Abstract
This work performs a historical reconstruction on the fundamentals of the equilibrium law. After a brief presentation of the evolving ideas that made possible the first mathematical equation of what we know nowadays as the equilibrium constant, the theoretical principles that initially grounded the quantitative study of chemical equilibrium are analysed. It is discussed how the use of mathematical language helped in the construction of the new chemical knowledge. The first interpretations accounting for how the equilibrium situation was eventually achieved are also studied.