QUANTITATIVE ANALYSIS OF MILK. 257 



cotton, and if the directions above given are followed closely 

 it will filter perfectly clear. After filtering the material is 

 to be collected in a sterilized flask. 



Fill a considerable number of test-tubes with the material 

 from the flask, placing in each test-tube about 10 c.c. of the 

 medium, carefully replacing the cotton stoppers. After the 

 test-tubes have been filled with the medium the whole quan- 

 tity, both that in test-tubes and in the flasks, should be 

 sterilized in a steam sterilizer for twenty minutes. To pro- 

 duce complete sterilization it is necessary to repeat the 

 steaming upon three successive days. The second and third 

 sterilization require a longer time than the first, inasmuch as 

 it requires some time to melt the agar, and until the agar 

 is thoroughly melted the sterilization is not effective. Upon 

 the second and third days, therefore, the material should be 

 steamed for at least half an hour. After the third steaming 

 the material in the test-tubes is ready for use. If an auto- 

 clave is at hand one sterilization for twenty minutes at 120 

 C. is sufficient. 



Method of Making Quantitative Analysis of Milk. In 

 order to make an analysis of the bacteria in milk it is neces- 

 sary to have an approximate idea of the number of bacteria 

 which are to be expected. The reason for this is that the bac- 

 teria are commonly so numerous that it is necessary to 

 dilute the milk highly with sterilized water in order that 

 reliable results may be obtained. In the quantitative analysis 

 of ordinary market milk it is commonly satisfactory to dilute 

 the milk one hundred times with sterilized water, provided 

 the medium to be used is the agar culture medium above 

 described. If the milk is old and contains large numbers of 

 bacteria a much higher dilution than this is desirable, but for 

 the kind of milk usually found in milk-distributing carts a 

 dilution of one hundred times is usually satisfactory for the 

 purpose here considered, 



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