Determination of Dissolved Oxygen. 63 



not hydrolyze as readily as most other soaps, and has therefore 

 been used extensively in our experiments. It is more toxic to 

 some bacteria than potassium or sodium stearate. This is prob- 

 ably due to the fact that castor oil soap is dialyzable and probably 

 dialyzes into the cell and disturbs the salt balance by precipitating 

 the calcium, magnesium and salts of the heavy metals. 



Bacteria such as the pneumococcus and streptococcus will not 

 grow on low tension media, while the organisms which inhabit the 

 intestinal tract grow abundantly on media of low tension. This is 

 not surprising since the contents of the intestines have a low S. 

 tension due to the presence of bile, soaps and other S. tension 

 depressants. It is well known that many of the intestinal bacteria 

 when inoculated in broth grow near the surface of the medium. 

 This is particularly true of the cholera vibrio. This selective 

 localization is probably due to the fact that the S. tension reducing 

 substances concentrate at the surface of the medium thus creating 

 a favorable environment for these bacteria. Incidentally it may 

 be pointed out that the bacteria which grow well in low tension 

 media are better antigens than the streptococcus, pneumococcus 

 and others which refuse to grow under such conditions. 



34 (1781) 



A micro- Winkler method for the quantitative determination of 



dissolved oxygen. 



By E. J. LUND. 



[From the Department of Animal Biology, University of Minnesota, 

 Minneapolis, Minn.] 



Winkler's method for quantitative determination of dissolved 

 oxygen may be applied to 10 c.c. or even 5 c.c. samples of water 

 in the following way. One tenth of a cubic centimeter of each 

 of the two solutions MnCl 2 and NaOH — KI are added from 1 c.c. 

 burettes graduated to 0.1 c.c. or less. The thiosulfate solution 

 of the usual concentration is diluted to ten times its volume. The 

 iodine is titrated in a tall dish using a 5 c.c. burette. The end 

 joint is just as definite as that in the ordinary procedure. The 

 percentage error is also the same, about 1 per cent. The distinct 



