374 ENZYMES 



a siphon tube, S ; the other end of this tube dips into a 

 beaker, B, which stands in a dish provided with a h'p, so that 

 the excess of mercury which falls from B is ultimately dis- 

 charged into a second beaker, C. Any mercury displaced from 

 N collects into the reservoir R, and is siphoned off into D, from 

 whence it overflows into the shallow dish, and is finally col- 

 lected in C. The mercury at R and H, therefore, remains 

 level with the mercury in B. This adjustment is made once 

 for all, so that the mercury extends to the bend of the tube 

 H, and thus the pressure in the flask is maintained at atmos- 

 pheric pressure. 



The volume of the gas given off is read on N at the 

 reduced pressure corresponding to the height of the column 

 of mercury NH, and is corrected by means of a table which 

 is drawn up as follows. The height of each graduation mark 

 of the nitrometer above H is measured, and subtracted from 

 760 mm., the normal pressure of the atmosphere ; the corrected 

 volume corresponding with each graduation is then calculated. 

 When great accuracy is required the reading must be corrected 

 for the atmospheric pressure obtaining at the time of the 

 experiment. 



In starting an experiment the nitrometer is filled with 

 mercury, and the siphon adjusted in R and started by means 

 of a pressure bulb which is attached to the short tube passing 

 through the stopper of the reservoir. The fermenting mixture 

 is placed in the flask F, together with a little toluene as an 

 antiseptic. When the temperature has reached that of the 

 thermostat and is constant, the flask is removed, filled with 

 carbon dioxide and thoroughly shaken, the process being re- 

 peated until the liquid is saturated. Owing to the fact that 

 the liquid easily becomes supersaturated the flask must be well 

 shaken for half a minute before a reading is taken. The flask 

 is replaced in the thermostat for a minute, in order to raise 

 the temperature to the proper degree, and then the volume of 

 gas collected in the nitrometer is read. 



At the expiration of the desired interval of time from the 

 last shaking the flask is again thoroughly shaken, after which 

 the volume of gas evolved during the interval is read. 



When the nitrometer is filled with gas, the tap of F is 

 closed, the siphon removed, the reservoir filled with mercury 



