1902-3- ] Bacterial Contamination of Milk. 493 



If we analyze this table we find that at 15° the increase during the 

 first half hour was 700, or 7 per cent., which would indicate that the 

 average duration of a generation is thirteen hours. In three hours more, 

 the increase is 15,000, or 150 per cent., which gives the average duration 

 of a generation as two hours. In the next three hours, the increase is 

 21,500, and the average duration of a generation about 3^ hours. From 

 the ninth to the twenty-fourth hour, the average is about 2>^ hours. At 

 25°, the times required for a doubling of the number are for the 

 successive periods, about half an hour, about an hour, and about 

 7^ hours, .'^t 35° the time occupied in a generation was about twenty 

 minutes at first, then about forty-five minutes, and at last thirty-seven 

 hours. These results are curious, but could only be explained by a 

 fuller knowledge of the species concerned, and of the cause influencing 

 the changes. 



The most important point brought out in this experiment is the 

 tremendous rate of increase at the higher temperatures ; therefore, much 

 may be done to restrain this rapid multiplication by cooling the milk as 

 rapidly as possible. Milk allowed to cool naturally takes some time 

 before it reaches the temperature of the air. Hence, measures should 

 be promptly taken to reduce the temperature quickly. 



Certified Milk. 



Of late years a number of sanitary, model dairies have been estab- 

 lished in the vicinity of large cities in various parts of the United 

 States and Canada, (see Figs. 16 and 77), which have placed on the 

 market, milk with a relatively low bacterial content. Such milk is 

 known as " hygienic," " sanitary," or " certified." It is interesting to note 

 that these establishments prosper, an indication that the discriminating 

 public appreciate the honest endeavour of these dairies to produce milk 

 which will fulfil the requirements of the most exacting sanitarian. 



These establishments put into practice the suggestions made by 

 various experimenters and investigators, as the result of their experi- 

 mental inquiries, and these have been more or less briefly outlined in 

 this paper. The freedom from bacteria obtained in these dairies 

 depends on the thoroughness with which all details are carried out. 

 Russell^^ has shown that when samples of milk are secured under as 

 nearly aseptic conditions as possible, the germ content was 330 organisms 

 per c.c; but when drawn under ordinary conditions, the bacterial 

 content was 15,500 organisms per c.c. Marshall®^ gives similar results. 



