12 



Scientific Proceedings (125) 



autocatalytic curve of growth. This took place at the end of 

 the fourth to the fifth day of incubation. The culture was then 

 filtered through filter paper to get it free from sulfur. 50 c.c. 

 portions of the uniform culture were then placed in small flasks 

 (100 c.c. Erlenmeyer), to each of which 1 gram of elementary 

 sulfur was added. The stimulating or repressive agents were 

 then added and the flasks placed in the incubator and kept at 

 30° C for about 20 hours. The change in hydrogen-ion con- 

 centration, titration or sulfate content during that period in- 

 dicates the amount of sulfur oxidized and can serve as an in- 

 dex of oxygen consumption or carbon dioxide assimilation. One 

 is particularly justified in doing that, since the ratio between 

 the sulfur oxidized and carbon dioxide assimilated is constant 

 under the conditions of the experiment, as pointed elsewhere. 

 The change in titration, using phenolphthalein as an indicator, 

 is the simplest index of the respiration of the culture, since at 

 the very high acid concentration at which the organism is active, 

 particularly in well buffered media, the change in hydrogen-ion 

 concentration is not a sensitive index. 



If the normal rate of respiration is taken as a 100, the respira- 

 tion of the organism as influenced by various treatments is as 

 follows : 



INFLUENCE OF GASES, NITRATES AND ORGANIC SUBSTANCES 

 UPON THE RESPIRATION OF SULF. THIOOXIDANS 



Control 100 



(NH 4 ) L ,S0 4 0.025 molar 108 



(NH 4 ) 2 S0 4 0.050 molar 112 



(NII 4 ) 2 S0 4 0.100 molar 112 



(NII 4 ) 2 S0 4 0.250 molar 108 



NaN0 3 0.025 molar 29.2 



NaN0 3 0.050 molar 12.5 



NaN0 3 0.100 molar 0 



NaN0 3 0.250 molar 0 



Ca(N0 3 ) 2 0.025 molar 27.3 



0a(NO 3 ) 2 0.050 molar 8.5 



0a(NO 3 ) 2 0.100 molar 10.0 



0a(NO 3 ) 2 0.250 molar 0 



Na.N0: { -f-(Nn 1 ) 2 S() 4 0.0125 molar each G2.5 



NaNO :{ f(NH 4 ) 2 S0 4 0.025 molar each 25.0 



Na.NO; { +(NH 4 ) 2 S0 4 0.050 molar reieh 16.5 



NaNO. { -f (NH 4 ) 2 S0 4 0.125 molar each 0 



KNO-j 0.005 molar 71 



KNO, 0.010 molar 57 



KNO., 0.020 molar 43 



KNO-. 0.050 molar 21 



Mg(N0 3 ) 2 0.050 molar 16 



NaCN 0.0004 molar 0 



