118 ELEMENTS OF WATER BACTERIOLOGY 



absorption of CO2. Keyes (1909) and others have 

 introduced more exact methods based on the collection 

 and analysis of all gases formed in a vacuum, and 

 Rogers, Clark and Davis (Journal of Infectious Dis- 

 eases, 1914, XIV, 411) in an important paper which we 

 discuss more fully on page 200, have shown that 

 the gas ratio when accurately determined is highly 

 characteristic for certain members of the colon 

 group. 



Not only is it true that little reliance can be placed 

 on the exact gas formula in the open dextrose broth 

 tube, but cultures of the colon group may be actually 

 overgrown and lost by the multiplication of other 

 forms. This is particularly true when the waters are 

 heavily polluted or when large samples are examined. 

 Hunnewell and one of us (Winslow and Hunnewell, 

 1902^) found that of 48 samples of certain polluted 

 river waters 18 showed B. coli when i c.c. was inoculated 

 directly into dextrose broth, while in only 4 cases was 

 a positive result obtained after preliminary treatment 

 of 100 c.c. in carbol broth. In 153 samples from 

 presumably unpolluted water B. coli was found 5 

 times in i c.c. and 11 times by the examination of 

 the larger sample. II will be noted that these results 

 were obtained with carbol broth for the enrichment of 

 the larger samples and carbol broth is less liable to 

 overgrowths than dextrose broth. 



Whipple (Whipple, 1903) notes that 2.9 per cent of 

 some samples of water examined by him gave positive 

 tests with .1 c.c. but not with i c.c, while 4.3 per 

 cent gave positive tests with .1 c.c. or i c.c. and negative 



