Important advances in chemistry are associated with the name of Lavoisier (1743-1794), who introduced into that science a principle which has become axiomatic, and which to-day remains the foundation of all work in the laboratory. To Lavoisier belongs the merit of introducing what is known as the "quantitative method" into chemistry, and thus establishing that science upon the exact--that is to say mathematical--basis, where it now rests and putting exact research in the place of vague reasoning. His principle was that _in all chemical combinations and reactions, the total weight of the various ingredients remains unchanged_; there is (in spite of appearances) neither loss nor gain of actual matter. "The quantity of matter is the same at the end as at the beginning of every operation." It was Lavoisier who finally established the correct theory of combustion; that it consisted in the combination of a special element called oxygen, with other bodies or elements. THE ATOMIC THEORY.--Lavoisier had opened a door to researches which naturally led the way to the establishment of the atomic theory of matter on an experimental, and not merely a theoretical basis. That theory is indeed nothing more than the elaboration of Lavoisier's own principle. John Dalton (1766-1844), a Manchester quaker, published in 1810 his _New System of Chemical Philosophy_, where highly important conclusions are drawn both from Lavoisier's facts and from experimental results of other chemists. Of these, Dalton gave an account and an explanation which has ever since been the soul of all chemical reasoning. This explanation is known as his Atomic theory. The two facts of which Dalton's theory is an explanation are as follows. _First_ (Lavoisier's fact), that the total weight of substances remains always the same, be they combined in ever so many different ways. _Second_, that all substances, be they in large or in small quantities, combine with each other, or separate from each other, in definite and fixed proportions. The theory of Dalton was that these combinations take place between independent particles of matter, which are indestructible and indivisible. These "atoms" of the various elements have definite weights which are responsible for the proportion in which they are found to combine. These facts of proportion in combination, or "chemical affinity," could not be accounted for by the theory which regards matter as "continuous," but only by the opposite theory that it is "discrete" (i.e. divided up into particles).