Chaptal calculates it at _three hundred and fifty-three_, and La Metherie at _three hundred and sixty_, cubic inches in an hour. Allowing these experiments to be just, the next question is, what part of the air is absorbed. Lavoisier says, that it is the oxygenous base, or the same with that which is absorbed in the calcination of mercury. But how comes he to know this? Surely not in the same way that he determines the absorption of it by mercury. In the latter case he takes a certain quantity of mercury, includes it in another known quantity of oxygen air, and heats the metal by means of a burning-glass or otherwise: the consequence is, that the air is absorbed, the mercury loses its fluidity, and is increased in weight. The metal gains the _whole_ weight of the air absorbed; and, by another process, _all_ the air and _all_ the metal, or very nearly so, may be obtained in their original form. This experiment is so decisive, that nothing can be said against it with any shadow of reason; but who _hath_ made, or who _can_ make, similar experiments with the blood of a living man? Such experiments indeed might be made, if _insensible perspiration_ did not stand in our way. Common atmospherical air is about _eight hundred_ times lighter than water. A cubic inch of distilled water, according to Dr. Kirwan, weighs _two hundred and fifty-three grains and a quarter_. Oxygen air is somewhat lighter than common air: we shall therefore suppose that six hundred inches of it are equal to an inch of water. If then the blood absorb three hundred and sixty inches of air in one hour, it will in twenty-four hours have absorbed eight thousand six hundred and forty inches, equal in weight to fourteen inches of water and two fifths, which according to Dr. Kirwan’s estimate is between seven and eight ounces. But the quantity of matter insensibly perspired in that time is so much greater, that no calculation can be made. Here is one mode of determining the quantity of oxygen inspired totally impracticable in the human body, though quite easy and practicable in the case of mercury. The other mode of determining it by the expulsion of oxygen from the blood is equally impracticable. Dr. Girtanner indeed has expelled oxygen from flesh; but we know not in what proportion, nor can we determine whence it came. With regard to this last, indeed, there are two sources allowed by Drs. Beddoes and Girtanner themselves; viz. the absorption of oxygen by the lungs, and the quantity taken in with the aliment. A third source was also manifest from Dr. Girtanner’s experiments; viz. absorption from the atmosphere; for, by exposure to the atmosphere, flesh, which had once parted with its oxygen, became again impregnated with it. In this case therefore we must acknowledge that the uncertainty of the absorption by the lungs must be extremely great. A certain quantity of oxygen is undoubtedly thrown out in fixed air. How are we to determine this quantity? Certainly not by the first