Like almost all others began with metaphysical discussions. The theory has advanced but the practical science is still in its infancy and the modern statesman is constantly short of facts on which he can base his speculations.
One part is absorbed in the blood to compensate for the losses that are constantly taking place through respiration and perspiration, and all the material that Nature cannot use is rejected in the form of excrement.
A man cannot live more than 24 hours unless he has at least three cubic meters of air that is being constantly replaced.