The protein-splitting reactions afford valuable tests in aiding in the recognition of bacteria. In the study of pathogenic bacteria the coagulation and digestion of milk, the digestion or liquefaction of blood serum, the liquefaction of gelatin and the production of indol and H₂S are those usually tested for. In dairy bacteriology the coagulation of milk and the digestion of the casein are common phenomena. Most bacteria which liquefy gelatin also digest blood serum and coagulate and digest milk, though there are exceptions. In soil bacteriology the whole range of protein changes is of the greatest importance. [Illustration: +----<----------------------+ | | | | v | _Nuclein of | animal cells_ | | | | | v | | | _Decomposition | bacteria_ | | \ | | \ ^ v \ | _Unknown _PH₃ | P oxidizes | compounds_ spontaneously | | to_ | | / | v / | _Phosphoric | acid_ | | | | | v | _Phosphates | in the | soil_ | | ^ | | v | _Green | plants_ | | | | | v | _Nuclein of | plant | cells_ | | | | | v | _Animals_----->----------------+ FIG. 68.--Diagram to illustrate the circulation of phosphorus through the agency of bacteria.] [Illustration: