The three physiological activities already discussed explain how bacteria break down the chief complex, energy-rich substances --carbohydrates, fats and proteins which constitute the bulk of the organic material in the bodies of plants and animals, as well as the waste products of the latter--into energy-free compounds like carbon dioxide, water, ammonia, nitric, sulphuric and phosphoric acids--mineralize them, as is frequently said. By so doing the bacteria act as the great scavengers of nature removing the dead animal and vegetable matter of all kinds which but for this action would accumulate to such an extent that all life, both on land and in the water, must cease. It is further to be noted that not only is all this dead organic matter removed; but it is converted into forms which are again available for plant growth. Carbon dioxide forms the source of the carbon in all green plants, hence in all animals; the sulphates and phosphates are likewise taken up by green plants and built up again into protein compounds; the ammonia is not directly available to green plants to any large extent but is converted by the nitrifying bacteria (Chapter XI) into nitrates which is the form in which nitrogen is assimilated by these higher types. Even the free nitrogen of the air is taken up by several kinds of bacteria, the symbiotic “root-tubercle bacteria” of leguminous and other plants, and some free-living forms, and made available. Hence bacteria are indispensable in nature, especially in keeping up the circulation of nitrogen. They are also of great service in the circulation of carbon, sulphur and phosphorus. Though some few kinds cause disease in man and animals, if it were not for the saprophytic bacteria above outlined, there could be no animals and higher plants to acquire these diseases. CHAPTER XI. PHYSIOLOGICAL ACTIVITIES (CONTINUED). PRODUCTION OF ACIDS. The production of organic acids has been sufficiently discussed in preceding chapters. It should be noted that not only these in great variety are produced by bacteria but that under certain conditions mineral acids, such as nitric, sulphuric and phosphoric may be formed (see Oxidation, p. 114). Acid production is of great value in the identification of bacteria in dairy and soil work and in connection with certain types of pathogenic bacteria. GAS PRODUCTION.