OSMOTIC PRESSURES OF AQUEOUS SOLUTIONS OF CALCIUM FERROCYANIDE. 195 



(2) A possible source of error is the presence of lubricant in the tap at the top of 

 the compressibility tube and also any unobserved air bubbles in the solution. 



It was considered that these errors were overcome when the same reading of the 

 mercury level in the compressibility tube was obtained for the same pressure reading, 

 whether when increasing the pressure or when decreasing it. On the other hand, it 

 was noticed that on increasing the pressure somewhat quickly the rapid rise of the 

 mercury in the stem of the compressibility tube caused it to enclose small globules of 

 liquid which further inci'ease of pressure did not seem to move, hence the apparent 

 constancy of the change in volume for a given change in pressure may be no 

 indication that the true change in volume has been obtained. 



It was found, however, that the enclosed globules of liquid were avoided when the 

 rate of change of pressure was slow. 



The following is from the laboratory book and gives the details of one of the 

 experiments with calcium ferrocyanide solution. 



The solution was made up by weight in the 400 c.c. flask and its density 

 determined as on p. 180. 



Date. 



March 18 

 March 19 



Weight of 

 U-tube. 



70-584gr. 



Time. 



9.0 A.M. 

 10.45 

 11.5 

 11.15 

 11.35 



12. 40 P.M. 70-612 gr. 



Remarks. 



Made up solution. 



Used No. II. compressibility tube. 



Filled bulb and connected up as before (i.e., as in fig. G). 



Started pumping. 



Opened bulb to pump for 5 seconds. 



Closed taps and opened compressibility tube to pump. 

 Closed compressibility tube and filled. 



Withdrew compressibility tube and placed its bulb in ice for 

 2 minutes while lower end dipped in solution and then 

 placed in apparatus which was immediately filled with ice- 

 cold water and covered with ice. 



During the afternoon the ice was rammed down at intervals of one hour, and at intervals of a quarter 

 of an hour between 4 and 5 P.M. 



Date. 



March 19 



Difference 



of Remarks, 



reading. 



32-55 



38-3 



37-9 



Rammed down ice. 



Rammed down ice. 



Rammed down ice. 



2 C 2 



