PHYSIOLOGICAL ACTIVITIES. The physiological activities of motion, reproduction and metabolism within the cell have been discussed in previous chapters. The objects in view in the discussion of the “physiological activities” (sometimes spoken of as “biochemical” activities) of bacteria in this and subsequent chapters are to familiarize the student to some extent with the great range of chemical changes brought about by these minute organisms, to show their usefulness, even their necessity, and to impress the fact that it is chiefly by a careful study of these “activities” that individual kinds of bacteria are identified. It should always be borne in mind that the bacteria, in bringing about these changes which are so characteristic in many instances, are simply engaged in their own life struggle, in securing the elements which they need for growth, in liberating energy for vital processes, or occasionally in providing conditions which favor their own development and hinder that of their competitors. FERMENTATION OF CARBOHYDRATES. By this is meant the changes which different carbohydrates undergo when subjected to bacterial action.[11] These changes are marked chiefly by the production of gas or acid. The former is called “gaseous fermentation” the latter “acid fermentation.” The gases commonly produced are carbon dioxide (CO₂) hydrogen and marsh gas (CH₄). Other gases of the paraffin series may also be formed as ethane (C₂H₆), acetylene (C₂H₂), etc. CO₂ and H are the ones usually formed from sugars by the few gas-forming bacteria which produce disease, though even here some CH₄ is present. The common _Bacterium coli_ forms all three, though the CH₄ is in smallest quantity. [Illustration: FIG. 60.--Cylinder to show the formation of gas by bacteria. The gauge shows 265 pounds. It went beyond 500 pounds.] [Illustration: FIG. 61.--A burning natural gas well at night. From a photograph colored.] In the fermentation of the polysaccharids--starch and especially cellulose and woody material--large amounts of CH₄ occur, particularly when the changes are due to anaërobic bacteria. This phenomenon may be readily observed in sluggish streams, ponds and swamps where vegetable matter accumulates on the bottom. The bubbles of gas which arise when the mass is disturbed explode if a lighted match is applied to them.