17. The modern discoveries in the composition of air, have tended greatly to elucidate the use of this fluid in the lungs, and its action on the blood in respiration. Dr. Priestly first determined it to be what he terms a _phlogistic process_, i. e. a process by which the parts of the blood no longer proper to be retained among the rest, or at least some of them, are carried off. That _something_ is carried off either from the lungs themselves, or from the blood circulating through them, is evident; for the air which is taken into the lungs in a dry state, comes out of them extremely moist, and loaded with vapour. An essential change is also made in the nature of the air itself; for it now assumes in a great measure the nature of what has been called fixed air, or the fume of charcoal, or fermenting liquor, and thus becomes unfit for being breathed a second time. This change is made by the addition of some terrestrial substance to the pure atmosphere, which the latter volatilizes and carries along with it.[48] [48] The discoveries of modern chemists have determined that the aerial fluid, termed _fixed air_ or _carbonic acid_, and which is nearly the same with the vapour arising from fermenting liquor, and is also largely contained in the fume of burning charcoal, is not a simple but a compound substance; one part consisting of the pure part of the atmosphere, or _oxygene_, the other of real charcoal. The proportions, according to M. Chaptal, are 12,0288 parts of charcoal to 56,687 of oxygen.