STUDY OF THE PHYSICAL CHEMISTRY OF ANAESTHESIA 171 
In using the apparatus, the two vertical tubes are first placed at the same level, 
the mercury-holder is filled with mercury, and, with the two glass stoppers out, the 
whole apparatus is filled with mercury, the two stoppers are next inserted, enough 
mercury being left above them to form a seal, and the mercury-holder is then 
lowered until the two vertical tubes become evacuated. The receiver is then raised 
again to the level of the stoppers, and any bubble of air found is discharged. 
The apparatus is now ready for an experiment, and, with stoppers out, the levels 
of mercury are adjusted until there is an equal volume left above the mercury on 
each side. A given volume of the solvent (say 5 c.c.) is now introduced on the one 
side (say left), and an equal volume of the solution of chloroform in the same solvent 
on the other side. In each case, immediately after the fluid has been introduced, the 
stopper is inserted, care being taken to prevent any air being included, either as a 
bubble at the mercury surface, or between the surface of the introduced fluid and the 
stopper. To achieve the latter end, we have almost always introduced above the 
mercury 2 or 3 c.c. more than the required quantity, so that it stood in the neck and 
slightly above, and then, by easing the stopper and gently adjusting the level of the 
mercury-holder, have brought the level of the mercury in the tube to the desired 
volume mark. After the solvent on the one side and the solution of chloroform of 
the desired strength on the other side have been successfully introduced in equal 
volume, and without any bubble of air, the mercury-holder is lowered until a space 
containing vapour has appeared on each side. The level of the mercury will be found 
to be lower on the chloroform side, and it is obvious, the instrument being inde- 
pendent of variations in atmospheric pressure, and the only different factor being the 
added chloroform on the one side, 1 that the difference in pressure will give the vapour 
pressure directly for that strength of chloroform solution at that particular temperature. 
There is hence no need to determine pressure due to dissolved gases on the two 
sides, 2 or pressure of aqueous vapour, since these balance, and the quickness with 
which readings can be directly obtained makes it possible to carry out a long series 
of determinations at varying strengths, without the proteid solutions having time to 
undergo bacterial change. 
Certain precautions have to be taken, however, and corrections made which mav 
here be mentioned : — 
1 . Before taking a reading it is essential to move the tubes vertically and adjust 
the levels until the volumes of the vapour spaces above the upper aqueous solution 
meniscus in each case are exactly equal, otherwise inequality in pressure of gases 
pumped off on the two sides gives rise to an error, which is greater the smaller the 
vapour space. 
1. There will be a small difference in the pressure of water vapour on the two sides, due to there being a stronger 
solution on the chloroform side, but this is in all cases too minute compared to the pressure of the chloroform vapour to make- 
any appreciable error. 
2. Slight differences in dissolved gases gave a disturbance with very dilute chloroform solutions, and this was later 
obviated by pumping the gases off (fide infra). 
