The knowledge of one elastic fluid, entirely different in its properties from air, very naturally suggested the probability of the existence of others. The processes of fermentation which had been observed by the ancient chemists, and those by which Hales had disengaged and collected elastic substances, were now regarded under a novel point of view; and the consequence was, that a number of new bodies, possessed of very extraordinary properties, were discovered. Mr. Cavendish, about the year 1765, invented an apparatus for examining elastic fluids confined by water, which has since been called the _hydro-pneumatic_ apparatus. He discovered inflammable air, and described its properties; he ascertained the relative weights of fixed air, inflammable air, and common air, and made a number of beautiful and accurate experiments on the properties of these elastic substances. Dr. Priestley, in 1771, entered the same path of enquiry; and principally by repeating the processes of Hales, added a number of most important facts to this department of chemical philosophy. He discovered nitrous air, nitrous oxide, and dephlogisticated air, (oxygen) and by substituting mercury for water in the pneumatic apparatus, ascertained the existence of several aëriform bodies which are rapidly absorbable by water; such as muriatic acid gas, sulphurous acid gas, and ammonia. Scheele, independently of Priestley, also discovered several of the aëriform bodies; he ascertained likewise the composition of the atmosphere; he brought to light fluoric acid, prussic acid, and the substance which he termed _dephlogisticated marine acid_, the oxy-muriatic acid of the French school, and the chlorine of Davy. Sir Humphry Davy, in the preface to his Chemical Philosophy, observes that Black, Cavendish, Priestley, and Scheele, were undoubtedly the greatest chemical discoverers of the eighteenth century; and that their merits are distinct, peculiar, and of the most exalted kind. He thus defines them: "BLACK made a smaller number of original experiments than either of the other philosophers; but being the first labourer in this new department of the science, he had greater difficulties to overcome. His methods are distinguished for their simplicity; his reasonings are admirable for their precision; and his modest, clear, and unaffected manner is well calculated to impress upon the mind a conviction of the accuracy of his processes, and the truth and candour of his researches.