76 MILK [vi 



cholera, glanders, malaria, and leprosy. The accom- 

 panying illustration shows the form of some of these 

 pathogenic germs (see Fig. 13, p. 77). Their pernicious 

 action is due to poisons, the so-called ptomaines, 

 which are produced within the body. The functions 

 performed by the other class of micro-organisms 

 are highly useful, and it is with them we are 

 mostly concerned when dealing with the bacteria 

 of milk. Among the most important conditions in 

 regulating their development is temperature. For 

 most the range of temperature is generally between 

 15 to 40 C. (59 to 104 Fahr.), about 32 C. (90 

 Fahr.) being the most favourable temperature. 

 Comparatively low temperatures (50 to 60 C. ; 122 

 to 140 Fahr.) are sufficient to kill most bacteria. 

 Similarly, low temperatures exercise a fatal effect. 

 On the other hand, the spores of many bacteria have 

 been found to possess great powers of resistance. 

 Some of them are able to resist the lowest possible 

 obtainable temperatures. Thus Pictet found that 

 the spores of certain bacteria were able to survive in 

 frozen oxygen at a temperature of 213 C. ( 353 

 Fahr.). Some can survive boiling temperature ; while 

 a few have been discovered that are able to resist 

 even considerably higher temperatures, viz. dry heat 

 of 150 C. (302 Fahr.). The important bearing 

 which these facts have on the practice of dairying 

 is obvious. They explain why heating milk to be- 



