126 



OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS. 



the effort to secure a cell of fixed capacity. The upward displacements 

 of the manometer in Series II are given, as an illustration, in Table]l6. 



Table 16. — Cane sugar, Series II. Upward displacements of the manometers (mm.). 



In Series I there was much actual slipping of the manometer in the 

 rubber stopper, in consequence of which the enlarged part of the manom- 

 eter — the bulb blown near the end— was frequently forced out of sight 

 into the stopper. It usually stopped, in such cases, just below the 

 constricted mouth of the glass tube. The principal purpose of the con- 

 stricted mouth of the tube and of the enlargement on the end of the 

 manometer was, originally, to prevent the instrument from being 

 pushed out of the cell. The slipping of the manometer in the stopper 

 was remedied by wrapping and tying the lower end of the latter in the 

 manner already described. 



The upward displacement of the manometer disappeared after the 

 fourth series. There was therefore some dilution in the first four series, 

 which was due to an enlargement of the capacity of the cells. 



3. The opening of the cell, after a measurement, like the closing of it, 

 was originally a process which required about 15 minutes. During 

 this time, the contents of the cell were again under a pressure which was 

 less than the osmotic pressure, and dilution of the solution necessarily 

 ensued. For reasons which will be given hereafter, dilution occurring 

 at this period was a much more serious matter than that which took 

 place during the closing of the cells or through an increase in their capa- 

 city under pressure. It was necessary, therefore, to suppress it as expe- 

 ditiously as possible. Several remedial measures were resorted to: (1) 

 Every effort was made to increase the rapidity of the necessary manip- 

 ulation ; (2) the rubber stopper was pierced with a hollow needle before 

 attempting to withdraw it; (3) "dipping" the cells before opening them 

 was practiced ; (4) a method was devised for slitting the stopper through- 

 out its whole length, which made it possible eventually to remove the 

 manometer in less than a minute, and without reducing the pressure upon 

 the cell contents below that of the atmosphere. 



The final result of all the measures taken to eliminate the three 

 known sources of dilution was the complete disappearance of loss in rota- 



