It will be sufficient merely to enumerate collectively the various gases mentioned in preceding paragraphs and to state that those commonly observed in the study of pathogenic bacteria are the first six mentioned. Most of them come in in dairy work either in the study of bacteria causing milk and cheese “failures” or as affecting the flavors of butter or cheese. In the study of soil organisms, any or all of them are liable to be of importance. The gases are: CO₂, H, CH₄, N, NH₃, H₂S, gaseous mercaptans, gaseous ptomaines, volatile fatty acids, ethereal salts or esters and others, both of pleasant and of foul odor, but of unknown composition. PRODUCTION OF ESTERS. The production of esters, as mentioned in Chapters IX and X, of various alcohols and aldehydes are activities which are sometimes of value in the study of bacteria, but need not be further discussed. PRODUCTION OF “AROMATIC” COMPOUNDS. These have been mentioned in discussing the putrefaction of proteins, as indol, skatol, phenol and various cresols. Of these only the first is ordinarily tested for in the study of bacteria, though others of the group become of value in certain special cases. [Illustration: FIG. 72.--Culture of phosphorescent bacteria in an Ehrlenmeyer flask photographed by their own light. Time of exposure twelve hours. (Molisch, from Lafar.)] PHOSPHORESCENCE OR PHOTOGENESIS. This is a most interesting phenomenon associated with the growth of some bacteria. The “fox fire” frequently seen on decaying wood which is covered with a slimy deposit is most commonly due to bacteria, though also to other fungi. Phosphorescent bacteria are very common in sea water, hence they are frequently found on various sea foods, especially when these are allowed to decompose, such as fish, oysters, clams, etc. The light is due to the conversion of the energy of unknown easily oxidizable compounds directly into _visible_ radiant energy through oxidation without appreciable quantities of heat. The light produced may be sufficient to tell the time on a watch in absolute darkness, and also to photograph the growths with their own light, but only after several hours’ exposure (Fig. 72). None of the phosphorescent bacteria so far discovered produce disease in the higher animals or man. PRODUCTION OF PIGMENT OR CHROMOGENESIS. One of the most striking results of bacterial activity is this phenomenon. The particular color which results may be almost any one throughout the range of the spectrum, though shades of yellow and of red are of more frequent occurrence.