THE VITAL PHENOMENA OF BACTERIA. 69 



tinuous sterilization. The explanation of this 

 process was first given by Tyndall. Many 

 germs resist the first heating, and subsequently 

 develop in the favorable nutrient solution ; but 

 the germs when in this immature condition are 

 less resistant and hence are surely destroyed by 

 a second application of heat. This method is 

 effective even when carried out at temperatures 

 of about 7o-75. Such temperatures are be- 

 low the coagulation point of proteids, and hence 

 blood and other proteid-containing fluids such 

 as milk and beer can be made germ-free by dis- 

 continuous heating, although their proteids 

 would be precipitated by the temperature of 

 boiling water. The use of temperatures from 

 65-68 in such cases is called the method of 

 pasteurization. 



The resistance of bacteria to drying is devel- 

 oped in different degrees, and depends in gen- 

 eral upon the character of the species, the extent 

 of zooglcea formation and upon the presence of 

 spores. Cholera bacteria vary in their resist- 

 ance to drying and often exhibit differences of 

 from a few hours to upwards of eight months. 

 Anthrax spores and other endospores have been 

 found to be still capable of development after 

 remaining in a dry condition for ten years. 

 The toxicity of cultures slowly desiccated 

 in the air decreases more or less rapidly, as 



