the case with any other elastic fluid except those with which oxygen has entered without undergoing combustion; and from the fire it produces in certain processes, and from the manner in which it is separated by positive electricity in the gaseous state from its combinations, it is not easy to avoid the supposition that it contains, besides its ponderable elements, some very subtle matter which is capable of assuming the form of heat and light.  My idea is that the common air inspired enters into the venous blood entire, in a state of dissolution, carrying with it its subtle or ethereal part, which in ordinary cases of chemical change is given off; that it expels from the blood carbonic acid gas and azote; and that in the course of the circulation its ethereal part and its ponderable part undergo changes which belong to laws that cannot be considered as chemical—­the ethereal part probably producing animal heat and other effects, and the ponderable part contributing to form carbonic acid and other products.  The arterial blood is necessary to all the functions of life, and it is no less connected with the irritability of the muscles and the sensibility of the nerves than with the performance of all the secretions.

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,