The field of chemistry that includes all chemical measurements, such as the measurements of atomic and molecular weights and sizes, gas volumes, vapor densities, deviation from the gas laws, and the structure of molecules. In the long struggle to determine the relative weights of the atoms, scientists relied upon combining ratios, specific heats, and measurements of gas volumes. All such measurements, and the calculations that relate them to each other, constitute the field of stoichiometry. Since measurements are expressed in mathematical terms, stoichiometry can be considered to be the mathematics of general chemistry. Thus, stoichiometry is not part of inorganic, organic, physical, or analytical chemistry, but is an essential part of all of them. Chemistry is an exact science, and it depends upon exact measurements of weights, lengths, and volumes, and on the amounts of energy which are absorbed or evolved in chemical reactions.