Physiology. 9 



by dry heat, a temperature varying from 260-300 F. is 

 necessary for an hour or more. Germ life may be more 

 rapidly destroyed in super-heated steam under pressure, 

 a temperature of 230-240 F. for fifteen to twenty min- 

 utes being sufficient to kill most species. Many of the 

 milk bacteria, like the sour milk germ, are very easily 

 destroyed by heat, as they do not form spores. Other 

 forms like the hay and potato bacilli are difficult to 

 eradicate on account of the great resistance that their 

 spores offer toward heat. High temperatures are by far 

 the most efficient means that can be used to render any 

 substance germ-free and are most often employed for 

 this purpose. 



2. Cold. While the maximum temperature that the 

 different spore-bearing species can stand has been accu- 

 rately determined in many cases, the minimum tempera- 

 ture has in most instances never been reached. But lit- 

 tle reliance is to be placed upon this method in attempting 

 to free any substance completely from bacterial life, 

 although freezing is able to destroy a large percentage. 

 Even an artificial degree of cold as low as 220 F. for 

 a day has been found to be insufficient to destroy some 

 forms, especially when in the spore stage. Bacterial 

 growth is held in abeyance when a liquid is congealed, 

 but development occurs very soon after it is melted. 



3. Desiccation. Different bacteria behave very differ- 

 ently when subjected to drying. The cholera germ dies in 

 three hours if it is dried, while anthrax retains its viru- 

 lence unimpaired for decades. Tuberculous sputum with- 

 stands drying and is often found to be infectious after 

 the lapse of eight or nine months. Those species that 

 form spores naturally resist desiccation much better than 

 those that do not form these latent structures. 



