Chemistry. In the early years of the eighteenth century Homberg, linking with the ideas of Sylvius, studied the reactions of acids and alkalirs to form salts-work which afterwards led to theories of chemical structure. In 1732 Boerhaave of Leyden published the best chemical treatise of the time and thus helped to co-ordinate existing knowledge.
Most early work on chemistry sprang from a desire to explain the phenomena of flame. When bodies are burnt, it seems that something escapes. This something, for long called sulphur, was given the name of ‘phlogiston’ the principle of fire, by Stahl (1660-1734) , physician to the king of Prussia. Rey and Boyle had shown that when metals were burnt the solid increased in weight; thus, as Venel pointed out, phlogiston must possess a negative weight-a return to Aristotle’s idea of a body essentially light. Chemistry, ignoring the achievements of physics, learned to express itself in terms of phlogiston, which dominated the chemical thought of the latter part of the eighteenth century.
Meanwhile many new substances were discovered. Oxygen had been obtained from saltpeter by borch in 1678 and it was again prepared and collected over water in 1729 by hales who still thought it was air modified by the presenc of some other substance. But about 1755 joseph black of Edinburgh discovered that a new pon-derable gas, distinct fron air, was combined in the alkalies. He described it as ‘fixed air’ ; it was what we now call carbon dioxide or car bonic acid. In 1774 Scheele discovered chlorine. Joseph Priestley(1733-1804) prepared oxygen by heating mercuric oxide, and rediscovered its unique power of supporting combustion and respiration. Cavendish demonstrated the compound nature of water in 1781, thus finally banishing it from the list of elements, though he still called its constituent gases phlogiston and dephlogisticated air. Thus we see men of the eighteenth century collecting chemical