136 GENERAL PHYSIOLOGY 



host (37 to 40 C.)- The third class are those which have a 

 high optimum temperature, frequently ranging from 50 to 

 70 C. and are known as the thermophilic bacteria. 



When the temperature goes above the maximum or falls be- 

 low the minimum, the bacteria go into a state which resembles 

 that of death and is spoken of as rigor, either heat rigor or cold 

 rigor. Unless the state of rigor continues too long the bacteria 

 are capable of recovering when brought into normal tem- 

 peratures. There is no definite change which takes place as 

 the result of exposure to extremes of temperature, but the 

 change is cumulative and may finally be destructive. The 

 zone of heat rigor is much narrower than that of cold rigor, 

 and hence the thermal death point more closely follows the 

 maximum than does the cold death point follow the minimum. 

 It is a popular notion that cold is frequently and rapidly de- 

 structive to bacterial life, but this idea is not borne out by ex- 

 periment, for it is found that when bacteria are subjected to 

 cold, either that of ordinary ice or much lower temperatures, 

 they can persist for very long periods of time. A number of 

 years ago Prudden froze B. prodigiosus and found that they 

 were living at the end of thirty-seven days in much reduced 

 numbers, but only on the fifty-first day had they entirely dis- 

 appeared. Sedgwick, likewise, a number of years ago, found 

 that the B. typhosus when frozen in ice persisted in consider- 

 able numbers even for a period of 103 days. From this it is 

 clear that ordinary cold cannot be depended upon to kill 

 bacteria, even those of the most delicate sort. When bacteria 



