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