_Eub_.--So that your researches confirm those of the French savants and Alexander von Humboldt, that in the respiration of animals which separate air from water, both principles of the atmosphere are absorbed. _Phil_.--I have heard so many and such various opinions on the nature of the function of respiration during my education and since, that I should like to know what is the modern doctrine on this subject. I can hardly refer to better authority than yourself, and I have an additional reason for wishing for some accurate knowledge on this matter, having, as you well know, been the subject of an experiment in relation to it which, but for your kind and active assistance, must have terminated fatally. _The Unknown_.--I shall gladly state what I know, which is very little. In physics and in chemistry, the science of dead matter, we possess many facts and a few principles or laws; but whenever the functions of life are considered, though the facts are numerous, yet there is, as yet, scarcely any approach to general laws, and we must usually end where we begin by confessing our entire ignorance. _Eub_.--I will not allow this ignorance to be entire. Something, undoubtedly, has been gained by the knowledge of the circulation of the blood and its aeration in the lungs--these, if not laws, are at least fundamental principles. _The Unknown_.--I speak only of the functions in their connection with life. We are still ignorant of the source of animal heat, though half a century ago the chemists thought they had proved it was owing to a sort of combustion of the carbon of the blood. _Phil_.--As we return to our inn I hope you will both be so good as give me your views of the nature of this function, so important to all living things; tell me what you _know_, or what you _believe_, or what others _imagine they know_.