New York Agricultural Experiment Station. 85 



Plate method. — The medium used throughout these comparative 

 tests was a lactose agar, made according to the following formula: 



Liebig's beef extract 5 grams 



Witte's peptone 10 grams 



Agar 15 grams 



Lactose 10 grams 



Distilled water 1000 cubic centimeters 



The acidity was determined and adjusted so that at no time 

 was it less than 1.3 per ct. nor more than 1.5 per ct. normal acid to 

 phenolphthalein. The agar was distributed in test tubes in 10 cubic 

 centimeter lots and sterilized. 



Methods of sampling. — The milk used was the ordinary market 

 milk produced for the Geneva city milk supply, and the samples 

 were taken in the morning directly from the 40-quart cans, brought 

 in by the farmers, as soon as they were placed on the milk-station 

 platform. The milk was thoroughly stirred and dipped with a clean, 

 long-handled dipper into sterilized pint bottles. In nearly every 

 case a separate sample was taken from both morning and night 

 milk. If the producer had several cans, then a composite sample 

 was taken from all the cans containing milk of the same age. It 

 usually required approximately an hour to procure the samples 

 and bring them to the laboratory. 



Smearing and plating. — The pint of milk was shaken 100 times 

 so as to thoroughly stir up the cream and sediment and to break 

 up the clumps of bacteria as far as possible. The making of the 

 smears was done at once as described on page 5. Then, with a 

 sterile pipette one cubic centimeter was transferred to a bottle 

 containing a measured amount of sterile water, preparatory to plating. 

 It required approximately from one-half to three-quarters of an hour 

 to complete this part of the operation with four samples. The 

 plating was done immediately. The dilutions used were determined 

 somewhat arbitrarily at first, according to the conditions under 

 which the milk was produced. Where studies of single farms were 

 continued during a considerable length of time, the dilutions were 

 changed according to the results secured, but an effort was made to 

 maintain uniformity throughout. The plates were incubated at 

 21° C. for five days and then counted with the aid of a lens magni- 

 fying four diameters. 



