THE CULTIVATION OF BACTERIA. 75 



If, by any accident, drops of infectious material should 

 fall upon a surface like the table, they should be covered at 

 once with bichloride of mercury solution i-iooo. A good 

 way is to cover the spot with a piece of blotting-paper wet 

 with the solution, place a bell-jar over it and leave for several 

 hours. If infectious material should reach the hands or 

 clothing, they should be thoroughly soaked in the bichloride 

 solution. When working with pathogenic bacteria it is well 

 to wash the hands in this solution and with soap and water, 

 as a routine procedure, before leaving the laboratory. 



To maintain their vitality bacteria need to be transplanted 

 from one tube to another occasionally; the time varies greatly 

 with different species. Many bacteria grow on culture-media 

 with difficulty at the first inoculation, but having become ac- 

 customed to their artificial surroundings, as it were, they may 

 be propagated easily afterward; this is especially true of the 

 bacillus tuberculosis. 



Some bacteria flourish better on one culture-medium than 

 on another. The bacillus tuberculosis grows best on blood- 

 serum and glycerin-agar; the bacillus of diphtheria grows 

 best on Loffler's blood-serum; the gonococcus on human 

 serum-agar. 



The virulence of most pathogenic bacteria becomes dimin- 

 ished after prolonged cultivation upon media. Sometimes 

 the virulence is lost very quickly, — for example, the strepto- 

 coccus pyogenes and micrococcus lanceolatus of pneumonia. 



Incubators. — Many bacteria flourish best at a temperature 

 about that of the human body, 38° C. Some species will 

 grow only at this temperature. The pathogenic bacteria in 

 particular, for the most part, thrive best at a point near the 

 body temperature. 



The incubator is a box made of copper, having double 

 walls, the space between the two being filled with water. 

 The outer surface is covered with some non-conductor of 



