In the first part of the paper, the author states that he still maintains the opinion expressed in his published work on respiration, viz., that the quantity of carbonic acid formed in respiration is exactly equal to that of the oxygen consumed; and he enters into a detailed critical analysis of the different experiments recorded by authors on this subject, placing, however, his chief reliance upon those of Berthollet, and of Allen and Pepys, in order to justify his opinion. He also holds the view that the exhalation of carbonic acid and consumption of oxygen are not independent processes, but are the immediate result of the direct union of carbonic and oxygen within the air-cells of the lungs, and he expresses doubts as to the accuracy of the experiments of Edwards, which have by many been held to establish the opposite view.