22 MICRO-ORGANISMS AND DISEASE. [CHAP. 



pointed out by Koch in his researches on pathogenic 

 bacteria. 1 



As solid media are used : 



1. Slices of Boiled Potato or Boiled White of Egg or Paste 

 (Fokker, Schroter, Cohn, Wernich). A boiled potato or a 

 boiled unshelled egg is cut in half with sterile scalpel, and 

 the cut surface is inoculated. Immediately after, it is placed 

 on a clean glass plate and covered with a bell-glass, the 

 edges of the latter being fixed on the former by vaseline or 

 grease, the chamber is kept moist by a piece of wet blotting 

 paper being placed inside the bell-glass. The progress of 

 the growth of a particular organism or of different organisms 

 sown out at a particular spot or line on the surface of these 

 substances can be easily watched with the unaided eye. 



2. Gelatine (Brefeld, Grawitz, Koch). This is used ad- 

 vantageously as a mixture with broth, peptone, beef-extract, 

 blood serum, or hydrocele fluid. Koch, who introduced 

 this mixture, used it for the cultivation of bacteria on solids, 

 to be exposed to the air ; the proportion of gelatine in the 

 mixture was 2 to 3 per cent. But this mixture, although 

 solid at ordinary temperature, does not keep solid in the 

 incubator, not even at 20 C. I have found that at least 

 7-5 per cent, of gelatine must be contained in the mixture 

 to keep it solid at 20 to 25 C. Above this temperature 

 not even 1 1 per cent, gelatine will keep solid. 



Nutrient Gelatine, most useful for the growth of all kinds 

 of bacteria, is prepared in this way : 



One pound of lean beef is cut up, to it is added one pint 

 of water, and is kept boiling in the digester or any other 

 vessel for from half to three-quarters of an hour. After 

 having been strained through fine calico it is filtered through 

 paper into a beaker ; bring up by adding water to 600 ccm. ; 

 1 Mittheilungen d. k. Gtsundheitsamtes, i. 1881. 



