82 MANUAL OF BACTERIOLOGY. 



bacillus tuberculosis grows best on boiled egg, blood-serum 

 and glycerin-agar; the bacillus of diphtheria grows best on 

 Lofflers blood-serum; the gonococcus on human serum-agar. 

 The virulence of most pathogenic bacteria becomes dimin- 

 ished after prolonged cultivation upon artificial media. Some- 

 times the virulence is lost very quickly, — for example, the 

 streptococcus pyogenes and micrococcus lanceolatus of pneu- 

 monia. These organisms are often very strongly patho- 

 genic for experiment animals, when inoculated directly from 

 one animal to another, but frequently lose all pathogenic 

 power, even on one transfer to artificial media. 



Incubators. 



High-temperature Incubator — ^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, and are consequently best 

 studied for many purposes when grown in an incubator, 

 regulated for this temperature. 



The high-temperature incubator used in laboratories con- 

 sists essentially of 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 heat, such as 

 felt, asbestos or linoleum. At one side is a door, which is also 

 double. The inner door is of glass, the outer door is of copper 

 covered with asbestos. At one side is a gauge which indi- 

 cates the level at which the water stands in the water-jacket. 

 The roof is perforated with several holes, some of which per- 

 mit the circulation of the air in the air-chamber inside the 

 box; some of them enter the water-jacket. A thermometer 

 passes through one of these holes into the interior of the air- 

 chamber, and often another into the water standing in the 

 water-jacket. A gas-regulator passes through another hole. 



