roduces 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