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 
Fie. 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 
