Very many chemical changes within living organisms are effected by the two polar processes of oxidation and reduction. We have discussed them repeatedly as hieroglyphs of much that occurs in nature by way of polarity. In accordance with the principle ruling the physical plane of nature, that differences of level tend to disappear, oxidation can occur by itself, whereas reduction requires the expenditure of energy. Let us from this point of view compare the transformation of oxidized into reduced iron, as it takes place inside and outside the realm of life.