CONDITIONS OF BACTERIAL LIFE 25 



make direct use oi the atmospheric nitrogen. Certain 

 inorganic salts, sulphates, phosphates, and sodium 

 chloride, also seem to be necessary for normal develop- 

 ment. These nutrient substances must be presented 

 to the bacteria in association with water, for without 

 water bacterial activity ceases, though in the dry state 

 many forms, and especially their spores, may retain their 

 vitality for a considerable time ; absolute desiccation, 

 however, is rapidly fatal to many. 



Temperature is also an important factor. Though the 

 growth of many species occurs through a wide range, 

 there is for almost all an optimum at which growth is best, 

 and of a range not exceeding 5 or 10. Growth usually 

 ceases below 10 C., but cold does not destroy bacterial 

 life : after exposure to the intense cold produced by the 

 evaporation of liquid oxygen ( 170 C.) for weeks, or of 

 liquid hydrogen ( 252 C.) for ten hours, bacteria and 

 their spores will grow and germinate, and their chromo- 

 genic and pathogenic properties seem to be unaltered. 1 

 On the other hand, bacterial growth usually ceases when 

 the temperature exceeds 40 C. or thereabouts, and most 

 bacteria without spores are destroyed within half an hour 

 by a temperature of 65 C. The spores are far more 

 resistant ; some may even be boiled for a short time 

 without losing their vitality, but prolonged boiling is fatal 

 to both bacteria and their spores. There is, however, 

 a group of so-called thermophilic bacteria, which thrive 

 best at a temperature of 60 to 70 C. They occur in the 

 soil and in water, and are probably of considerable import- 

 ance in the natural fermentations accompanied by the 

 evolution of heat, such as are met with in manure heaps 

 the heating of hay, and firing of moist cotton. 2 



1 Macfadyen and Rowland, Proc. Roy. Soc. Land., 1900. 



2 Macfadyen and Blaxall, Journ. of Path, and Bact., November, 1894, 

 and Trans. Jenner Inst. of Prev. Med., vol. ii, 1899, p. 162. 



