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8 a hundred years. A theory is only useful when it explains the truths of science and helps the advancement of knowledge. Many theories have disappeared from the realms of science, and many are undergoing modification even at the present day.

About eleven years (1785) after the succession of Louis XVI. to the throne of his grandfather, the phlogistic doctrine was completely overthrown—and this was entirely due to the researches, philosophical deductions, and writings of the great Frenchman.

Lavoisier gave the present accepted definition of an "element," although it may be remarked en passant that the researches of Lockyer, Ramsay, and others in the present day have somewhat modified our views concerning the nature of the chemical elements.

About the years 1785-87 Lavoisier, aided by De Morveau (a convert to the nouvelle chimie), Berthollet, and De Fourcroy, formulated a new system of chemical nomenclature which was greatly needed for the further development of the Lavoisierian chemistry: elements, compounds, oxides (peroxides and protoxides), acids, salts, etc., were defined, thus forming the basis of the chemistry of to-day. Lavoisier was the general who marshalled the isolated facts and co-ordinated them into one harmonious and stupendous whole. The work of the four French chemists and the teaching of Lavoisier were, by the close of the eighteenth century, universally accepted by chemists.