94 SCIENCE AND PRACTICE OF DAIRYING. 



power of growth in another way, viz., by shedding seed-like bodies, 

 the so-called spores, which, however, do not multiply as such. During 

 this process, as a rule, there can be seen in the inside of the vessels 

 themselves, brightly glittering bodies, chiefly pear-shaped, which sub- 

 sequently develop into spores. While the vegetative cells are easily 

 killed, the spores exhibit a high degree of resistance to unfavourable 

 external conditions. The spores or the lasting cells, or lasting 

 spores, as they are named, are cells which possess a thin but very 

 compact membrane. Under favourable conditions they germinate 

 and grow into a new and much larger vegetative form of fission 

 fungi. 



The life of bacteria is to a great extent dependent on temperature. 

 With reference to this, every bacterium has a maximum and minimum, 

 even an optimum degree of temperature at which it flourishes, and 

 further, a point below or above which it dies. With reference to 

 the low death point, it may be remarked that the influence of cold, 

 especially repeated freezing and repeated thawing, according to 

 late researches, is able to destroy many kinds of bacteria. The 

 temperature above which death ensues lies, for the vegetative cells 

 of the majority of bacteria, between 50 and 60 C., while their 

 spores are able to withstand a much higher temperature. Most 

 spores remain capable of germination even after being heated for a 

 short time in liquids at 100 C., and many resist for a comparatively 

 short time even a dry heat of 130 to 150 C. 



These facts, which have been discovered by careful experiments 

 under reliable conditions, possess the greatest practical importance. 

 They teach that vegetative cells of almost all kinds of bacteria 

 present in liquids are certain to be destroyed by heating for a 

 comparatively long time (about two hours) at a temperature of 60 

 to 70 C., and that a liquid may be rendered perfectly sterile, i.e. 

 free from resistant spores, if heated at 120 to 130 C., for a similar 

 period. 



In addition to temperature, the life of the lower organisms is still 

 further influenced by the reaction and by the concentration, that is, 

 the percentage of water of the nourishing liquid or the nutrient 

 soil. Further, it is affected by the presence of bodies which exert 

 a deleterious action on the cells, by the free access or otherwise of 

 the oxygen of the atmosphere to the cells, and finally by electricity 

 and by light. The ferments proper prefer a slightly acid reaction 

 in their nutrient liquid or nutrient soil. The fission fungi, on the 



