Adeney — Dissolved Gases and Fermentative Changes. 553 



again partly detached from the collar, and some mercury driven out, and the 

 vessel again replaced in position. These operations of causing the mercury to 

 overflow the top, lowering the reservoir so as to nearly empty, and again rapidly 

 refilling the vessel, are continued till the sharp click, indicative of complete 

 exhaustion, is obtained. Grenerally, this is obtained after two or three trials, the 

 operation being extremely easy. 



During the time occupied in these manipulations, the water in the laboratory 

 flask is kept violently boiling to disengage the dissolved gases. 



These are drawn into the burette from the flask, and stored in the laboratory 

 vessel in a similar manner to that described for exhausting the flask in the first 

 instance, with the exception that the condensed aqueous vapour, drawn into the 

 burette with the dissolved gases, is not allowed to pass into the laboratory vessel, 

 but is returned to the flask. When the dissolved gases have been completely 

 extracted, a few minutes delay are allowed for the small quantities of water, 

 adherent to the sides of the burette, to rise to the top, when it is returned to the 

 flask. The measurement and analysis of the gases stored in the laboratory vessel 

 is then proceeded with. 



The method of adjusting the level of the mercury in the burette, after the 

 gases have been passed into it, will require some explanation. The simple plan 

 of closing the stopcock, and then raising and lowering the reservoir so as to bring 

 the mercury to tlie level of the division line selected cannot always be followed, 

 since, after transferring the gas, a small bubble of it sometimes remains in the 

 laboratory vessel under the rubber collar, and cannot be removed therefrom. 

 When, therefore, the level of the mercury in the burette is being adjusted, the 

 stopcock must be left open, and the adjustment be made by working the two 

 reservoirs conjointly. To transfer the gases into the burette in the first instance, 

 its reservoii- is lowered, and that of tlie laboratory vessel is raised, if necessary, 

 until nearly all the gas is passed over, a little, however, remaining in the 

 laboratory vessel. It must be observed at this point whether the gas will requii'e 

 to be compressed or expanded in order that the mercury in the burette shall stand, 

 after final adjustment, at the level of the division line selected for measurement, 

 and in order that, at the same time, the laboratory vessel shall be completely filled 

 with mercury, with the exception, possibly, of the small bubble of gas above 

 referred to ; with careful manipulation, however, no bubble of gas need remain. 

 With a little practice, the adjustment may be effected easily and quickly. An 

 indication that all the gas possible has been transferred to the bm-ette at the final 

 adjustment, is obtained when the mercury, or more generally water, since a small 

 quantity always collects in the labortatory vessel during the operation of extract- 

 ing the gases from the laboratory flask, and storing them, as above described, 

 rises a little in the capillary branch tube. 



