_The Unknown_.--The powers of the organic system depend upon a continued state of change. The waste of the body produced in muscular action, perspiration, and various secretions, is made up for by the constant supply of nutritive matter to the blood by the absorbents, and by the action of the heart the blood is preserved in perpetual motion through every part of the body. In the lungs, or bronchia, the venous blood is exposed to the influence of air and undergoes a remarkable change, being converted into arterial blood. The obvious chemical alteration of the air is sufficiently simple in this process: a certain quantity of carbon only is added to it, and it receives an addition of heat or vapour; the volumes of elastic fluid inspired and expired (making allowance for change of temperature) are the same, and if ponderable agents only were to be regarded it would appear as if the only use of respiration were to free the blood from a certain quantity of carbonaceous matter. But it is probable that this is only a secondary object, and that the change produced by respiration upon the blood is of a much more important kind. Oxygen, in its elastic state, has properties which are very characteristic: it gives out light by compression, which is not certainly known to be 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.