Glucose and Mannitol by B. coli communis. 



79 



The nitrogen which enters through A is filtered through a plug of cotton 

 wool. 



The tube B is in connection with a wash-bottle C of 500 c.c. capacity, 

 which contains a solution of sodium hydroxide free from CO2. If the 

 strength of the solution is about normal, it is found that the one bottle of 

 this capacity suffices to absorb the carbon dioxide from the evolved gases 

 although these niay pass through the solution at a considerable rate. A 

 second small bottle D has been introduced for safety ; it contains about 100 c.c. 

 of the sodium hydroxide solution. The mixture of nitrogen introduced at the 

 beginning of the experiment and the hydrogen evolved from the fermentation 

 is collected in the reservoir F. By lowering the vessel H the difference of 

 level between F and H may be adjusted to counteract the weight of the 

 column of sodium hydroxide solution which has to be lifted by the gases 

 issuing from the fermentation flask. The gases are thus evolved under 

 atmospheric pressure. The method of collecting the gases here described is 

 much simpler than that in which they are collected in an evacuated flask 

 over mercury. As far as the estimation of CO2 is concerned the method is 

 just as accurate, but the estimation of hydrogen is not so accurate owing to 

 the difficulty of accurately graduating a vessel of the form F and also 

 because of the solubility of hydrogen in water. 



But apart from its simplicity the present method has now been adopted 

 because the rate of evolution of the gases is too great to permit of them being 

 collected through a column of mercury into a vacuum even if the wide form 

 of tube recommended by the writer in an earlier communication be adopted. 

 It may be pointed out, however, that a combination of the sodium hydroxide 

 absorption bottles with the mercury collecting apparatus employed by Harden 

 would be still better than the system here employed if an accurate estimation 

 of small amounts of gases other than carbon dioxide was to be made. The 

 gases in this case woiild, after passing through the sodium hydroxide solution, 

 pass on into the mercury gas-collecting apparatus, the size of which could be 

 cousiderably reduced. 



A further point to be noted in connection with the form of apparatus now 

 employed is that it is suitable for the study of aerobic as well as anaerobic 

 fermentation, for oxygen can be readily admitted during the course of the 

 experiment by opening the pinchcock A. 



In the series of experiments described below, the practice has been 

 adopted of stopping the fermentation at the time desired by raising the 

 temperature of the water-bath to about 70° C. and maintaining it at that 

 temperature for about half-an-hour. At the same time hot distilled water 

 has been introduced to displace the gases above the solution in the flask. 



