In the same volume of the _Memoirs_ of the Academy for 1778, another of Lavoisier’s papers--“On Combustion in General”--is to be found. In this he showed that oxygen gas is the only substance which supports combustion; that during the burning of combustible substances in air a portion of the oxygen disappears, and converts the burning substance into one of two kinds of compounds: either an acid, such as sulphuric acid from sulphur, phosphoric acid from phosphorus, or carbonic acid from carbon (for in those days the term “acid” was applied to what we now term an anhydride); or in the case of metals a calx, or compound of oxygen with the metal. The processes are analogous, but differ in the rate at which they take place; for the calcination of metals is a much slower operation than the combustion of sulphur or phosphorus. It is the rapidity of the action which leads to actual inflammation. He next examined and attacked the theory of phlogiston, and maintained that the existence of phlogiston is purely hypothetical, and quite unnecessary for the explanation of the phenomena. But his papers were received with doubt. The change demanded was too great; the trammels of custom were too firmly bound. He gained no converts. Until the true nature of hydrogen had been explained, the attack on the phlogistic theory could not be said to be complete. This combination of hydrogen and oxygen to form water was first proved by Cavendish. And as soon as Sir Charles Blagden, in 1783, had communicated Cavendish’s results to Lavoisier, the latter at once saw their bearing on the new theory which he was endeavouring to uphold, and perceived how they would give a final blow to the adherents of the theory of phlogiston. For it had been frequently adduced as an objection to his new views, that they were incapable of explaining why hydrogen should be evolved during the solution of metals in acids, or why it should be absorbed during the reduction of calces to the metallic state. Lavoisier at once repeated Cavendish’s experiments on a large scale, and was assisted on that occasion by Laplace, Sir Charles Blagden also being present. A considerable quantity of water was produced, and the volumes of the combining gases were found to be 1 of oxygen to 1·91 of hydrogen. Shortly after, in conjunction with Meunier, he performed the converse operation, in decomposing steam by passing it over iron wire heated to redness in a porcelain tube. The iron withdrew the oxygen from the water, while the hydrogen passed on and was collected in the gasholder.