 In the 18th and early 19th centuries, chemical analyses of substances consisted in the decomposition of substances into their elemental components. Careful weighing combined with an application of the law of constant proportions allowed chemists to characterize substances in terms of the mass ratios of their constituent elements, which is what chemists mean by the composition of a compound. During this period, Berzelius developed a notation of compositional formulas for these mass ratios where letters stand for elements and subscripts stand for proportions on a scale which facilitates comparison of different substances. Although these proportions reflect the proportion by weight in grams, the simple numbers are a result of reexpressing gravimetric proportions in terms of chemical equivalents. For example, the formulas ‘H2O’ and ‘H2S’ say that there is just as much oxygen in combination with hydrogen in water as there is sulphur in combination with hydrogen in hydrogen sulphide. However, when measured in weight, ‘H2O’ corresponds to combining proportions of 8 grams of oxygen to 1 gram of hydrogen and ‘H2S’ corresponds to 16 grams of sulphur to 1 of hydrogen in weight.