HOW THE TEST IS MADE 29 
(6) The dishes are put in an incubator where 
the temperature is the same as that used in 
hatching hens’ eggs. They remain there for two 
days. 
COLONIES OF BACTERIA 
The jelly is perfectly clear at first. After 
two days there are developed white and gray spots 
on the jelly where the bacteria in the milk have 
grown into masses. Each germ has surrounded 
himself with millions of descendants, making a 
spot visible to the naked eye. It is easy to 
count these spots, and thus count the number 
of bacteria present in the milk sample. The 
