TIME AND TEMPERATURE 189 



commencement the milk contained 812,000 bacteria per c.c. After 

 four hours it contained 2,066,000 (at 5° C), 3,650,000 (at 15° C), 

 and 6,116,000 (at 37° C). 



Secondly, speaking in a general way, the following great principle 

 became evident, namely, that there is at each temperature (a) a 

 sudden rise, (6) a sudden fall, (c) a steady rise to maximum, and (d) 

 a steady fall ultimately to sterility. In other words, there are tides 

 of organisms, and this was found to occur invariably in our study 

 of " natural " milks. It is a variable phenomenon in ordinary milks, 

 but is the rule in respect to " natural " milk examined immediately 

 after milking. It is obvious that if we had commenced our examina- 

 tion, as is frequently the case in the study of town milks, twelve 

 or twenty hours after milking, we should, even if we had obtained 

 the same figures, have drawn very different deductions, because the 

 initial rise and initial fall would have been lost sight of. The 

 sudden fall occurred in forty-eight hours at 5° C, in twelve hours 

 at 15" C. and 37° C. 



Thirdly, the maximum number of bacteria occurred in ten days 

 at 5° C. (406,400,000 bacteria per c.c), in six days at 15° C. 

 (84,000,000), and in seventy-two hours at 37° C. (8,360,000). The 

 maximum was lowest at blood-heat and highest at 5° C. It is 

 evident, therefore, that what occurs in a short time at a high 

 temperature occurs in a longer period at a low temperature, but at 

 a low temperature ■ the bacteria eventually become most numerous. 

 These facts are of great importance in relation to the time which 

 milk is kept before use, and to the injurious properties which it 

 may acquire during such a period in the direction of increased 

 bacterial toxin production. 



Fourthly, marked acidity commenced between the twelfth and 

 sixteenth hours in the sample at 37° ; between the twentieth and 

 twenty-fourth hours at 15°; and between the seventy-second and 

 ninety-sixth hours at 5° C. At the end of these particular stages 

 it will be noticed that there is a rising tide following the " low-water 

 mark " of organisms at each temperature. The relation which the 

 degree of acidity bears to the bacterial content is an intimate one. 

 As far back as 1878 Lister pointed out the marked inhibitory effect 

 which the presence of a high degree of lactic acid had upon common 

 moulds and ordinary saprophytic bacteria.* When the lactic acid 

 decUnes, these other forms commence growth, and eventually 

 enormously preponderate. 



Fifthly, as the flasks of milk were kept intact, we were able to 

 repeat the experiment in every particular after the lapse of exactly 

 two years from the commencement. The milk was the same milk, 



* Path. Soc. Trans., 1878, p. 440, 



