two elements A and B form more than one compound, the various weights of B which combine with a given weight of A are in small whole-number ratios.