MATILDA M. BROOKS 7 



For experimentation, 2 cc. of the emulsion of bacteria were placed 

 in the apparatus, and the air was caused to circulate. The CO2 

 produced by the bacteria was thereby carried over into the indicator, 

 and the reciprocal of the time required to change the color from pH 

 7.78 to 7.60 was taken as the rate of respiration or production of 

 CO2. When the indicator had reached the end-point required, the 

 stop-cock was closed, thus allowing the CO2 to be washed out of the 

 system again by passing the air through the NaOH, and thereby 

 returning the indicator to its original color. In this way, a series 

 of readings could be taken. In general these were remarkably con- 

 stant. It was found that the rate of respiration, under normal 

 conditions {i.e. of bacteria placed in 0.75 per cent dextrose solution), 

 was practically constant for about 6 hours. The experiments, how- 

 ever, lasted only 70 minutes. This included the time necessary for 

 the establishment of the normal rate, usually 10 minutes, and the 

 time for determining the effect of the salt. 



When the normal rate had been determined, the salt was added 

 and the change in the rate was observed. The first reading was 

 discarded owing to the possibiHty of experimental error, as the result 

 of CO2 dissolved in the salt solution. In adding the salt solution, 

 the system was opened because it was found that no appreciable 

 error was introduced by exposing the system for a moment to contact 

 with the air. 



When the bacteria had been in contact with the salt for an hour, 

 the respiration seemed to have reached an equihbrium, as the rate 

 then decreased very slowly during the next few hours. This decrease 

 was faster in the higher concentrations of the salt; the lower con- 

 centrations remained constant for hours at a time. In adding the 

 salt, 2 cc. (of double the strength desired for experimentation) were 

 added to 2 cc. of the bacteria in 0.75 per cent dextrose solution, so 

 that the volume remained constant throughout all the experiments. 



It is generally known that a reduced pressure of oxygen has Httle 

 effect for a considerable time upon the rate of oxidation, so that the 

 slight change in the oxygen content of the system during the experi- 

 ment does not introduce an experimental error. 



Fig. 1 shows the manner in which the rate of CO2 production 

 changes under the influence of NaCl in the concentrations of 0.15, 



