This forms a tolerable approximation to the truth. It is now known, that the quantity of oxygen in the oxide of iron formed by the combustion of iron in oxygen gas is not quite uniform in its composition; sometimes it is a compound of Iron 3½ Oxygen 1⅓ While at other times it consists very nearly of Iron 3·5 Oxygen 1 and probably it may exist in all the intermediate proportions between these two extremes. The last of these compounds constitutes what is now known by the name of _protoxide_, or _black oxide of iron_. The first is the composition of the ore of iron so abundant, which is distinguished by the name of _magnetic iron ore_. Lavoisier was aware that iron combines with more oxygen than exists in the protoxide; indeed, his analysis of peroxide of iron forms a tolerable approximation to the truth; but there is no reason for believing that he was aware that iron is capable of forming only two oxides, and that all intermediate degrees of oxidation are impossible. This was first demonstrated by Proust. I think it unnecessary to enter into any details respecting two papers of Lavoisier, that made their appearance in the Memoirs of the Academy, for 1783, as they add very little to what he had already done. The first of these describes the experiments which he made to determine the quantity of oxygen which unites with sulphur and phosphorus when they are burnt: it contains no fact which he had not stated in his former papers, unless we are to consider his remark, that the heat given out during the burning of these bodies has no sensible weight, as new. The other paper is "On Phlogiston;" it is very elaborate, but contains nothing which had not been already advanced in his preceding memoirs. Chemists were so wedded to the phlogistic theory, their prejudices were so strong, and their understandings so fortified against every thing that was likely to change their opinions, that Lavoisier found it necessary to lay the same facts before them again and again, and to place them in every point of view. In this paper he gives a statement of his own theory of combustion, which he had previously done in several preceding papers. He examines the phlogistic theory of Stahl at great length, and refutes it.