Lavoisier was one of the few chemists of his time to fully appreciate the importance of careful measurements of reactants and products. In order to make careful measurements Lavoisier invented a balance which was accurate to about .0005 grams or about 1/100 of a drop of water.

By precise measurements of the reactant and the product using his newly invented balance Lavoisier showed that the mass of reactants had to equal the mass of the original products thus proving the Law of Conservation of Mass.

Lavoisier wrote in 1785, "Nothing is created, either in the operation of art or in those of nature, and it may be considered as a general principle that in every operation there exists an equal quantity of matter before and after the operation; that the quantity of the constituents is the same, and that what happens is only changes, modifications. It is on this principle that is founded all the art of performing chemical experiments; in all such must be assumed a true equality or equation between constituents of the substances examined, and those resulting from their analysis."