APM - Analisi Prove Materiali

Nothing is created neither in the operations of art, nor in those of nature, and it can be established as a principle that in every operation there is an equal amount of raw material and after the operation; that the quality and quantity of the principles are equal and that there are only their changes and modifications. The whole art of experience in chemistry is based on this principle; in all of them one is obliged to assume an identical equality or equivalence between the principles of the bodies studied and those obtained from analysis.

A. LAVOISIER
"Traité elementaire de chimie!", 1789