24 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS. 



with, and the union between the manometer attachment and the cell 

 is effected by means of the brass rings (a, a, a, a) and the packing 

 which is placed between them. The packing is compressed in the ver- 

 tical direction and made to expand horizontally against the cell on the 

 one side and the brass tube (b, b) on the other by the sliding piece (c, c). 

 The collar (e, e) and the nut (/,/) do not differ essentially from the corre- 

 sponding pieces seen in Figures 9 and 10. The vent and the arrange- 

 ments for fixing the manometer are the same in Figure 11 as in Figure 10. 

 The adjustment of pressure within the cell is effected by means of the 

 nut (g, g). Turned to the right, it drives the tube (b, b), and with it the 

 manometer, into the cell, increasing the pressure. If it is turned to the 

 left, the tube and manometer are raised and the pressure diminished. 

 The principal advantages of the arrangement seen in Figure 11 over that 

 shown in Figure 10 are in the better means of adjusting the pressure 

 and in the substitution of packing for rubber tubing in making the 

 joint with the cell. 



In measuring the osmotic pressure of electrolytes, it is desirable to 

 avoid, as far as possible, any contact of the solutions with metallic sur- 

 faces, even though the same are protected by plating with the more 

 resistant metals. The covering is often imperfect in spots, notwith- 

 standing the care which is taken in the plating. Accordingly, a number 

 of schemes have been devised for joining the manometer and the cell, in 

 which the solution comes in contact only with glass and rubber. 



In Figure 12, the hollow glass cone (a) serves the same purpose as the 

 brass cones seen in Figures 9 and 10. It is set in the brass piece (6) 

 with litharge-glycerine cement. Its use, in connection with the usual 

 collar (d) and nut (c), is apparent. The cone, like those in Figures 9 

 and 10, is covered with rubber tubing, which is wound and tied at the 

 upper and lower ends with twisted shoemakers' thread. The side tube 

 (e), which serves as a vent for the escape of surplus solution, is embedded 

 with cement in a brass tube, which is threaded externally to receive the 

 cap (/). The packing in the bottom of the cap closes the vent. It will 

 be seen that the means of adjusting pressure within the cell is the same 

 in all three of the instruments represented in Figures 9, 10, and 12. 



Another form of glass cone which has rendered good service is seen 

 in Figure 13. The cone, which is made of a solid piece of glass, is bored 

 excentrically for the manometer (a) and the vent (6). The vent is 

 closed at the lower end by the rubber disk (e), which is attached to and 

 controlled by a platinum rod running through the hole in the stopper. 

 The upper end of the rod is threaded and provided with a nut, as seen 

 in the figure. To prevent the solution which escapes through the vent 

 from coming into direct contact with the cement, all exposed parts of the 

 latter are painted with a solution of rubber. The solid glass cone has 

 some advantages over the hollow one, as will appear when the manipu- 

 lation connected with filling and closing the cells is explained. 



