48 The Story of the Bacteria 



on, the ice closes around both bubbles and 

 bacteria, and we have formed, to use the 

 language of the geologist, an air and fossil- 

 bearing stratum. Only our bacterial fossils 

 are not dead, and all we have to do in order 

 to find out what forms of life were present 

 in our successive geological periods, limited per- 

 haps only by a night, is to melt a bit of the ice, 

 mix it with our culture gelatin, and in a day 

 or two we shall have a whole garden of grow- 

 ing plants, which we can study at our leisure. 



But many bacteria can survive a tempera- 

 ture much below freezing. Experiments with 

 liquid air, which affords extreme degrees of 

 cold, have shown that exposure to more than 

 190 C. below zero leaves some bacteria quite 

 unharmed. While many bacteria thus sur- 

 vive extreme cold, they do not multiply at 

 low temperatures, so that the use of ice in 

 preserving foods is of great importance. 



On the other hand, there are bacterial 

 species which prefer high temperatures. Thus 

 some forms flourish in the water of hot springs 

 at a temperature of 170 Fahrenheit. These 

 are called thermophylic or "heat lovers." 



