**3. Amount of a substance as key category for thermodynamic and chemical description of photon absorber**

The concept of "amount of substance" arose from corpuscular knowledge about the structure of matter. Dalton based his hypothesis on the number of atoms, Avogadro counted molecules. The number of substances had not differ fundamentally from the masses until the moment, when the quantum theory and the theory of relativity have shown that the mass of the atoms and molecules of the same substance in different energy states and with various total mass is not proportional to their number. For chemists, this difference of masses is minor, metrologists take it into account. The fundamental difference between the concepts of "amount of substance" and the "mass" was relatively recently justified in Metrology. In 1971 XIV General Conference on weights and measures in its decision confirmed a fundamental difference between the mass and volume of a substance: in the International system of units was entered the seventh basic unit, which is designed to measure the amount of a substance. Its name-mol, marked with the symbol ν.
