OSMOTIC PKESSURES OF AQUEOUS SOLUTIONS OF CALCIUM FERROCYANIDE. 193 



way by means of mercury. The stem was made 40 cm. long, and its bore such that 

 an appreciable change in reading could be observed even when caused by only a 

 change of pressure of a few atmospheres. 



The bottom of the Robax tube is filled for some few centimetres with mercury and 

 matters are so arranged that the mercury rises in the stem of the compressibility 

 tube H a centimetre or two above the level of the mercury outside when the whole 

 apparatus has been brought to a constant temperature at atmospheric pressure. 



Fig. 5. 



Fig. 6. 



General Method of Experiment. 



The compressibility tube is filled as shown in fig. G, which represents the method 

 adopted to fill with air-free liquid. The compressibility tube A is joined at its lower 

 end to a bulb, B, containing the liquid, and at its upper end to the U-tube C 

 containing pumice moistened with sulphuric acid; this U-tube is weighed before and 

 after the experiment. The U-tube is connected at D to a Fleuss exhaust pump by 

 flexible pressure tubing ; the T-piece E serves to connect it to the other end of the 

 bulb containing the solution through the three-way tap F. By closing tap G, 

 turning F so as to have through communication, and opening tap H, the liquid in B 

 can be caused to boil. Care, however, must be taken only to create a vacuum 

 above B sufficient to get rid of the air (shown by air bubbles coming off), otherwise 

 an unnecessary increase of concentration of the solution in B will take place. When 



VOL. ccix. A. 2 c 



