138 GENERAL PHYSIOLOGY 



the warm-blooded animals, which are known as homothermic. 

 Furthermore, it is found that some of the bacteria are able to 

 grow in a wide temperature range. That is, their minimum 

 temperature is widely separated from their maximum tem- 

 perature, and their optimum is usually a wide zone. These 

 organisms are spoken of as eurythermic. Most of the sapro- 

 phytes belong to this class and are thus distinguished from the 

 class of bacteria which are described as stenothermic, which have 

 a narrow temperature range. Such bacteria are the patho- 

 genic or disease-producing ones. Many of these, on account of 

 their parasitic mode of life have become adapted to a partic- 

 ular temperature, and only slight variations from this interfere 

 with their life processes in a very striking way. Such a germ is 

 the Bact. tuberculosis, which has become adapted to a parasitic 

 mode of life and is dependent upon a very narrow tempera- 

 ture range. The psychrophilic bacteria were found by Foster 

 growing in melting ice. Fischer found them in sea water. 

 Many others have found that they exist in soil and milk and 

 various other materials. They grow quite rapidly even at a 

 temperature only slightly above zero. These psychro- 

 philic bacteria are similar to the red snow plants found in 

 the north, and the diatoms that swarm the Arctic Seas so far 

 as their resistance to cold is concerned. 



Thermal Death Point. The determination of the thermal 

 death point is a matter of so much practical importance that 

 it is worth while to consider it at some length here. It is 

 important because it is desirable to apply as low a temperature 



