226 BACTERIOLOGY. 



multiplies clusters are formed about it at the spot where 

 it was fixed at the moment of solidification. The num- 

 ber of colonies of bacteria (Fig. 28) thus indicate to 

 us roughly the number of living bacteria in the quan- 

 tity of fluid added to the liquid agar. Nutrient gelatin 

 is used exactly as agar, except that, as it does not 

 congeal until cooled below 22 C., we have no fear of 



FIG. 28. 



Photograph of a large number of colonies developing in a layer of gelatin 

 contained in a Petri dish. Some colonies are only pin point in size ; some as 

 large as a pencil. The colonies here appear in their actual size. 



its cooling too rapidly. In order not only to count 

 the number of colonies which develop, but also to 

 obtain a characteristic growth, it is desirable not to 

 have them too near together. As it is impossible to 

 determine accurately the number in any suspected fluid, 

 it is usual to make a set of four different plates, to each 

 of which a different amount of material is added, so that 



