MEASUREMENT OF OSMOTIC PRESSURE 551 



to some extent guarded against) by Pfeffer ; in particular, the 

 German botanist was aware of the errors due to leakage of the 

 solution through the membrane and to dilution of the solution 

 by inflowing water ; he saw the importance of using a manometer 

 of small bore and stoppers of slight compressibility in order to 

 diminish the inflow and actually invented the method of applying 

 pressure mechanically in order to reduce this factor to the 

 smallest possible dimensions. 



The osmotic pressures developed in the apparatus were 

 measured by means of an air-manometer (fig. 1) ; this had a 

 closed limb graduated over a range of 200 mm. and a short open 

 limb also graduated from the same zero and provided with a bulb 

 to act on a mercury reservoir. In order to secure rapid adjust- 

 ment, the bore of the tube was small, about i'2 mm. ; the air was 

 renewed after every five experiments lest water should have 

 crept into it ; a joint at a, by which the long limb could be 

 disconnected, also served as a tap by which the manometer 

 could be cut off from the osmotic apparatus. 



As is shown in the figure, the final sealing of the apparatus, 

 after it had been completely filled with solution, was effected by 

 fusing the capillary point of the glass tube shown atg; the tube 

 g was then forced down a little, in order to hasten the attainment 

 of a steady pressure and reduce the quantity of water entering 

 the cell. The inflow of water into the most concentrated 

 solutions, due to the displacement of 100 mm. of mercury, was 

 about o"i 1 c.c. on a total of 16 c.c. ; the compression of the rubber 

 stoppers amounted to 0*05 c.c. at 2 atmospheres and 0*09 c.c. at 

 4 atmospheres ; but the total inflow can scarcely have exceeded 

 0*14 c.c. or less than 1 per cent, when the glass tube g was 

 pressed down after sealing. 



The rubber stopper holding the tube g was wired down when 

 using higher pressures up to 7 atmospheres. Steady conditions 

 of temperature were secured by immersing the whole apparatus 

 in water or in a dilute solution of a membrane-former ; thus a 

 0*09 per cent, solution of copper nitrate was often used, a o - i 

 per cent, of ferrocyanide being placed inside the cell to balance it. 



The concentrations of the solutions were checked by mea- 

 suring their densities both before and after they were used for 

 the osmotic experiments ; in the case of sugar solutions the 

 polarimeter was used to check both the concentration and the 

 purity of the sugar. The substances examined were cane sugar, 



