the cholera spirillum is destroyed by ten minutes' exposure to a temperature of 52° C.; the typhoid bacillus by 56°; the micrococcus of pneumonia by 52°; the streptococcus of erysipelas (S. pyogenes) by 54°; etc. According to Loeffler, the bacillus of glanders is destroyed in ten minutes by a temperature of 55° C.; the bacillus of diphtheria by 60°. The experiments of Yersin show that the tubercle bacillus does not survive exposure for ten minutes to a temperature of 70° C. The practical value of such knowledge is apparent. Articles of clothing infected with any of the pathogenic bacteria mentioned would be speedily disinfected by immersion in water heated to 70° C. or above, and water or milk recently heated to the same temperature would evidently be without danger so far as infection by these "disease germs" is concerned. The recommendation of sanitarians that water or milk or food suspected of being contaminated by pathogenic bacteria should be exposed to a boiling temperature before it is used is based upon the experimental data referred to; and the knowledge that organic liquids can be sterilized by heat constitutes the foundation upon which the bacteriology of the present day has been established. To obtain reliable information with reference to the biological characters of any particular micro-organism it is necessary to experiment with pure cultures, and this requires a sterile culture medium.
It is hardly necessary to call attention to the fact that an immense industry in the preservation of food products depends upon the sterilization of these products by heat, and their preservation in hermetically sealed receptacles.
When Pasteur demonstrated the fact that sterile organic liquids, when protected by a sterilized cotton air-filter, can be kept indefinitely without undergoing any putrefactive or fermentative change, he also proved that such changes are due to the presence of micro-organisms; and, extending his investigations, he found that certain definite kinds of change are due to particular species of low organisms. Thus the alcoholic fermentation of a saccharine liquid was found to be due to a torula (Torula cerevisiæ), the acetic fermentation of an alcoholic liquid to a bacterial ferment (Pasteur's Mycoderma aceti), etc. Subsequent researches show that alcoholic fermentation may be induced by several species of torula, and even by certain bacteria; while the number of bacterial ferments now known to science is very considerable and is constantly being added to. Among the most important of these we may mention the Bacillus acidi lactici, which is the usual cause of the acid fermentation of milk; the various anaërobic bacilli which give rise to the formation of butyric acid in solutions containing starch, dextrin, sugar, or salts of lactic acid; the bacteria which cause the alkaline fermentation of
