TECHNICAL APPENDIX. 185 



media (agar and gelatin) and for the following rea- 

 sons: 



(a) They may be used as fluids and as solid media : 

 as fluids they permit the separation, as solid sub- 

 stances the fixation, of the isolated germs and their 

 separate growth into colonies. 



(b) On account of their transparency they permit a 

 macroscopic as well as a microscopic observation of 

 the cultures; they permit a differential diagnosis of 

 the varieties and an early recognition of any im- 

 purities. 



They are used particularly : 



(a) For plate cultures, i.e., as a proof of positive 

 separation and for the enumeration of individuals and 

 varieties. 



(b) To secure characteristic macroscopic cultures, 

 which will serve for differential diagnosis. 



(c) For permanent cultures or collections of living 

 bacteria. 



The special advantages of agar and gelatin are : 



(a) Gelatin. Advantages : Easily produced, easily 

 formed into plates (at 25) ; its property of liquefac- 

 tion by certain bacteria possesses great diagnostic 

 importance. Disadvantages : As it melts at 25, it 

 cannot be used in hot weather and at incubating tem- 

 perature. 



(b) Agar. Advantages : Practicable at incubating 

 temperature (i.e., for the rapid culture of bacteria 

 [spores] and particularly of thermophile bacteria). 

 Disadvantages: Difficulty of preparation; not so 

 easily formed into plates. The cultures are often not 

 very characteristic. 



2. Blood serum and glycerin agar. Used for the 



