§ =71.= Chemistry as distinct from Alchemy and Iatro-chemistry commenced with Robert Boyle (see plate 15), who first clearly recognised that its aim is neither the transmutation of the metals nor the preparation of medicines, but the observation and generalisation of a certain class of phenomena; who denied the validity of the alchemistic view of the constitution of matter, and enunciated the definition of an element which has since reigned supreme in Chemistry; and who enriched the science with observations of the utmost importance. Boyle, however, was a man whose ideas were in advance of his times, and intervening between the iatro-chemical period and the Age of Modern Chemistry proper came the period of the Phlogistic Theory--a theory which had a certain affinity with the ideas of the alchemists.
§ =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.