396 



Prof. B. Moore and Mr. H. E. Eoaf. 



[Apr. 12, 



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 independent of variations in 

 atmospheric pressure, and the only different factor being the added 

 chloroform on the one side,* 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,! 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 may 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. 



* 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. 



f Slight differences in dissolved gases gave a disturbance with very dilute 

 chloroform solutions, and this was later obviated by pumping the gases off {vide 

 infra). 



