MATILDA M. BROOKS 195 



of respiration than cultures between 18 and 24 hours old. The time 

 required to change the pH value from 8.3 to 8.1 by respiration may 

 be only a few minutes when the culture is young, while as much as 6 

 hours may be necessary in the case of an older culture. This might 

 be due to the fact that in the older culture the bacteria have gone 

 into the resting stage or have produced spores. In this stage ether 

 has little or no action upon the respiration of the bacteria. 



The temperature maintained was from 18-20°C. as this was found 

 to be sufficiently constant. Presumably the respiration of bacteria 

 has the ordinary temperature coefficient of 2. A few experiments 

 were tried at about 1°C. and it was found that exposure to this tem- 

 perature for 15 minutes had a marked after effect, as little or no 

 respiration was observed for about 6 hours. 



In the earlier experiments it was thought desirable to permit res- 

 piration to produce a definite change in pH value and then wash 

 out the CO2 with H2 or with air free from CO2. But this was found to 

 injure the bacteria (owing probably to mechanical or chemical dis- 

 turbances), so that the rate of respiration was greatly diminished. 

 Therefore it was necessary to use separate tubes for the control and 

 for each concentration of ether (instead of finding the normal rate of 

 respiration of a given tube and afterward exposing it to ether). It 

 was possible to do this as a number of readings showed a fairly con- 

 stant rate of respiration. 



The rate is obtained by taking the reciprocal of the time required 

 to produce a given change in pH value. It is expressed as per cent 

 of the normal rate which is always taken as 100 per cent. 



Fig. 1 shows a comparison between the respiration of Bacillus 

 suhtilis in tap water and in four concentrations of ether (by volume) . 

 In every case the rate is more rapid at first and becomes slower as the 

 reaction of the medium becomes more acid. The normal curve, in- 

 dicated by the dotted line, shows a slower rate than any of the other 

 curves. In no case does the indicator show the pH value to be below 

 6.8 to 6.4. 



When higher concentrations are used, the respiration is more rapid 

 during the first few pH intervals and much slower during the last 

 few. In fact, concentrations of ether as high as 1.46 per cent and up- 

 ward never reach so low a pH value as concentrations below 1.46 per 



