62 THE BACTERIA IN ASIATIC CHOLERA. [CH. 



present in the air of ordinary city laboratories, which can be 

 made visible by simply exposing a number of sterilised glass 

 dishes, in which previously a thin layer of nutritive gelatine 

 or Agar-agar mixture liquefied by warmth has been poured 

 out and allowed to set over a layer of cold water. Each glass 

 dish is kept covered up with another glass dish, and the whole 

 kept in a moist chamber, or in other words on a glass plate 

 covered up by a bell-glass to which a piece of wet blot- 

 ting-paper has been fixed. This for all practical purposes 

 represents a good plate-cultivation after Koch's method. 

 On exposing for from several seconds to several minutes the 

 solidified layer of the nutritive medium, by lifting off the 

 upper covering glass dish, then closing it again and placing 

 the bell-glass in position and keeping the whole at a tempera- 

 ture of i9-22 c C. in the case of gelatine, or 35-36 C. in the 



FIG. 20. PLATE-CULTIVATION iv GELATINE 48 HOURS OLD, SHOWING YOUNG 

 COLONIES OF CHOLERAIC COMMA-BACILLI. 



case of Agar-agar mixture, for two, three, or more days, 

 various spots or colonies will show themselves on the surface 

 of the plate-cultivation, some small, others large, some round, 

 others irregular, some spherical, others disc-shaped, some 

 whitish or greyish, others yellow, orange, or of other colours, 

 some not liquefying the gelatine, others liquefying it ; of the 



