BACILLUS-COLI-LlKE BACTERIA FROM *FECES AND SEWAGE 5 



The body temperature was selected for incubation, because, as was shown 

 by Browne, 1 acid-production by B. coli is most rapid at this temperature. He 

 also showed that with certain carbohydrates and alcohols the maximal amount 

 of acid is formed in less than 24 hours. Thirty-six hours' incubation was 

 employed for convenience in this study. With the alcohol, glycerol, and the 

 glucosid, salicin, the 36-hour .incubation period was not sufficient. Acid- and 

 gas-formation from these substances were therefore determined after 72 

 hours' growth. 



Titration. As the acidity of distilled water varied on different days, the 

 following technic was adopted in order to obviate tedious subtractions of 

 checks. To a pail of distilled water (6 to 8 liters) was added 1% phenol- 

 phthalein solution (5 gm. phenolphthalein in 1 liter of 50% alcohol). The 

 water was boiled vigorously for 15 minutes and then neutralized with sodium 

 hydroxid. Of this neutral distilled water, containing the indicator, 45 c.c. were 

 dipped out into an evaporating dish or casserole, 5 c.c. of the test culture were 

 added, and the amount of acid determined by titration with N/20 NaOH 

 without boiling. 4 



TREATMENT OF RESULTS 



A few extremely high or low results will influence considerably 

 the average acid-production of a collection of organisms. The use of 

 unqualified averages may therefore lead to a misconception of the 

 acid-producing properties of a group. To supplement the arithmetic 

 mean, or numeric average, some statement should be made as to the 

 distribution of the variates about the average. This may be indicated 

 by the probable error or by the standard deviation. The coefficient 

 of variability (the ratio of the standard deviation to the mean) is an 

 excellent abstract measure of variability. The modal acid-production 

 (the amount of acid most frequently formed) may, under certain con- 

 ditions, be of greater significance than the average amount of acid 

 formed. 



In this study the mean, the probable error of a single variate, the 

 standard deviation, the coefficient of variability, and the empirical 

 mode are employed. 



The standard deviation is the measure of variability most com- 

 monly employed, particularly by mathematicians. It may be expressed 

 mathematically as 



is the number of variates, 

 or observations, "d" the deviation of the individual variates from the 

 mean, and "f" the frequency of a deviation "d". The standard devia- 

 tion serves to indicate whether the departures from the mean are 



