As the capacity of one individual is rarely able to bring to perfection different branches of science, it is no wonder that, in the investigation of so complex a subject as the component parts of the blood, Hewson failed, Hunter fell short in the attempt, and Dr. Young, after the interesting discovery of the colouring matter being readily separated from the red globules, made no farther advance. This view of the subject, let me, at the time I ventured upon this enquiry, to engage as my associates, Mr. Bauer, whose microscopical observations have been too long put to the proof, to admit of their being disputed; and Professor Brande, whose chemical researches, and the judgement with which they have been pursued, are fully established. In two former Lectures, assisted by these skilful coadjutors, I have been enabled to prove, that the human blood in the act of coagulation, evolves aeriform matter, so as to pervade the coagulum in every direction; and that such currents, passing through the serum, form permanent tubes, which are immediately afterwards filled with red blood, when the circumstances in which the coagulum is placed, admit of their being so.