56 Prof. J. P. Cooke on Lecture Experiments. 



also screwing on to one side an ordinary " elbow-joint," and 

 at the same time by carefully rimming out the three apertures 

 so as to present smooth surfaces for the adhesion of the rubber 

 stoppers. Into one of the openings of the "return bend" we 

 fasten with a perforated rubber stopper a common straight eudi- 

 ometer-tube (such, for example, as is ordinarily used in Bunsen's 

 method of gas-analysis), while into the second opening we fasten 

 in a similar way an open glass tube. In the opening of the 

 elbow-joint we secure with a perforated rubber stopper as before, 

 that convenient substitute for a stopcock so well known to che- 

 mists as a nipper-tap. Last of all we obtain from the glass- 

 house a tubulated tube, like that shown in fig. 8, of such size 

 that it will cover the eudiometer-tube and fit the Fig. 8. 



upper end of the same rubber stopper by which Jf^^ 



the last is secured in its place. .The upper end 

 of this outer tube is also closed with a perforated 

 stopper, as shown in the figure; and fine platinum 

 wires connecting with the wires of the eudiometer 

 pass between tho stopper and the glass. The 

 lower end of the outer tube should not fit too 

 tightly around the rubber stopper, so that it can 

 easily be removed when not wanted; moreover 

 all the stoppers should be made of such length 

 that, while perfectly tight, they can easily be re- 

 moved for cleaning the tubes*. These rubber 

 joints, if well made, give great flexibility to the ap- 

 paratus, and enable it to withstand quite rough 



HJ 



The eudiometer being thus mounted, mercury 

 is first poured into the open limb, which is then 

 closed with a rubber stopper and the mercury 

 transferred by inclining the apparatus to the 

 closed limb, this process being repeated until the 

 eudiometer-tube is full. The excess of mercury is now drawn 

 out of the open tube by the tap, "and, this tube having been re- 

 moved, the instrument is left as represented in fig. 9. The eudi- 

 ometer is now ready for receiving the mixed gases, which are 

 passed up into it from a simple decomposing-cell, as shown in 

 the figure, the excess of mercury overflowing into any conve- 

 nient vessel. The open tube is now replaced, and the outer tube 



* It is also best to place around the upper end of the eudiometer-tube a 

 rubber ring, quite narrow, but sufficiently thick to fill the annular space 

 when the outer tube is in its place, and also perforated with a number of 

 small holes, so that the steam or vapour employed in the experiments may 

 pass freely. This ring confines the eudiometer-tube and gives greater so- 

 lidity to the apparatus. 



