280 



Dr. W. M. Bayliss. 



[Apr. 19, 



of congo-red, as described above ; a faint haze, with bright particles here 

 and there, these bright points being much more numerous than in congo-red. 

 I am inclined, therefore, to attribute the low osmotic pressure found to 

 incomplete removal of electrolytes, the solution really being one of single 

 molecules, but, owing to the electrolytes, a large number of these are 

 aggregated to particles, the rest being single. 



The free acid of congo-red, as already stated, forms a deep blue colloidal 

 solution, with comparatively large particles, easily resolved bv the ultra- 

 microscope. These particles are of a very nearly uniform size. In view of 

 the debated question as to the source of osmotic pressure, it is of interest to 

 see whether an unquestionable suspension, such as this is, has an appreciable 

 osmotic pressure and, at the same time, to determine the number of particles 

 present in unit volume. Since, as shown above, the salts of this acid have 

 so considerable an osmotic pressure, while that of the particles of the acid 

 would probably only be small, it is obviously of the greatest importance to 

 ensure the absence of traces of alkali, whether in the water used or from 

 glass vessels. The apparatus shown diagrammatically in fig. 2 was devised 

 for this purpose. It will probably be found of use in the investigation of 

 other colloids sensitive to traces of electrolytes, such as arsenious sulphide. 

 All the tubes in contact with the fluids in the osmometer are of quartz and 

 the containing vessel of glass is thickly coated with paraffin inside. The 

 membrane is a Schleicher and Schiill parchment-paper thimble, as used by 

 Waymouth Eeid and by Hiifner. It is necessary to test these thimbles very 

 carefully for holes and, for my purpose, to extract thoroughly before use 

 with hydrochloric acid and distilled water to remove soluble salts. In future 

 experiments it would perhaps be better to use membranes of collodion. 



The distilled water must be carefully prepared. I found it best to distil 

 tap water (New River) after adding potassium permanganate and sulphuric 

 acid, and again after the addition of barium hydroxide. When essential to 

 exclude carbon dioxide entirely, the water from the quartz condenser drops 

 directly into the osmometer, as shown in the figure. 



In an actual experiment with congo-red acid, the outer water was at first 

 made acid with hydrochloric acid to ensure the absence of any salt of congo- 

 red in the contents of the osmometer. The solution in the inside had been 

 diabysed for a long time against distilled water. The concentration at the 

 end of the experiment was - 465 per cent, of the free acid. At the beginning 

 of the experiment no measurable osmotic pressure was obtained ; the acid 

 was in the form of a precipitate, due to the presence of hydrochloric acid. 

 Distilled water, from potassium permanganate and sulphuric acid, was then 

 allowed to flow from the condenser, gradually displacing the hydrochloric 



