METHODS OF CULTURE AND OF EXAMINATION. 



97 



stat or incubator consists of a double-walled copper or sheet- 

 iron chamber surrounded with felt or asbestos. The space 

 between the two walls is filled with water, which is kept con- 

 stantly at an equable temperature by means of a thermo-regu- 

 lator. The layer of water transmits its heat to the interior of 

 the thermostat, the actual culture-chamber, which constantly 

 should have a temperature of 37 C. 

 (98.6 F.) for agar, or from 24 C. 

 (75.2 F.) to 26 C. (78.8 F.) for 

 gelatin. The culture-tubes, which 

 are to be kept for a considerable 

 length of time in the thermostat, 

 must be provided with rubber caps 

 sterilized with mercuric chlorid, in 

 order to protect them from evapora- 

 tion. 



The growth in a number of these 

 cultures constitutes for many bacteria 

 a definite characteristic, which is of 

 the highest significance in their iden- 

 tification. Least typical, as a rule, is 

 the growth upon agar. Attention 

 must be directed to the thickness, 

 the transparency, and the color of 

 the deposit, etc. Jn bouillon-cultures 



there may be distinguished bacteria that develop only 

 upon the surface in the form of a thick or a thin membrane, 

 others that render the bouillon more or less homogene- 

 ously turbid, and still others that grow only at the bottom 

 as a crumbling or a viscid sediment. Milk is unaltered 

 by many bacteria, while others cause coagulation through 

 the formation of acid. The most important peculiarities 

 are furnished by gelatin-cultures. In the first place the 

 liquefaction that results through the activity of a peptoniz- 

 ing ferment is to be looked for. This occurs in part only 

 superficially, and is in part funnel-shaped or stocking-shaped 

 at a depth. If liquefaction does not take place, varied 

 and often characteristic peculiarities of growth occur (nail- 

 culture, tree-like division, etc.). 



Fig. 27. Method of inocu- 

 lating a test-tube containing 

 sterile nutrient jelly. 



