EXPERIMENTS OF AUTHORS 



133 



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. In this particular series of experiments 

 we estimated the commencement of acidity by sourness of taste. 

 But in order to check the results, we carried out some acid-testing 

 experiments on similar milk, as follows : — Three flasks of milk, 

 each containing i pint, were taken at the byre. The milk was 

 milked direct from the cow into sterile flasks, and drawn from one 

 quarter of the udder only. 



One flask of the milk was kept at 37° C. 



„ 15^ c. 

 10° c. 



For the purpose of acid-testing 50 cc of milk was placed in a 

 porcelain vessel, and 2-5 cc. of a -i per cent solution of phenol- 

 phthalein was added to and well mixed with the milk. The 

 milk was then titrated with a solution of deci-normal caustic 

 soda from a burette until a faint pink colour was obtained. 



Generally speaking, about 15 to 20 cc. of ^ alkali is required. 



10 



N 



Each cc. of — alkali is termed "one degree of acidity" (Richmond). 



Hence a milk requiring 20 cc. will have 20° of acidity. Each cc of 



N 



— soda = -009 grammes of lactic acid. The results were as follows : 



Stage at which the milk became " sour " to the taste. 



