Portrait of French chemist Antoine Laurent Lavoisier (1743-1794). Lavoisier was one of the first to recognise the importance of accurate measurement in the analysis of chemical reactions. By careful weighing he showed that air is taken in by substances during combustion &, more generally, that any mass gained by one reactant is balanced by a loss elsewhere; the law of conservation of mass. He was the first to realise the nature of the combustion-supporting gas discovered by Priestly & named it oxygen. In 1787 Lavoisier published a book which revolutionised chemical nomenclature by assigning every substance a name based on the elements of which it was composed.