Under ordinary conditions the carbohydrate is never completely fermented, since the accumulation of the product--acid--stops the reaction. If the acid is neutralized by the addition of an alkali--calcium or magnesium carbonate is best--then the sugar may all be split up. Where such fermentation occurs under natural conditions, the products are further split up, partly by molds and partly by acid-destroying bacteria into simpler acids and eventually to carbon dioxide and water, so that the end-products of the complete fermentation of carbohydrate material in nature are carbon dioxide, hydrogen, marsh gas, and water. In all of these fermentations the bacteria are utilizing the _carbon_ both as building material and for oxidation and the fermentations are incidental to this use. As a rule, the acid-forming bacteria can withstand a higher concentration of acid than the other bacteria that would utilize the same material, and in a short time crowd out their competitors or inhibit their growth, and thus have better conditions for their own existence, though finally their growth is also checked by the acid. SPLITTING OF FATS. The _splitting of fats_ into glycerin and the particular acid or acids involved may be brought about by bacteria. An illustration is the development of rancidity in butter at times and the “strong” odor of animal fats on long keeping and of many kinds of cheese--“limburger”--in this country. Generally speaking, however, fats are not vigorously attacked, as is illustrated by the difficulties due to accumulation of fats in certain types of sewage-disposal works. The chemical change is represented by the equation: Fat. Glycerin. Fatty acid. C₃H₅(C{_n_}H₂{_n_}₋₁O₂)₃ + 3 H₂O = C₃H₅(OH)₃ + 3 (C{_n_}H₂{_n_}O₂). CHAPTER X. PHYSIOLOGICAL ACTIVITIES (CONTINUED). PUTREFACTION OF PROTEINS. The word “_putrefaction_” is now restricted to the action of bacteria on the _complex nitrogen-containing substances_, proteins, and their immediate derivatives. The process is usually accompanied by the development of foul odors. Bacteria make use of proteins chiefly as a source of nitrogen, but also as a source of carbon and other elements. Proteins contain nitrogen, carbon, hydrogen, oxygen, sulphur and frequently phosphorus. Some of the metals--potassium, sodium, calcium, magnesium, iron and manganese and the non-metal chlorine--are nearly always associated with them more or less intimately. Since these bodies are the most complex of natural chemical substances it follows that the breaking up of the molecule to secure a part of the nitrogen gives rise to a great variety of products.