_Eub_.--No one can be more convinced than I am of the very limited extent of our knowledge in chemical physiology, and when I say that, having been a disciple and friend of Dr. Black, I am still disposed to prefer his ancient view to your new one, I wish merely to induce you to pause and to hear my reasons; they may appear insufficient to you, but I am anxious to explain them. First, then, in all known chemical changes in which oxygen gas is absorbed and carbonic acid gas formed, heat is produced. I could mention a thousand instances, from the combustion of wood or spirits of wine to the fermentation of fruit or the putrefaction of animal matter. This general fact, which may be almost called a law, is in favour of the view of Dr. Black. Another circumstance in favour of it is, that those animals which possess the highest temperature consume the greatest quantity of air, and, under different circumstances of action and repose, the heat is in great measure proportional to the quantity of oxygen consumed. Then those animals which absorb the smallest quantity of air are cold-blooded. Another argument in favour of Dr. Black's opinion is the change of colour of blood from black to red, which seems to show that it loses carbon. _The Unknown_.--With the highest respect for the memory of Dr. Black, and for the opinion of his disciple, I shall answer the arguments I have just heard. I will not allow any facts or laws from the action of dead matter to apply to living structures; the blood is a living fluid, and of this we are sure that it does not burn in respiration. The terms warmth and cold, as applied to the blood of animals, are improper in the sense in which they have been just used; all animals are, in fact, warm-blooded, and the degrees of their temperature are fitted to the circumstances under which they live, and those animals, the life of which is most active, possess most heat, which may be the result of general actions, and not a particular effect of respiration. Besides, a distinguished physiologist has rendered it probable that the animal heat depends more upon the functions of the nerves than upon any result of respiration. The argument derived from change of colour is perfectly delusive; it would not follow if carbon were liberated from the blood that it must necessarily become brighter; sulphur combining with charcoal becomes a clear fluid, and a black oxide of copper becomes red in uniting with a substance which abounds in carbon. No change in sensible qualities can ever indicate with precision the nature of chemical change. I shall resume my view, which I cannot be said to have fully developed. When I stated that carbonic acid was formed in the venous blood in the processes of life, I meant merely to say that this blood, in consequence of certain changes, became capable of giving off carbon and oxygen in union with each other, for the moment inorganic matter enters into the composition of living organs it obeys new laws.