32 MICRO-ORGANISMS AND DISEASE [chap. 



the growth of the organisms, they remain more limited 

 to the spot or spots on which they are sown, and therefore 

 can be watched more easily; besides, an accidental con. 

 tamination — i.e. a growth appearing at a spot at which no 

 sowing was made, can be recognised at once. These advan- 

 tages are perhaps of the greatest use when it is intended to 

 grow the organisms on a surface exposed to the influence 

 of air — of course protected from contamination with other 

 organisms. 



These advantages of solid media have been very minutely 

 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, or a glass capsule 

 covered with another, both sterile, receive the potato. The 

 progress of growth of a particular organism or of different 

 organisms sown at a particular spot or line on the surface 

 of these substances can be easily watched with the unaided 

 eye. 



Blocks of potato cut with a sterile cork-borer from a clean- 

 cut potato are placed into test-tubes over a cushion of sterile 

 cotton-wool, the test-tubes are then plugged and steamed on 

 two successive occasions for 20 minutes each time. 



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

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



' Mittheilungen d. k. Gesimdheitsamles, i. 1881. 



