THE BACTEEIOLOGIC EXAMINATION OP MILK 



417 



with difficulty. If litmus is used and a differential count made, 

 some colonies forming but slight amounts of acid may escape proper 

 classification. 



There are many devices for counting colonies, some of great 

 simplicity, others more or less complex. The counting apparatus 



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recommended by Slack requires a small school slate on which a 

 circle 4| inches in diameter is cut. The circle is divided into 10 

 equal segments and the lines filled with red lead. The surface is 

 occasionally rubbed with vaselin. Another apparatus is the one 

 designed by Ayers (Fig. 185). It consists of a wooden box, 7 



27 



