THE CHEMISTRY OF BACTERIA 43 



required for the growth of different bacteria just as various 

 higher plants require different soils. Most bacteria grow well in 

 beef tea. However, we must prevent the organisms from swim- 

 ming about and thus mixing during growth. This is accom- 

 plished by adding to the medium one per cent of a seaweed, agar- 

 agar. Such a medium is placed in large flasks, in test tubes stop- 

 pered with cotton plugs, or shallow dishes covered with a second 



Fig. 21. — Sterilizers used in bacteriology. 



dish. These are known as petri dishes. The medium is sterilized 

 by heating in steam under pressure to a termperature of I20° C. 

 for fifteen minutes or at a lower temperature for a longer time. 

 Such a medium melts at 98° C. and does not solidify until the 

 temperature drops to 30° C. Hence, such a mixture, while at 

 40'' C. can be seeded with a few bacteria, thoroughly shaken, and 

 poured into sterile petri dishes. When it solidifies the bacteria 

 are anchored in isolated spots. These grow and become visible 



to the naked eye. 



We are thus using in another field a project which has been 

 urged with a good deal of persistency by various individuals m 

 the past for finding out whether there were inhabitants upon 

 Mars and perchance be able to communicate with them. 



