Through his development of quantitative experimental methods, the chemist Antoine Lavoisier (1743-1794) implemented a principle that many regard as the cornerstone of modern science: in every operation there is an equal quantity of material before and after the operation. The origin of Lavoisier's methods, however, has remained a missing piece in this remarkable episode of scientific history, perhaps because the talented young scientist himself was not prepared for the journey his discoveries would set before him. In this book, Frederic Lawrence Holmes suggests that Lavoisier gradually came to understand the nature and power of his quantitative method during the year 1773, when he began to carry out a research program on the fixation and release of airs. Drawing upon Lavoisier's surviving laboratory notebooks, Holmes presents an engaging portrait of a scientist whose innovative experiments would make him the leader of one of the great upheavals in the history of science.