142 PATHOGENIC BACTERIA. 



covering the ice- water of the levelling apparatus. The 

 plug of cotton closing the mouth of tube No. i is re- 

 moved, and to prevent contamination during the outflow 

 of the gelatin the mouth of the tube is held in the flame 

 of a Bunsen burner for a moment or two. The gelatin 

 is then cautiously poured out upon the plate, the mouth 

 of the tube, as well as the plate, being covered by the 

 bell-glass to prevent contamination by germs in the air. 

 The apparatus being level, the gelatin spreads out in an 

 even, thin layer, and, the plate being cold from the ice 



beneath, it immediately solidi- 

 fies, and in a few moments can 

 be removed to the moist cham- 

 7^ ber prepared to receive it. As 



FIG. 25. Glass bench. 



soon as plate No. i is prepared, 



the contents of tube No. 2 are poured upon plate No. 2, 

 allowed to spread out and solidify, and then superimposed 

 on plate No. i in the moist chamber, being separated from 

 the plate already in the chamber by small glass benches 

 (Fig. 25) made for the purpose and sterilized. After the 

 contents of all the tubes are thus distributed, the moist 

 chamber and its contents are allowed to stand for some 

 hours, to permit the bacteria to grow. Where each or- 

 ganism falls a colony develops, and the success of the 

 whole method depends upon the isolation of a colony 

 and its transfer to a tube of culture-medium where it 

 can grow unmixed and undisturbed. 



The description must have made evident the fact that 

 only such culture-media can be used for plate-cultures as 

 can be melted and solidified at will viz. gelatin, agar- 

 agar, and glycerin agar-agar. Blood-serum and L,6frler's 

 mixture are entirely inappropriate. 



The great drawback to this excellent method is the 

 cumbersome apparatus required and the comparative im- 

 possibility of making plate-cultures, as is often desirable, 

 in the clinic, at the bedside, or elsewhere than in the 

 laboratory. The method therefore soon underwent mod- 

 ifications, the most important being 



