§ =72.= The phlogiston theory was mainly due to Georg Ernst Stahl (1660-1734). Becher (1635-1682) had attempted to revive the once universally accepted sulphur-mercury-salt theory of the alchemists in a somewhat modified form, by the assumption that all substances consist of three earths--the combustible, mercurial, and vitreous; and herein is to be found the germ of Stahl’s phlogistic theory. According to Stahl, all combustible bodies (including those metals that change on heating) contain _phlogiston_, the principle of combustion, which escapes in the form of flame when such substances are burned. According to this theory, therefore, the metals are compounds, since they consist of a metallic calx (what we now call the “oxide” of the metal) combined with phlogiston; and, further, to obtain the metal from the calx it is only necessary to act upon it with some substance rich in phlogiston. Now, coal and charcoal are both almost completely combustible, leaving very little residue; hence, according to this theory, they must consist very largely of phlogiston; and, as a matter of fact, metals can be obtained by heating their calces with either of these substances. Many other facts of a like nature were explicable in terms of the phlogiston theory, and it became exceedingly popular. Chemists at this time did not pay much attention to the balance; it was observed, however, that metals increased in weight on calcination, but this was “explained” on the assumption that phlogiston possessed negative weight. Antoine Lavoisier (1743-1794), utilising Priestley’s discovery of oxygen (called “dephlogisticated air” by its discoverer) and studying the weight relations accompanying combustion, demonstrated the non-validity of the phlogistic theory[89] and proved combustion to be the combination of the substance burnt with a certain constituent of the air, the oxygen. By this time Alchemy was to all intents and purposes defunct, Boerhave (1668-1738) was the last eminent chemist to give any support to its doctrines, and the new chemistry of Lavoisier gave it a final death-blow. We now enter upon the Age of Modern Chemistry, but we shall deal in this chapter with the history of chemical theory only so far as is necessary in pursuance of our primary object, and hence our account will be very far from complete. [89] It should be noted, however, that if by the term “phlogiston” we were to understand energy and not some form of matter, most of the statements of the phlogistics would be true so far as they go. Boyle and the Definition of an Element.