The principal object of this Paper is to give an account of a singular combination of chromic acid, and chlorine, which I discovered about a year and a half ago. But as the investigation of this compound led me to a more careful examination of the oxides of chromium, and a more accurate knowledge of their composition than had been hitherto attained, I shall also state the facts which I have ascertained with respect to these bodies. In a Paper inserted in the Annals of Philosophy (1st Series, vol. 16, p. 321), I have shown that the atomic weight of chromic acid is 6·5. Two other compounds of chromium and oxygen being known, namely, the green oxide , and the brown oxide , I was induced, from analogy, to consider the atomic weight of the former to be 4·5, and that of the latter 5·5; and to make the atom of chromium 3·5. These views were rather favoured by some experiments on the chloride and muriate of chromium, which I have stated in my “First Principles of Chemistry," vol. ii. p. 52. But these experiments deviating more than two per cent from the theoretical number, could not be considered as decisive. How far my former notions on this subject were correct, will appear from the following statements. I. Chromium The metallic chromium, which I employed in the following experiments, had been reduced by Mr. Cooper, of London, who possesses great skill in the management of furnaces. He was kind enough to give me a specimen of this rare metal several years ago; and I take this opportunity of thanking him for a present, which has been of considerable service in my investigations.