552 Adeney — Dissolved Gases and Fermentative Changes, 



sensitive means of gauging the degree of exhaustion which can be obtained in 

 the laboratory flask. The flask is usually exhausted by the time the burette has 

 been filled with mercury, and emptied four or five times. The flask being 

 exhausted, preparations are immediately made to transfer a known volume of the 

 water to be examined, into the laboratory flask. 



A few minutes' delay is first allowed to allow the surplus water left on the sides 

 of the burette to collect at the top. 



This is then transferred to the laboratory vessel, which is next detached from 

 the capillary branch tube, care, however, been taken before doing so, to leave the 

 branch tube full of mercmy. 



The rubber collar is detached from the end of this tube, which is then 

 connected by means of a piece of indiarubber tube with a glass tube of the same 

 thickness and bore as itself (27 ctms. long), which has been previously completely 

 filled with the water to be examined. The lower end of the latter tube is 

 immersed in the water for examination, the burette reservoir lowered a little, 

 and the stopcock opened, the water then passes into the burette. 



By gradually lowering the reservoir, 250 c.cs., or a less volume if required, of 

 the water may be drawn into the burette. 



When this has been effected, the stopcock is closed, and opened to the 

 laboratory flask. The water then passes into it from the burette, care being 

 necessary to prevent mercury getting into the flask during the operation. The 

 glass tube is now detached from the capillary branch, the rubber collar reattached, 

 and the laboratory vessel refixed in position. It is now necessary to clear the labo- 

 ratory vessel of small bubbles of air, and of any decided quantities of water that 

 may remain on its sides or in the reagent tube from the process of washing after 

 the previous analysis. This is rapidly done by I'aising the reservoir decidedly 

 above the level of the side tube of the vessel, and unscrewing the clip which closes 

 the rubber tube attached to the outer end of the reagent tube, so as to allow 

 mercury to pass into the pipette attached to the other end of the rubber tube and 

 supported by a clip, fixed to the stand, in the position shown in the drawing. 

 The reservoir is then dropped considerably, and the screw-clip being cautiously 

 opened, the greater portion of the mercury in the pipette is drawn back into 

 the laboratory vessel, carrying with it any air-bubble or water which may have 

 lodged in the reagent tube. The clip being closed, the reservoir is raised 

 sufficiently to cause the mercury to overflow the laboratory vessel when it is 

 slightly withdrawn from the rubber collar on the capillary branch tube, thus 

 driving out the air and water from the top of the vessel. The laboratory 

 vessel is refixed in position, and the reservoir is then lowered so as to empty 

 the vessel almost completely, and being rapidly raised, note is made as to 

 whether the sharp click is heard when the vessel refills completely. If not, it is 



