In his fifth Bakerian lecture, “On some of the Combinations of Oxymuriatic Gas and Oxygene, and on the chemical Relations of these Principles to inflammable Bodies,” read before the Royal Society on November 15th, 1810, he still further developed his ideas respecting the nature of chlorine. Gay Lussac and Thenard, who had convinced themselves that potassium and sodium are not hydrates of potash and soda, had made known the fact that potassium can combine with oxygen in more than one proportion; and Davy had confirmed their conclusion, seeing in it a further proof of his views concerning the constitution of the hyper-oxymuriate of potash. He then studied the behaviour of a large number of the metals and their oxides with chlorine, making in many cases quantitative determinations, from which very fair approximations to the combining proportions or atomic weights of the substances may be deduced. Thus, he says “the number representing the proportion in which mercury combines must be about 200,” and that “the quantity of chlorine in corrosive sublimate is exactly double that in calomel, and that the orange oxide contains twice as much oxygen as the black, the mercury being considered the same in all.” The atomic weight of silver deducible from the amount of chlorine taken up by that metal during its conversion into horn-silver is almost exactly the value obtained by the most rigorous analyses of modern times. It is, however, noteworthy that in this paper Davy is brought into sharp conflict with Dalton, and there is a characteristic exhibition of temper in the way in which he protests against the manner in which Dalton had sought to use certain of his numerical estimations in deducing the weights of atoms. The comparative merits of Mr. Higgins and John Dalton as the real authors of the explanation of the laws of chemical combination have now been fully and finally assessed, but it was wholly unnecessary for the purpose of Davy’s contention to underrate the originality of the Manchester chemist. Dalton was no doubt wrong in the assumption that 47 represented the weight of the atom of nitrogen, and Davy was right in pointing out the invalidity of the basis on which this assumption rested, and in his statement that 13·4 more nearly represented the smallest proportion in which nitrogen is known to combine. Davy says:-- “I shall enter no further at present into an examination of the opinions, results, and conclusions of my learned friend; I am however obliged to dissent from most of them, and to protest against the interpretations that he has been pleased to make of my experiments; and I trust to his judgment and candour for a correction of his views.