232 GENERAL SCIENCE 



a greater number of bacteria than this is unfit for food, and should 

 be looked upon with suspicion. Of course it would be impossible 

 to count the bacteria in 1 cubic centimeter of milk, but an estimate 

 is made in a very simple way. One cubic centimeter of milk is put 

 into 999 cubic centimeters of sterilized water, making a total of 1000 

 cubic centimeters of diluted milk. This is thoroughly agitated and 

 one drop of the mixture is placed on sterilized gelatin beef broth. 

 In a few days little spots or molds appear. These are little colonies 

 of bacteria, each one representing the place where one bacterium 

 from the milk started a whole colony. If the colonies are counted, 



PLATE No. 1 PLATE No. 2. 



DIRTY MILK. CLEAN MILK. 



FIG. 147. Bacterial Plates of Milk. Bacteria are tiny plants which grow in 

 proper soil. The round glass dishes contain a beef jelly in which is mixed a 

 little milk. The microscopic bacteria in two days develop colonies indicated 

 by the spots. The number of spots indicate the number of bacteria originally 

 in the milk. One cubic centimeter of milk is diluted with 99 c.c. of water, 

 and higher dilutions are made when necessary before planting the bacteria in 

 these glass plates. The total number per c.c. is obtained by multiplying the 

 number of spots by the dilution. 



and multiplied by the number of times the milk was diluted, the 

 number of bacteria in each drop of milk is easily estimated. Bac- 

 teria growing in milk cause the milk to sour, for they change the 

 milk sugar to an acid. The common notion that thunder-storms 

 cause milk to sour is a fallacy. 



QUESTIONS 



1. Why is milk one of the essential foods? 



2. Why should milk be well cared for? 



3. What precautions should always be taken regarding the 

 handling of milk? 



