Phosphorus is present especially in the nucleoproteins and nucleins. Just what the intermediate stages are, on whether there are any, so far as the phosphorus is concerned, in the splitting up of nucleic acid by bacterial action is not determined. The phosphorus may occur as phosphoric acid in such decompositions, or when the conditions are anaërobic, as phosphine (PH₃), which burns spontaneously in the air to phosphorus pentoxide (P₂O₅), and water.[13] The hydrogen in proteins appears in the forms above indicated: H₄C, H₃N, H₃P, H₂S, H₂O and as free H. The oxygen as CO₂ and H₂O. In the breaking down of the complex protein molecule even by a single kind of bacterium there is not a perfect descending scale of complexity as might be supposed from the statement that there result proteoses, peptones, polypeptids, amino-acids. These substances do result, but at the time of their formation simpler ones are formed also, even CO₂, NH₃ and H₂S. It appears that the entire molecule is shattered in such a way that less complex proteins are formed from the major part, while a minor portion breaks up completely to the simplest combinations possible. A more complete knowledge of these decompositions will aid in the further unravelling of the structure of proteins. The presence or absence of free oxygen makes a difference in the end-products, as has been indicated. There are bacteria which oxidize the ammonia to nitric acid and the H₂S to sulphuric acid. (See Oxidation, Chapter XI.) Bacteria which directly oxidize phosphorus compounds to phosphoric acid have not been described. It does not seem that such are necessary since this is either split off from nucleic acid or results from the spontaneous oxidation of phosphine when this is formed under anaërobic conditions. Not only are proteins decomposed as above outlined, but also their waste products, that is, the form in which their nitrogen leaves the animal body. This is largely urea in mammals, with much hippuric acid in herbivorous animals and uric acid in birds and reptiles. These substances yield NH₃, CO₂ and H₂O with a variety of organic acids as intermediate products in some cases. The strong odor of ammonia in stables and about manure piles is the everyday evidence of this decomposition. Where the putrefaction of proteins occurs in the soil with moderate amounts of moisture and free access of air a large part of the products is retained in the soil. Thus the ammonia and carbon dioxide in the presence of water form ammonium carbonate; the nitric, sulphuric and phosphoric acids unite with some of the metals which are always present to form salts. Some of the gases do escape and most where the oxygen supply is least, since they are not oxidized.