1909.] 



The Properties of Colloidal Systems. 



271 



more accurately. Dilute solution of sodium hydroxide was then run 

 through the lower chamber in order to convert the free acid into the sodium 

 salt. This solution was replaced at intervals of 24 hours until the outside 

 solution remained permanently slightly alkaline. During this process the 

 osmotic pressure rose gradually to about 40 mm. Hg. Kepeated changes of 



H 



Fig. 1. 



A, Osmometer of Moore and Eoaf. 



B, Mercury manometer, read by means of a reading microscope. 



C, T-tube, with inner smaller tube, to allow a current of water, or other fluid, through 



the outer, lower, chamber of the osmometer. 



D, Tube, connecting the two chambers when the stopcock is opened. This is done in 



order to control the zero of the manometer at any time. 



E, Inlet tube to the lower chamber. F, Outflow. 



G, Soda-lime tube to exclude atmospheric C0 2 . 



H, Tube from flask of distilled water or other fluid. 



The osmometer was immersed in a thermostat. 



distilled water were then similarly run through as long as the pressure 

 continued to rise. After ten days, equilibrium was attained with a pressure 

 of 79-3 mm. Hg at a temperature of 30 o- 2 C. 



The molecular weight of congo-red (the di-sodium salt of benzidine- 

 tetrazo-di-naphthylamine-di-sulphonic acid) is 696 - 47. On the basis of an 

 osmotic pressure of 22"4 atmospheres for a molar solution (as true solution) 

 at 0°, a 1-per-cent. (=10 grammes per litre) solution at 30 o- 2 should have a 

 pressure of 



22-4 x 760 x 273 +f' 2 x -ig- = 271'4 mm. Hg. 



